What was the biggest insect on Earth?

A dragonfly that lived about 310 million years ago. The insect had a wingspan of 271/2 inches (69.9 cm)! That’s about the distance from the tip of your nose to the end of your arm. The biggest dragonfly today has a wingspan of less than 5 inches (12.7 cm).

Which is the meanest fly?

A kind of gall midge fly. Gall midge flies are born inside their mother’s body. Once born, they start eating her insides! In about two days they have eaten her whole body. The newborns then crawl out, leaving behind the hard, empty shell of the mother. Talk about rotten kids!

Which flies are like helicopters?

The hoverfly, bee fly, and flower fly. Most flies can only fly forward. But these three can fly forward and backward. They can even hover in place.

Are flies dirty?

Not all of them. Flies that live in dirty surroundings are dirty. A fly may walk around on rotting food, garbage, and other filth. Germs stick to its legs, mouth parts, and hairy body. Its legs alone may have over one million germs! If you see a fly near your sandwich, shoo it away. Flies that touch your food may leave harmful germs behind.

Do flies taste with their tongues?

No—mostly because they don’t have tongues! Flies taste with their feet. First they step on food. If their feet tell them it tastes good, they eat it. But flies can’t eat solid foods. They can only sip liquids. House flies and fruit flies soak up liquids with their mouth parts and then sip the liquid. Or they drop saliva on solid food, changing it into a liquid that they then drink. Sand flies, stable flies, and female mosquitoes have sharp mouth parts. They stab the victim and then sip its blood.

How did army ants get their name?

From the way they march along in huge troops of between 10,000 and 30,000,000 soldiers! Their feet make a loud rustling noise as they walk. Some people find this the scariest animal sound in the world. Army ants also give off a smell like rotting meat. Often they kill and eat other insects, spiders, or larger animals that do not escape in time.

Are ants strong?

You bet. An ant can lift a weight 50 times as heavy as its body. If you were that strong, you could pick up a car weighing nearly 2 tons (2.1 t)! Ants are also very fast walkers. The hotter it is, the faster they walk. Try shading some ants with a piece of cardboard on a hot, sunny day, and watch them slow down.

Do ants have good eyesight?

Most ants can see nearby objects very well. But some are blind. To survive, all ants depend on a good sense of smell. Ants smell with their antennae. Their antennae lead them to food and help them recognize friends and foes. Ants can live without seeing well. But if they lost their sense of smell, they’d die.

How do ants find your lunch in the park?

By smell. An ant finds a crumb you dropped. It carries the crumb back to the nest. As it goes, it presses its abdomen to the ground. This leaves a smell trail. Back in the nest, the ant pokes the other ants. They run out and pick up the scent. By instinct, they follow the trail. Soon there are dozens of ants looking for more lunch crumbs.

Do all bees sting?

No. Only female bees can sting. Also, some kinds of bees have no stingers. Some have stingers but don’t use them. A honeybee has a barbed stinger at the back of its abdomen. Usually the bee keeps it hidden, but it’s ready for use if needed. A honeybee stings when in danger. It plunges the stinger into the victim’s flesh. But as the bee flies away, the barbs hold fast. The stinger pulls out of the bee’s body—and the bee soon dies. Bee stings can be dangerous as well as painful. In the United States alone, about 40 people a year die from bee stings.

Why do honeybees make honey?

To feed the bees in their colony. A large colony of honeybees has up to 80,000 bees. They eat nearly 500 pounds (225 kg) of honey a year. Honeybees collect nectar from flowers and bring it back to the hive, or nest. Other bees place the nectar into six-sided cells, where it changes into honey. The bees eat some honey and feed some to the larvae. They store the rest for winter feeding or for rainy days. The stored honey is what beekeepers collect—and you eat.

How busy is a bee?

Very. A honeybee needs both nectar and pollen to live. To get these foods, the honeybee may visit 500 flowers in a single trip. The insect makes about 15 such trips on a sunny day, covering about 31/2 miles (5.6 km). A bee must collect nectar from about 22 million flowers to make 1 pound (0.5 kg) of honey!

What gives moth and butterfly wings their color?

Tiny, colorful, overlapping scales. Under a microscope they look like shingles on a roof. Some scales are colored red, orange, yellow, brown, black, or white. Others bend and scatter light, so they look blue, silver, violet, or green. The colors may blend in with a background and help hide the butterfly. Or they may stand out and scare enemies away. Often the colors are meant to catch another butterfly’s eye.

How can you tell a moth from a butterfly?

When at rest, a moth spreads its wings out flat to form a triangle. A butterfly holds its wings straight up, like sails on a boat. Moths have feathery antennae without knobs. Butterfly antennae are slender, with knobs at the tips. The bodies of most moths are thicker and furrier than those of butterflies. Moths fly mostly at night, while butterflies like sunlight and fly during the day.

Which insects live in your house?

The flies, mosquitoes, and ants you sometimes see, of course. But also many insects that keep out of sight. The holes in your wool sweater were probably made by moth caterpillars or beetle larvae. Your dog scratches to get rid of tiny fleas. Hidden in your bookcase may be “bookworms,” which are beetle larvae. And those fuzzy creatures you can find inside bad apples are the caterpillars of certain moths.

Do all insects go through metamorphosis?

No. A few, such as silverfish and springtails, simply grow bigger, much as you do. The newborns look just like their parents—except smaller. And from the time they hatch until they become adults, these animals hardly change their form or shape.

When do nymphs grow wings?

The time varies. Grasshopper nymphs take about two months. During this time, the nymphs keep eating, growing, and molting. With each molt, they develop bigger and bigger wings. After about five molts, the nymphs are full grown. Now they have wings, just like their parents do. Most dragonfly nymphs live in the water. Some take up to 5 years and 12 or more molts to become adults. They then crawl up out of the water, on plant stems. Soon they shed their final skin, open their wings, and fly away.

Do all insects pass through four stages?

No. Grasshoppers, crickets, termites, mayflies, cockroaches, dragonflies, and some others go through only three stages: egg, nymph, and adult. This is known as incomplete metamorphosis. The egg stage and adult stage are the same as in complete metamorphosis. Only the nymph stage is different. Nymphs have unformed wings, are smaller than their parents, and are often not the same color. The nymphs gradually grow bigger and develop wings and reproductive organs.

How long does the pupal stage last?

Anywhere from a few days to over a year. The pupae are forming new body parts. Many kinds of insects spend the winter as pupae and emerge as adults in the spring. As adults, the insects are ready to mate and start the whole process all over again: egg, larva, pupa, and adult.

What are pupae?

A stage of development when insects are protected by a case, or covering. Most pupae look lifeless, but inside the case many big changes are taking place. The pupae are slowly turning into adult insects. Larvae usually find well-hidden places in which to change into pupae. Beetle pupae can be buried deep in the soil. Most moth pupae are covered by cocoons. They might be hidden under a leaf or behind the bark of a tree. Butterfly pupae, called chrysalides (kruh-SAL-uhdeez), are usually attached to weeds or tree branches. Mud, stones, dead leaves, or rubbish protect some pupae.

What do larvae do?

Eat and eat and eat. One kind of moth larva eats 86,000 times its weight in the first two months of life. If you did the same, you’d put away 300 tons (305 t) of food! Day by day, the larvae grow bigger. Since their outer skins can’t expand, the insects molt. The young insects crawl out of the old skin. Then the new skin, which has been forming under the old skin, hardens. The insects may eat the skin they shed or leave it behind. It looks just like a real insect—except that it is hollow! After larvae have molted several times, they finally stop eating and molt for the last time. Now when the skin splits, they are in an entirely new stage of life. They are pupae.

What hatches from insect eggs in complete metamorphosis?

Wormlike or grublike larvae. Different species’ larvae have different names. The larvae of moths and butterflies are called caterpillars. The larvae of some flies are called maggots. You might see them on dead animals or decayed meat. Some beetle larvae are known as grubs. Mosquito larvae are called wrigglers.

What do insect eggs look like?

Most are round in shape and light in color. But they can also be long or short, have ridges or be smooth. The eggs of very small insects are so tiny that they can be seen only under a microscope. But the eggs of big insects can be up to 1/2 inch (13 mm) long. Insects lay their eggs either separately or in clumps.

What happens during metamorphosis?

Insects change. In butterflies, moths, beetles, flies, bees, wasps, ants, and most other insects, the change has four completely different stages: egg, larva (plural is larvae), pupa (plural is pupae), and adult. This is known as complete metamorphosis.

Do newborn insects look like their parents?

Not usually. Most newborn insects do not resemble the adults at all. In fact, they look so different that you can’t tell they’re the same species. When they grow up, all insects of the same species will look alike. But first, almost every insect passes through a number of stages. The process is called metamorphosis.

Do insects ever sit on the eggs they lay?

No, but the adults of a few species do stay with the eggs. In certain of these species, adults protect and feed the young for some time after they hatch. But most female insects lay their eggs and then either leave or die.

Where do insects lay their eggs?

In soil, on plants, in and on animal bodies, and in water. The place varies with the kind of insect. But each place supplies food to the insects that hatch from the eggs. For example, the female horse botfly sticks her eggs to hairs on a horse’s legs. The horse licks off the eggs. The eggs hatch and the young insects, called maggots, start to grow inside the horse’s stomach!

How are insects born?

Most hatch from tiny eggs laid by female insects. A few insects give birth to living young. These newborns hatch from eggs inside the female’s body.

Do insects fall in love?

No. But insects do have ways of finding one another. Some female moths and male butterflies give off a special odor. Male grasshoppers, crickets, cicadas, and katydids sing. Both sexes of fireflies produce flashing lights. Female mosquitoes whirr their wings. And some male insects give their mates tasty bits of food to eat.

Can insects harm you?

Fewer than 10 percent of all insects bite or sting humans. Yet insects can—and do—cause  enormous suffering. They can carry germs that cause yellow fever, cholera, typhus, and many other diseases. For example, every 10 seconds a person dies of malaria, a disease carried by certain mosquitoes. It is said that one-half of all human deaths throughout history were caused by mosquitoes. Insects can be big pests. They eat about 10 percent of all food and fiber crops. They also harm cattle and sheep by spreading disease among them. Farmers spend about $7 billion a year to control pesky insects.

Why don’t hibernating insects freeze to death?

The blood of several kinds of insects contains a kind of antifreeze called glycerol (GLIHSuh-rohl). This helps to keep them alive until warm weather returns. The African midge can survive the very lowest temperatures. One was dipped in liquid helium at a temperature of –452 degrees Fahrenheit (–269°C), and it lived!

What happens to insects in the winter?

Many have laid eggs by then and died. In the spring, the eggs hatch and newborns emerge. Others hide or hibernate in attics, cellars, barns, leaf piles, holes in trees, under bark, in caves, or in underground tunnels. While hibernating, the insects breathe more slowly and don’t eat. When warm weather returns, they become active again. Honeybees form big balls inside the hive. The bees on the inside shake and shiver to raise their body temperatures. The heat spreads out and warms all the bees. Some insects migrate for the winter. Monarch butterflies fly south about 2,000 miles (3,200 km). At the beginning of spring, they head north.

Which insects run the fastest?

Cockroaches. They can reach speeds of 21/2 miles (4 km) an hour. You may not think that is very fast. But at that speed they cover 40 body lengths a second. Compare this with human runners, who cover only four body lengths a second.

How fast can insects fly?

Faster than you can run! Yellow jacket wasps can fly 15 miles (24 km) an hour. That’s fast enough to catch you if you disturb a nest. Dragonflies are probably the fastest, at about 60 miles (96 km) an hour. A no-see-um midge holds the record for wing speed. It flaps its wings nearly 63,000 times a minute!

Which is the smallest insect?

The fairyfly. It is only about 1/100 of an inch (0.25 mm) long and is nearly invisible to the naked eye. In fact, the fairyfly is so tiny, it can fit through the eye of a small needle! Nearly 150 million of its eggs together weigh only 1 ounce (58 g).

Which is the biggest insect?

The Goliath beetle. At over 4 inches (10 cm) long, this insect is the size of a computer mouse! Also, it weighs nearly 1/4 pound (100 g). This makes it the heaviest insect as well. Another big insect is the Atlas moth. It has a wingspan of 12 inches (30 cm) from tip to tip. About 1 foot (30 cm) in length, the tropical walkingstick is the longest insect on record. If you include its legs, the insect measures 20 inches (51 cm). This stick insect lives in the rain forests of Borneo.

How do insects defend themselves?

Usually by escaping. They fly, run, or jump away. Many use camouflage. They blend in with their surroundings. Green caterpillars look like leaves. Gray and brown moths resemble the bark or moss on trees. When walking-stick insects sit on a branch, they look like twigs. The caterpillars that become giant swallowtail butterflies look like bird droppings. Some insects fight back. Ladybugs, stick insects, cockroaches, and certain beetles give off bad-smelling liquids when enemies come too close. Some ants and beetles bite with their powerful jaws. Bees, wasps, and some ants sting. Other insects have bright colors that warn away their enemies. Monarch butterflies taste bad, and birds have learned to leave them alone. Viceroy butterflies don’t taste bad, but they look like monarchs and this keeps them safe.

What eats insects?

Birds, frogs, lizards, skunks, anteaters, fish, and many other kinds of animals eat insects and insect eggs. Insects also eat other insects. Humans eat insects, too—like locusts, ants, caterpillars, and beetle larvae. There are about 500 kinds of insect-eating plants. Perhaps you know the Venus flytrap best. It can catch an insect in the blink of an eye! Then it slowly digests the unlucky bug.

What do insects eat?

Many different things. Butterflies, moths, flies, and mosquitoes are sucking insects. They feed on liquids. These insects use their mouth parts to suck up nectar and other fluids. Grasshoppers, crickets, beetles, and termites are chewing insects. They eat plants and other solid foods. These insects use one pair of jaws to cut off bits of food and grind them down. Another pair of jaws helps to push the food down the throat. A few insects, such as mayflies and some moths, never eat. That’s because their lives are over in just a few hours or days. These insects become adults, lay eggs, and die. Some insects are very heavy eaters. A silkworm eats enough leaves to increase its weight more than 4,000 times in just 56 days. A locust eats its own weight in plants every day. Just imagine eating your weight in food every day.

Do insects have tongues?

No. But insects have other ways to pick up various flavors. Butterflies, moths, bees, and flies taste with their feet. Ants, wasps, and some bees taste through their antennae. Crickets and some wasps taste with the tips of their abdomens to find a good place for laying eggs.

Do insects have a sense of touch?

Yes. Insects have a sense of touch that is far sharper than yours. Some of the short hairs that cover insects’ antennae and bodies are connected to nerves and are very sensitive. They pick up the lightest pressure—even a little breeze. The keen sense of touch helps most insects fly away before you can swat them. As soon as you move your hand, they feel the air moving. And away they go!

Do insects make sounds?

Yes. Many insects hum, buzz, or sing. But they don’t make sounds the way you do. They have no vocal cords. Whirring sounds come from rapidly flapping wings. Clicking and other sounds are made by rubbing body parts together—usually wing against wing or leg against wing. Male cicadas vibrate a thin skin on their abdomens. Their sounds can be heard for more than 1/4` mile (0.4 km). Sounds often help insects keep in touch with one another. But they’re also used to warn of danger or to woo a mate.

How do insects smell?

With antennae. The antennae of May beetles, for example, have 40,000 tiny pits. Each one is like a little nose for smelling. We wonder: If they catch cold, do they have 40,000 runny noses? Many insects give off special chemicals that only other insects can sense. Antennae let each kind of insect find food, tell friend from foe, and spot danger. Some male moths can find female moths that are up to 7 miles (11.2 km) away—just by their smell!

How do insects hear?

Not through ears like ours! Crickets hear through tiny openings on their front legs. Locusts, cicadas, and some kinds of moths and grasshoppers hear through little flat “ears” on their abdomens. Ants and mosquitoes hear with hairs on their antennae. Caterpillars receive sounds through hairs all over their bodies. All sound is made by vibrations in the air. Insects pick up these vibrations and hear very well—even without ears like ours!

Do some insects have extra eyes?

Yes. Most adult insects also have three tiny simple eyes called ocelli (oh-SEL-eye). You can find them between the two compound eyes. The simple eyes cannot form images. They help the insect tell light from dark.

How do insects see?

With two large eyes that can take up most of an insect’s head. Insect eyes are called compound eyes. Each compound eye is made up of many tiny lenses. A housefly’s eye, for example, has 5,000 lenses. But dragonflies take the prize, with 30,000 lenses in each eye! Insects can spot anything that is moving. Yet most don’t see very well. The world looks blurry to them. And since insects don’t have eyelids, their eyes are always open.

How do insects breathe?

Through tiny holes along their sides. Insects have no lungs. Instead, the air passes from the holes into a large tube. This tube divides into small tubes. The small tubes divide into still smaller tubes. These very tiny tubes bring oxygen to every part of the body.

Do insects have blood?

Yes. But the blood is usually not red like your blood. Insect blood is generally light green, yellow, or colorless. And it doesn’t flow through veins and arteries. The insect’s heart pumps blood through all the empty spaces inside the insect’s body. Slap a mosquito and you may see red blood. But that’s not the blood of the mosquito. It’s your blood—or the blood the mosquito got from another person or from an animal.

How do insects walk on six legs?

Easily. They move the front and back right legs at the same time as the middle left leg. Then they switch, moving the front and back left legs and middle right leg. This way they’re always balanced on three legs. Each of their six legs has five parts. Muscles attached to the thorax move the legs. Does it sound complicated? Be glad you have only two legs to worry about!

Are insects strong?

Very. Some have as many as 4,000 separate muscles. That’s a lot more than the 600 muscles in your body! A bee, for example, can lift a load 300 times its own weight. If you were that strong, you could pick up a 10-ton (10.2 t) truck!

Do insects have bones?

No. Instead, every insect has an outside skeleton, called an exoskeleton. Attached to the exoskeleton are the insect’s muscles. The exoskeleton protects the insect like a suit of armor. As the insect grows bigger, its exoskeleton gets too tight. It splits open and the insect comes out. This is called molting. Then, a new and bigger exoskeleton hardens around the insect. Molting occurs again and again, until the insect is a full-sized adult.

Skaters and Swimmers

In the calm water of ponds, deadly predators are on the move. Diving beetles scud through the shallows, grabbing small fish and insects with their sharp claws. Water boatmen hang just beneath the surface, waiting to stab flying insects that crash-land. On the surface itself, pondskaters wait for their victims, feeling for tiny ripples that pinpoint struggling prey. About one in 20 insect species live in watery surroundings such as ponds, lakes, rivers, and streams. Some spend their whole lives in freshwater, while others grow up in it and then fly away.

SURFACE TENSION
Instead of floating, pondskaters use surface tension to walk on water. Surface tension is a force that pulls water molecules together. When the water is calm, it makes the surface behave like a thin sheet. Pondskaters have light bodies and waterrepellent feet, so they can stand on the surface without sinking through it. This photograph, taken under a special light, shows how the water makes dimples around a pondskater’s feet.

WATER SCORPION
Although they live in water, most freshwater insects breathe air. The water scorpion gets its air supplies through a long snorkel, which it pushes up through the surface. The snorkel passes air to its tracheal system, which delivers oxygen throughout its body. Water scorpions are carnivorous bugs, and they stalk small fish and insects in muddy water. Their weapons are stealth, good camouflage, and two strong front legs that grip and spear their prey.

PONDSKATER
Seen from the side, this pondskater shows its piercing mouthparts and long slender legs. Pondskaters eat insects that have become stranded on the surface. They use their front legs to grip their food, their middle legs to swim, and their back legs to steer. Pondskaters are true bugs, and most of them have well-developed wings. They can fly, so it is easy for them to spread from pond to pond.

MAYFLY NYMPH
Mayfly nymphs that live in running water have flattened bodies and strong legs – features that prevent them from being washed away. Instead of breathing air from the surface, they collect oxygen through two rows of feathery gills. The nymphs spend up to three years underwater, preparing for an adult life that lasts less than a day.

GREAT DIVING BEETLE
With bodies up to 5 cm (2 in) long, diving beetles are powerful freshwater hunters. They swim with their back legs, speeding through the water. Before each dive, they store air under their wing cases, and they have to kick hard to stop themselves bobbing up to the surface. Diving beetle larvae are even more aggressive than their parents, with powerful jaws that can kill tadpoles and small fish.

SAUCER BUG
Like most freshwater bugs, saucer bugs are hunters, and they grip their victims with their front legs, which can snap shut like a pair of penknives. Saucer bugs lurk on the bottom of ponds, and their camouflage helps them to hunt. They surface to breathe, but afterwards they quickly dive back to the bottom, to hide among plants or in mud.

WHIRLIGIG BEETLES
Predatory insects often lie in wait, but whirligig beetles are always on the move. Like tiny black boats, they spin around on the surface, watching for small insects that have fallen in. A whirligig’s eyes are divided into two parts. One part looks above the surface, while the other part looks at the water underneath. This all-round view means that whirligigs can dive after food, and spot danger from above and below. Adult whirligigs spend the winter buried in mud at the bottom of ponds.

PHANTOM MIDGE LARVA
With its transparent body, the phantom midge larva is an almost invisible hunter. It hangs motionless in water, and snags small animals with its hook-shaped antennae. To change its depth, it adjusts two pairs of onboard buoyancy tanks, which make it rise and fall like a submarine. In summer, the adults often gather in dense swarms that look like clouds of smoke.

BACKSWIMMER
Hanging beneath the water’s surface, the backswimmer spends its entire life upside down. Like pondskaters, it eats insects that have crashlanded, but it attacks them from below. Its extra-long hindlegs work like a pair of oars, and it uses them to swim towards its prey. Backswimmers have big eyes, and they always keep their fronts towards the light. If they are put in a tank that is lit from the bottom, they swim the right way up.

DRAGONFLY NYMPH
Young dragonflies – known as nymphs – rely on stealth to hunt. They crawl along the bottom of ponds and lakes and up water plants, watching for prey. If a tadpole or small fish wanders near, they spear it with their telescopic jaws. The nymphs breathe by sucking water in and out of their abdomens. If danger threatens, they squeeze water out of their abdomens like jet engines – the perfect high-speed getaway.

Predatory Insects

Insects have many enemies, but the most deadly are often other insects. Some chase their prey in the open, while others use stealth, taking their victims by surprise. Some do not feed until their prey is dead, but praying mantises start straight away, while their meal is still struggling to escape. Predatory insects eat a huge range of small animals, including other insects, spiders, mites, fish, and frogs. Some of these are troublesome pests, so predatory insects can help to keep them under control.

HUNTING IN A PACK
When predators hunt together, they can attack prey much larger than themselves. These army ants are doing just that, as they swarm over a caterpillar. Army ants live in tropical forests, and pour over the ground like a wolf pack, overpowering anything that is too slow to get away. A single army can contain more than a million ants, advancing in a column up to 15 m (49 ft) wide. The ants have tiny eyes, so they find their prey by touch.

AMBUSHED!
For a praying mantis, a fly makes a tasty meal. Using her superb eyesight, this female mantis spots a fly, and then makes a lightning-fast attack. As she lunges forwards, her front legs open straight and then snap shut, pinning the fly between two rows of sharp spines. With the insect imprisoned, the mantis starts to feed. Male mantises are smaller than the females. When mantises mate, the female sometimes dines on her partner, starting with his head.

WASP STING
To make a kill, predatory insects have to overpower their victims. Many use their legs or mouthparts to grab their prey, but wasps often follow this up with a deadly sting. The sting slides out of the wasp’s abdomen, and a muscular pouch pumps venom through the sting and into the victim’s body. Unlike wasps, bees are not predatory. Their stings often have barbed tips, but they use them only in self-defence.

LIVING LARDERS
Some insects hunt on behalf of their young. This wasp has caught a spider, and has paralysed it with her sting. She will drag the spider back to an underground nest, where it will be a living food store for one of her grubs. There are many species of hunting wasps, and they specialize in different types of prey, from caterpillars to tarantulas. Only the females hunt – the males usually feed at flowers.

Dragonflies and Damselflies

Speeding over fields and ponds, dragonflies are some of the fastest-flying hunters in the insect world. They feed on other insects, overtaking their prey and then grabbing them in mid-air. There are about 5,500 species of dragonflies and damselflies, and all of them have large eyes, long bodies, and two pairs of transparent wings. Dragonflies usually rest with their wings held out, but damselflies fold theirs along their backs. Young dragonflies and damselflies live in freshwater, and take up to three years to grow up. During their underwater development, they feed on other animals, catching them with a lightning-fast stab of their jaws.

MID-AIR KILLER
With its strong wings and sturdy legs, this dragonfly is superbly equipped for ambushing and catching its prey. Inside its extra-large thorax are powerful flight muscles that beat its wings. Unlike most insects, a dragonfly’s wings beat in opposite directions, which means that it can fly backwards or hover on the spot. Dragonflies have very long abdomens, and people often imagine that they can sting; however, they cannot. Instead, dragonflies and damselflies kill their prey with their powerful legs and jaws.

HUNTING UNDERWATER
This dragonfly larva has caught a stickleback fish. It hunts by stealth, ambushing or stalking its prey. When it is close enough, it shoots out a set of hinged mouthparts, known as a mask. The mask is tipped with two spiky claws, and it works like a harpoon, stabbing and then pulling in the prey. Young damselflies are less powerful and eat smaller water animals.

HUNTING IN THE AIR
Adult dragonflies usually spot their prey by patrolling through the air. This dragonfly has just caught a meal, and has settled down to feed. It uses its feet to catch its prey, and also to hold it down as it starts to eat. Damselflies use a different technique – they either sit and wait for insects to fly past, or snatch them from waterside plants.

PAIRING UP
Dragonflies and damselflies have a unique way of mating – these two damselflies show how it is done. The male, on the left, grips the female behind the head, using a pair of special claspers on his tail. Meanwhile, the female’s tail reaches forwards to touch the male, so that her eggs can be fertilized. The mating pair can fly like this, and they often stay paired up while the female lays her eggs.

Insect Flight

Insects are some of the most impressive fliers in the animal world because of their small size. Dragonflies dart through the air after prey, and bees speed over fields and gardens in search of flowers. Hover flies can stay rock-steady in mid-air, while butterflies can migrate across entire continents. To do all this, insects use special flight muscles that power their wings. These muscles are packed inside an insect’s thorax, and they can work for hours without a rest. Large butterflies flap their wings slowly, so each wingbeat is easy to see. But many insects beat their wings hundreds of times a second, making them vanish in a blur. When wings move this quickly, they make the air vibrate. This creates a buzzing or whining sound.

POWER FOR FLIGHT
Some insects – including dragonflies – have flight muscles that are attached directly to the wings. But in more advanced fliers, such as wasps, they are attached to the thorax. These muscles work by making the thorax change shape. One set pulls vertically, making the top of the thorax move down. When this happens, the wings flick up. Another set pulls horizontally, making the wings drop back down. Once the wings start beating, the muscles carry on automatically until the insect decides to land.

BUTTERFLY IN FLIGHT
This time-lapse sequence shows a butterfly speeding through the air. Beneath it, a time bar shows how long each wingbeat lasts. Butterflies have four wings, but they beat like a single pair. Most of the push comes when the wings flick down, but because the wings twist slightly, some extra push comes when they flick up. In windy weather, butterflies are easily blown about, so they keep close to the ground.

WARMING UP
Insect flight muscles work best when they are warm. When the temperature drops below about 10°C (50°F), many insects are too cold to take off. But not all insects are like this. Bumble bees shiver to warm up their muscles – after a few minutes their flight muscles can be 20°C (68°F) warmer than the air outside. This Arctic bumble bee is feeding on flowers in Greenland, which is less than 750 km (465 miles) from the chilly North Pole.

UNDERCARRIAGE
Many flying insects use their legs to launch themselves into the air. This scorpionfly has taken off with a helpful kick. Scorpionflies are quite weak fliers, so they choose a high point from which to jump. Crickets and grasshoppers give a bigger push – once they are airborne, they can open their wings and fly away. During flight some insects fold their legs away, but many spread them out. This helps them to balance, and also makes it easier to land.

FLIGHT SPEEDS
Insects often fly in short bursts, which makes it difficult to measure their speeds. Many cruise quite slowly, but speed up if they are in danger, or if they are chasing their prey. This chart shows flight speeds for a range of different insects. At 58 km/h (36 mph), dragonflies can overtake most other insects, and even some small birds. However, they cannot keep flying at top speed for long, because their bodies begin to overheat.

Insect Behaviour

Compared to humans, insects have simple nervous systems, and their brains are often smaller than a full stop. But despite this, they have quick reactions, and they often behave in complex ways. All of them know how to search for food, how to escape danger, and how to track down a mate. Some can perform much more impressive feats, such as navigating their way across featureless sand or building elaborate nests. Insect behaviour is controlled mainly by instinct. Instinct is like a computer program that is built into an insect’s brain. It tells an insect what to do, how to do it, and often when to do it as well.

RAPID REACTIONS
The instant a house fly senses danger, it takes emergency action, and launches itself into the air. To do this, it relies on its fast-acting nervous system. The trigger for launch usually comes when its eyes spot movement overhead. Special nerves flash signals from the eyes to the insect’s flight muscles, powering up its wings. At the same time, the fly stows away its tongue and pushes up with its legs. By now, its wings are already buzzing, and in fraction of a second, it is on its way.

BRAINS AND MINI-BRAINS
Like all insects, this cockroach has a brain in its head, and a nerve cord that runs the length of its body. The nerve cord works like a data cable. It collects signals from sense organs and carries them to the brain, and it carries signals from the brain to the muscles. The nerve cord also has a series of ganglia (mini-brains) that control regions of the body, so parts of the body can work on their own. However, the brain is in overall command.

BUILT-IN CLOCKS
These two cockroaches have been caught on camera, feeding after dark. Like all insects, cockroaches cannot tell the time. Instead, their activities are controlled by a chemical clock that ticks away inside their brains. This built-in clock keeps insects in step with the world around them, and it makes sure that they come out at night. If cockroaches are kept in 24-hour daylight, they still come out at night, even though it is not dark.

INSECT REFLEXES
Clinging on to a potato stem, these Colorado beetle larvae look like easy targets for predatory birds. The larvae do not have wings, and their legs are small, so they cannot run away. But if anything touches them, the larvae carry out a simple but effective trick – they let go of the stems with their legs, and drop to the ground. Once the coast is clear, they slowly make their way back up the plant. This kind of behaviour is called a reflex. It can save an insect’s life, but it needs almost no brainpower at all.

INSECT INTELLIGENCE
Holding a pebble in her jaws, this female sand wasp is hammering shut the entrance to her nest. It is a remarkable piece of behaviour, because tool-users are practically unknown in the insect world. Once the nest is sealed up, the wasp puts the pebble back on the ground. Tool-using makes sand wasps look intelligent, but they are not quite as smart at they seem. When a sand wasp picks up a pebble, it is simply following its instincts. Unlike a human or a chimp, it does not understand how tools work.

Insect Senses

If insects were as big as we are, some of their eyes would be as large as footballs and their antennae would be up to 2 m (7 ft) long. Fortunately, insects never reach this size, but their senses play a vital part in their lives. For us, sight is the most important sense, and it is for many insects too. Most insects also have a superb sense of smell, and some can hear sounds more than 1 km (3⁄4 mile) away. Insects use their senses to find food, track down a mate, and avoid being caught.

COMPOUND EYES
Unlike vertebrates (animals with backbones), insects have compound eyes. A compound eye is split into lots of separate facets (units), each with its own lens. Each facet works like a mini-eye, collecting light from a small part of the view. Some insects have a few facets in each eye, but horse flies and dragonflies have many thousands. This gives them a detailed picture of their surroundings – although not quite as good as ours.

THREATENING GAZE
This horse fly’s compound eyes cover most of its face. Unlike our eyes, its eyes cannot move, but because they bulge outwards, it gets a good allround view. As well as compound eyes, many insects have three small eyes, or ocelli, on the top of their heads. These eyes each have a single lens. They register light levels, but they do not form a picture.

NECTAR GUIDES
Insects see fewer colours than we do – for example, they are not nearly so sensitive to red. However, many of them can sense ultraviolet light, a colour that we cannot see. Plants often use ultraviolet markings to attract insects to their flowers. These markings are called nectar guides. They steer insects towards the centre of a flower, so that they can collect a meal of nectar, and carry pollen from one flower to the next.

EARS AND ANTENNAE
Many insects communicate by sound, but their ears are not always on their heads. Crickets have their ears on their legs, while grasshoppers and moths have them on the sides of their abdomens. Moths use their ears as an early warning system, to listen out for flying bats. An insect’s antennae (feelers) are multipurpose sense organs. They can smell, touch, and taste, and they can also pick up vibrations in the air.

Insect Habitats

Wherever you are in the world, insects are not far away. They live in every type of habitat on land, from steamy tropical rainforests to the darkness and silence of caves. Many insects grow up in freshwater, and plenty spend their adult lives there as well. Some insects live along the shore, and a few even skate over the surface of the waves. Only one habitat – the ocean depths – is entirely insect-free.

COASTS AND SEAS

The coast is a difficult place for insects. Many live in dunes or on clifftop grass, but very few can survive in places that get soaked by salty spray. Beach insects include bristletails, which scuttle among stones and rocks. Long-legged bugs called sea skaters are the only insects that live on the open sea.

TEMPERATE WOODLANDS
Every spring, temperate woodlands burst into leaf, creating a gigantic banquet for insect life. Caterpillars chew their way through this tasty food, while predatory insects, such as hornets, harvest huge numbers of caterpillars and other grubs to feed to their young.

GRASSLANDS
The most numerous grassland insects are termites and ants. They scour every inch of the surface for food, collecting seeds and leaves and carrying them back to their nests. Dung beetles are particularly useful in this habitat. They clear up the droppings that grazing mammals leave behind.

FRESHWATER
Lakes, rivers, ponds, and streams teem with insect life. Mosquito larvae feed on microscopic specks of food, but some freshwater insects, such as water bugs, are big enough to kill tadpoles and even small fish. On the water’s surface pondskaters pounce on insects that have crash-landed, grabbing them before they have a chance to fly away.

CAVES AND MOUNTAINS
Caves are home to some unusual insects. Cave crickets are almost blind and use their extra-long antennae to find their way in the dark. Mountains are often cold and windswept, but many insects use them as a home. Beetles scavenge for food among rocks, while butterflies and bees pollinate flowers. High above the snowline, wingless scorpion flies scuttle about under the snow.

DESERTS
Compared to many animals, insects are well suited to desert life. Some of them feed during the day, but many wait until after dark. Desert insects include hawk moths, antlions, and giant crickets, as well as many kinds of ground-dwelling beetles. Some of these animals never have to drink, but this darkling beetle, from the Namib Desert, collects droplets of moisture from fog that rolls in from the sea.

TROPICAL FORESTS
The world’s tropical forests have more kinds of insects than all other habitats put together. They range from microscopic wasps to giant butterflies, like this Cairns birdwing, whose wings measure 28 cm (11 in) from tip to tip. In tropical forests, many bees and flies feed at flowers, while termites and beetles feast on rotting wood. Columns of army ants swarm over the floor, overpowering any other insects in their path.

Why are there so many insects?

few reasons. Insects multiply very fast. Most females lay up to 200 eggs in a lifetime. A queen termite can lay more than 30,000 eggs a day! Insects can survive the most difficult conditions on Earth. You can find insects at the North and South Poles and at the equator, in deserts and in jungles, under the ground and high in the air—and almost everywhere in between. Insects are small. This means that each one needs little food and can easily hide from its enemies.

How many kinds of insects are there?

More than one million different species, or kinds. And scientists are still counting. Every year, experts find up to 10,000 new species. They think there may be as many as 30 million species yet to be discovered. At this rate, it will take another 1,000 years to locate and identify all the insect species in the world!

Who has been on Earth longer: insects or humans?

Insects, by far. The oldest insect fossils are at least 400 million years old. Compare that to the earliest humans. They appeared no more than four million years ago.

How many insects live in your backyard?

About 2,000 in every square yard (square meter) of soil. Suppose you dug up 1 square mile (2.6 km2) of land. You’d find more than five and onehalf billion insects. That is about the total number of people in the whole world! Scientists say insects outnumber people one million to one!

Are spiders insects?

No. Spiders belong to another group of small animals, called arachnids (uh-RAK-nidz). Other arachnids include ticks, mites, and scorpions. Unlike insects, spiders have eight legs and only two parts to their body. Also, they have neither wings nor antennae. So never call a spider an insect!

Are flies insects?

Yes. So are ants, bees, ladybugs, mosquitoes, butterflies, moths—and about one million other kinds of small animals. All adult insects have three parts to their body: head, thorax, and abdomen. The head has the eyes, mouth parts, and two antennae, or feelers. On the thorax most insects have six legs and either two or four wings. The abdomen is where the insect digests food and breathes. Even with these three parts, most insects are less than 1/4 inch (6.4 mm) long.

How do flies walk upside down?

Easily! Flies have tiny claws at the ends of their feet that grip the rough spots on ceilings, windows, or walls. Also, their feet have hairy pads covered with a sticky substance that helps them cling to any surface. It’s a little like walking with chewing gum on the bottom of your shoes.

Between the claws and the sticky stuff, flies can walk anywhere they want!

Aflatoxin Poisoning

Aflatoxin poisoning, like botulism poisoning, is a case of food poisoning . However, it is slightly different than the carcass - maggot cycle of botulism poisoning, wherein the wildlife are affected by feeding on toxin - containing insects. In the case of aflatoxin poisoning, the wildlife ingest plant products that contain high levels of afl atoxin. Like botulism, this disease is not truly infectious, but caused by a biotoxin.

So what is the role of insects in this disease? Aspergillus fungi develop in cereal grains when plants are stressed or damaged. Hot, dry weather is stressful to plants, as is damage by insects, hail, and early frost. The products normally affected by aflatoxin are cereal grains and oil seeds, particularly corn, rice, sorghum, cottonseed and peanuts. Insects also serve to move the fungus around from plant to plant in the field, or within stored grains as they feed. Even aflatoxin - free grain can become heavily contaminated if it is not stored properly after harvest. Insect infestation in stored grain causes increase in moisture levels within grain. Increased moisture levels facilitate growth of fungi including Aspergillus , and production of heat, which promotes more growth of Aspergillus , as optimal growth of this fungus occurs at 26 – 32 ° C.

How important is aflatoxin poisoning to free - ranging waterfowl? Following are some examples of poisoning in the USA. In the winter of 1977 – 1978, approximately 7500 ducks and geese were killed in Texas following ingestion of crop residue, with corn and rice recovered from the digestive tract of the dead birds containing 500 ppb of AFB 1 . Similarly, in 1998 – 1999 approximately 10,000 waterfowl died in Louisiana after eating affected corn. Corn intended for consumption by mature poultry is not allowed to have over 200 ppb of aflatoxin, so clearly these wild birds ingested too much aflatoxin. Survey of residual corn in farmers ’ fields, or planted for wildlife, has shown that such grain may contain high levels of aflatoxin. For example, 32% of the corn growing near the Mississippi Sandhill Crane National Wildlife Refuge was found to contain over 200 ppb of AFB 1 , and corn grown in southern Georgia and northern Florida to provide food for quail and other wildlife was found to contain up to 1200 ppb of AFB 1 . Corn used to feed deer in North Carolina and South Carolina was found to contain 750 ppb of AFB 1 , and even birdseed sold for home bird feeding has been found to contain aflatoxin.

Considerable potential exists for wildlife to be poisoned unintentionally by cultivated crops. Indeed, it is estimated that 25% of the world ’ s food supply of grains is contaminated with aflatoxin, with contamination being higher in developing than industrialized countries. Also, the rate of aflatoxin infection is higher in warm, wet production areas such as the southeastern USA, where phytophagous insects abound. Numerous species have been poisoned by afl atoxins, including fi sh, birds, and mammals. Young animals are more susceptible than old animals, and animals such as rats and rabbits that metabolize the metabolites rapidly are more susceptible than those that metabolize the aflatoxin slowly. Serious poisonings have involved snow geese, Chen caerulescens ; Ross ’ s geese, Chen rossi ; greater white - fronted geese, Anseralbifrons ; mallards, Anas platyrhynchos ; northern pintails, Anasacuta ; and ring - necked pheasants, Phasianus colchicus , among others.

Avian Botulism

Although bacteria are often transmitted when insects feed on their wildlife hosts, transmission of the toxin that causes avian botulism can occur in a different manner. Avian botulism is the most signifi cant disease of migratory birds, especially waterfowl and shorebirds. Not surprisingly, it was formerly called ‘ duck sickness. ’ The bacterial complex known as Clostridium botulinum produces protein neurotoxins that cause food poisoning when animals (or humans) eat toxin - laden food. There are several different neurotoxins produced by Clostridium , with type C 1 being most common among birds, though loons and gulls are typically affected by type E toxin. Although over 250 species of birds have been found to experience botulism poisoning (scavengers such as vultures and crows seem to be resistant), it is filter feeding, dabbling, and fish - eating birds that are especially likely to ingest toxin. Apparently, the poison originates with invertebrates and plants living under anaerobic (depleted of oxygen) conditions in marshes and mud flats, because these bacteria only develop under anaerobic conditions. Decomposing vertebrate carcasses also support high levels of toxin production, producing a secondary form of poisoning called the carcass - maggot cycle of botulism . Decomposing animals are a very suitable substrate for growth of Clostridium , as decomposition also generates anaerobic conditions and the high temperatures that favor bacterial growth and toxin production. The dead animals are not attractive to waterfowl, but these birds will readily ingest any mature fly larvae (maggots) that disperse from the carcass, and most maggots do move a considerable distance as they leave the carcass and search for dry pupation sites.

Maggots developing in carcasses can have very high levels of toxin, and cause death to birds that feed upon them. Thus, all that is needed to initiate a prolonged cycle of botulism poisoning is initial death of some animals following a storm, collision with power transmission lines, algal poisoning, or feeding on invertebrates or decaying plant material rich in Clostridium. The bacteria normally found within the bodies of animals in wet habitats are then free to multiply in their carcasses, which are then assimilated into blowflies, which are fed upon by other birds, which then die and become available to more blowflies. Thus, outbreaks of botulism can originate in several ways, and persist for long periods. It is important to note that the animals are not killed because they became infected with the Clostridium botulinum bacterium, they die after ingesting their chemical metabolites, which happen to be poisonous neurotoxins. Thus, Avian Botulism is not really an infectious disease, but a biotoxin, and not quite comparable to most bacterial diseases.

Plague

Yersinia pestis affecting mammals is called plague, and is renowned for causing three pandemics that killed about 200 million people in past centuries. This disease, and its hosts and vectors, have had profound effects on human history. The first pandemic, known as the Justinian Plague , affected the Byzantine Empire, specifi cally Egypt, the Middle East, and Mediterranean Europe. It lasted from about 540 to 700 A.D. and caused about a 50% – 60% reduction in human populations in the affected area. The second pandemic, called the Black Death , occurred in the 1300s and killed about 75 million people, or about one - third of the people in Europe. The third (unnamed) pandemic started about 1855 in China, but was soon spread to seaports in many countries around the world. India was particularly affected, realizing about 12.5 million deaths in 20 years. It was during this outbreak that the bacterial cause of the disease was discovered. It was not until 1908 that the involvement of rats was established, though it was suspected for centuries because massive deaths of rats typically preceded the occurrence of disease in humans. Technically, the third pandemic has not been terminated as there are continuing cases, particularly in Vietnam, where up to 250,000 cases are estimated to have occurred in the second half of the 1900s. Thus, plague remains an occasional problem, especially in less developed countries. Plague outbreaks typically subside when most susceptible hosts perish, or surviving hosts develop immunity.

Among humans, this bacterium causes three forms of plague, based on the nature of the infection. Bubonic Plague is transmitted by fleas to the skin, swellings called buboes occur, and lymph nodes are commonly affected. Septicemic Plague occurs when the flea injects the bacilli directly into a capillary vein, and the infection becomes general rather than localized in the lymph system. Pneumonic Plague , is caused by transmission of the bacilli through coughing or sputum following involvement of the lungs.

Plague normally resides in wildlife, specifically rodents, and is transmitted by fl eas. Virtually all mammals can become infected with the pathogen, though susceptibility varies widely, and nearly all non - rodent and non - lagomorph species are considered to be incidental hosts. Maintenance of the plague at relatively low levels in wild rodents is sometimes referred to as Sylvatic Plague or Campestral Plague . In the USA, it persists at low levels in arid western rural environments but occasionally erupts to attain higher levels, even affecting humans in urban environments as far east as New York City. In the enzootic cycle the reservoir consists of 30 – 40 species of rodents, with birds, rabbits, carnivores, and primates unaffected. In urban areas, outbreaks usually involve urban rats. Outbreaks based on urban rat hosts is referred to as Murine Plague . Around the world, more than 200 species of rodents can be affected.

Lyme Disease

Lyme Disease, the clinical manifestation of Lyme borreliosis,
is a chronic, debilitating disease of humans,
and presently infects more humans than any other
tick - borne infection. Lyme borreliosis is caused by the
spirochaete Borrelia burgdorferi and is transmitted by hard ticks in the genus Ixodes . The important vector species vary geographically: I. scapularis and I. pacificus are the important species in the eastern and western regions of North America, respectively, whereas I. ricinus and I. persulcatus are the important species in Europe and Asia, respectively. Lyme disease now occurs in most temperate areas of the Holarctic region, and also from Australia and occasionally from South America and Africa. Lyme borreliosis has become increasingly important to humans in recent years due to increased contact between ticks borne on wildlife, and humans. Due to its increased prevalence, it is often designated an emerging pathogen.

The host range of Lyme borreliosis is impressive, with over 50 species of wild mammals known to support the pathogen. The important mammalian reservoirs in North America are eastern cottontail, Sylvilagus fl oridanus ; wood rats, Neotoma spp.; kangaroo rats, Dipodomys californicus ; white - footed mice, Permyscus leucopus ; eastern chipmunks, Tamias striatus ; black rat, Rattus rattus ; and Norway rat, Rattus norvegicus . In Europe, the important mammalian reservoirs include mountain hare, Lepus timidus ; squirrels, Sciurus spp.; wood mice, Apodemes sylvaticus ; yellow - necked mice, Apodemes fl avicollis ; bank voles, Clethrionomys glareolus ; and Eurasian hedgehog, Erinaceus europaeus . Also, a number of birds support the bacterium, including ring - necked pheasant, Phasianus colchicus ; wild turkey, Meleagris gallopavo ; Atlantic puffi n, Fratercula arctica ; house wren, Troglodytes aedon ; robin, Erithacus rubcula ; yellowthroat, Geothlypis trichas ; song sparrow, Melospiza melodia ; house sparrow, Passer domesticus ; and orchard oriole, Icterus spurious . Migratory birds are important in spreading the disease and ticks. Adult ticks feed on larger animals, and various deer are favored. Although the deer are not good hosts for the bacteria, they are important because they are the principal source of nutrition for ticks that are already infected with B. burgdorferi . Often overlooked is the potential role of lizards and other reptiles to support ticks. However, research in Hungary showed that lizards were common hosts for Ixodes ricinus . Further, Borrelia burgdorferi was found in several species of lizards. The Lyme disease cycle is shown diagrammatically in Fig. 8.2 .

Vectors of this disease are said to be three - host ticks because they take three blood meals, each from a separate vertebrate host. They feed as a larva, again as a nymph, and then as an adult. Fully fed (replete) female ticks detach from their fi nal host to oviposit, and larvae hatch from the eggs. Typically, larvae ingest the bacteria during their fi rst blood meal but are not capable of transmitting disease, whereas nymphs and adults developing from these infected larvae can infect new hosts. Nymphs and larvae usually feed on the same hosts; thus, nymphs are important in continuing the cycle because they inoculate vertebrate hosts that are fed upon by uninfected larval ticks. Rodents and some birds are persistently infected and fed upon by a high proportion of the immature tick population. In contrast, lizards and deer do not become persistently infected so do not act as such important reservoirs of the disease.

The primary site of infection is typically the skin, and a small infl ammatory response at the site is normal. The spirochaetes soon spread to the lymph, blood, heart, bone and other organs. Signs of infection generally are absent from naturally infected wildlife, and most wildlife seem to tolerate the infection, though some mice display neurological disturbances. In general, there is no impact of B. bergdorferi on wildlife populations. In contrast, skin rash, arthritis and cardiac disease are prevalent in humans following infection.

The key to protecting humans from Lyme disease is personal protection. Hikers should seek to avoid tick infested habitats, wear light clothing to enable detection of ticks, wear long pants and sleeves to minimize skin exposure, tuck pants into boots to keep ticks away from skin, use insect repellents, and search for and remove ticks regularly. Tick control is of limited value. Habitat modifi cation is useful if it decreases contact of humans with wildlife, and the ticks borne by wildlife.

Anaplasmosis

This disease is caused by rickettsiae in the genus Anaplasma , with A. marginale the best known, and found throughout the tropical and subtropical regions of the world. It is an infectious, but not contagious, disease of ruminant animals that is transmitted mostly by ticks, but to a much lesser degree by tabanid fl ies. The host range of Anaplasma is considerable, including deer, Odocoileus spp.; elk, Cervus elaphus ; giraffe, Giraffa camelopardalis ; pronghorn, Antilocapra americana ; American bison, Bison bison ; cape buffalo, Syncerus caffer ; Asian water buffalo, Bubalus bubalis ; wildebeest, Connochaetes spp.; bighorn sheep, Ovis canadensis ; and many African antelope species in addition to domestic cattle, sheep, and goats.

Anaplasma infect erythrocytes (red blood cells) exclusively, causing anemia and reduced hemoglobin concentrations due to damage to the erythrocytes. However, although domesticated ruminants are quite susceptible to infection, wild ruminants are quite resistant. In either type of ruminant, survivors that have been exposed to acute anaplasmosis usually regain normal hematologic parameters, and possess antibodies for some time.

Cattle, and to a lesser degree sheep, are affected by anaplasmosis. Wild ruminants, though substantially immune, play a major role in maintenance and spread of the disease. As noted previously, ticks are largely responsible for inoculating domesticated livestock, and when wild and domesticated animals share the same pastures, the wild animals become a problem for ranchers. Only where effective tick vectors are absent is the problem diminished.

Tularemia

The bacterium Francisella tularensis affects primarily lagomorphs (rabbits and hares) and rodents in the northern hemisphere, especially western North America, central Europe, and the former USSR, but also in Asia and Africa. However, it has a broad host range, affecting 190 species of mammals and 23 species of birds in addition to a few amphibians and numerous invertebrates. Among the more important hosts in North America are hares, Lepus spp.; New World rabbits, Sylvilagus spp.; water voles, Arviocola sp.; muskrat, Ondatra zibithecus ; American beaver, Castor canadensis ; lemmings, Lemmus spp.; voles, Microtus spp.; hamster, Cricetus cricetus ; and red - backed voles, Myodes spp. In Europe, the disease is common among European brown hare, Lepus europaeus ; varying hare, Lepus timidus ; common vole, Microtus arvalis ; house mouse, Mus musculus ; common shrew, Sorex araneus ; and others. Birds are relatively resistant, though gallinaceous game bird species such as grouse, as well as hawks, owls, and some waterfowl are susceptible.

Francisella tularensis is a highly infectious agent that enters the body in several ways, but primarily via inoculation by blood - feeding arthropods. Other routes of infection include inhalation of aerosols or handling and ingestion of contaminated water or meat. Among the important arthropod vectors are mosquitoes, fleas, tabanid flies, and ticks. A number of ticks from several genera are associated with transmission. Tularemia is primarily an acute disease, infecting the blood and causing infl ammation and necrosis in wildlife. The liver, spleen, bone marrow, and lungs are affected.

In western North America, various host and vector systems are evident. In the eastern USA, cottontail rabbit, Sylvilagus fl oridanus , is infected by ticks and biting fl ies, with tularemia serving as a regulatory mechanism for rabbit populations. In Canada and northern USA, however, muskrat, Ondatra zibethicus, and American beaver, Castor canadensis , are most affected, with the disease apparently water - borne. In both cases, humans are also at risk, both from tick bites and from handling rabbits. The effect on wildlife populations, especially beaver, is signifi cant in North America. In Europe, a different pattern occurs, with the vertebrate host primarily hares and the vector primarily mosquitoes.

Tularemia is rarely a problem for domesticated animals, though cats, dogs, horses, and sheep are occasionally affected. However, it is an issue for humans in areas where tularemia occurs. In some cases, it is advisable to delay the hunting season until cold weather reduces the density of ticks, and the probability of human infection. Also, trappers handling wildlife need to be aware of the risk. Finally, wildlife managers need to be sensitive to the population shifts due to incidence of tularemia, and may need to adjust harvest quotas in furbearers.

Bacteria

Bacteria are small, but not nearly so small as viruses. They are single - celled organisms. Bacteria are prokaryotes , which means that they do not have a cell nucleus or other membrane - bound organelles. They contain a single chromosome with double - stranded DNA. Some bacteria are saprophytic, and others are parasites. Some survive for years under adverse environmental conditions. Traditionally, they are divided into one of two groups based on their staining characteristics when exposed to the ‘ gram stain. ’ Bacteria that stain blue are said to be gram - positive; bacteria that stain pink are said to be gram - negative. Bacteria multiply rapidly in infected organisms, but can be eliminated by the host’s immunological system. Rickettsiae are small, gram - negative, intracellular bacteria that cannot live outside the cell, so unlike many bacteria they cannot be cultured on artifi cial media. They are variable in form but often rod - shaped, have DNA, RNA, and cell walls. Due to their small size, they were once thought to be viruses or positioned somewhere between viruses and bacteria. They are found in one family (Rickettsiae) and several genera (e.g., Rickettsia , Ehrlichea , and Anaplasma ). They are natural parasites of certain arthropods (lice, fleas, ticks, mites) and mammals, and cause many important diseases in humans and animals. Spirochaetes are also gram - negative bacteria, but are motile and free - living. Table 8.2 shows some bacteria that involve wildlife and insects.

The role of insects in transmitting bacteria is quite important for some diseases, but not for others. For example, avian cholera is a very important disease of birds, affecting over 190 species around the world, and sometimes causing massive death in both wild and domestic birds. About 35 species of ducks, geese and swans are susceptible, though this is the most susceptible group, with fewer than 10 species known to be susceptible in each of the following: wading birds, gallinaceous birds, doves and pigeons, woodpeckers, shorebirds, hawks and eagles, and owls. Transmission occurs principally by inhalation of aerosols (droplets), or ingestion of contaminated food or water. However, insects such as chewing lice, poultry mites, soft ticks, and tabanid flies have been shown to harbor and transmit the bacterium. Likewise, avian tuberculosis can be found in fowl tick ( Argas persicus ) and its feces, so it is implicated in mechanical transmission, though this is thought to be of little consequence as compared to direct contact among birds and contamination of food and water. In some cases, such as with heartwater, long - distance relocation of wildlife by humans can result in movement of the disease, and the ticks that vector it effectively, because the host animals are not sufficiently screened prior to transport.

Hemorrhagic Disease

This disease is caused both by bluetongue virus and epizootic hemorrhagic disease virus, with the results of infection referred to collectively as hemorrhagic disease. These are similar orbiviruses in the family Reoviridae, and exist as many strains that differ in pathogenicity. Biting midges (Diptera: Certatopogonidae) of the genus Culicoides are the only known vectors of this disease, though many different species are involved around the world.

The name ‘ bluetongue ’ is based on the tendency of the tongue and mucosal membranes to turn blue, a result of cyanosis. This also can occur with epizootic hemorrhagic disease infection. Other common signs of infection include an arched back, lameness, and painful or cracked hooves in affected animals. Affected animals can appear disoriented, weak and staggering, or asymptomatic but followed by sudden death. Death may result from congestion of the lungs, necrosis, and internal hemorrhaging. The digestive tract and the organs are typically involved, including the kidneys, thymus, spleen, and lymph nodes. Reproduction is also affected, resulting in aborted or stillborn animals, particularly livestock. Because the viruses can be transmitted in semen, mandatory testing of semen is required prior to artifi cial insemination. Thus, economic loss results from the disease in livestock, the inability to export semen to countries that lack the disease, and from reduced abundance of wildlife.

Hemorrhagic disease affects wild and domestic ruminants in many parts of the world, including temperate and tropical areas. In the USA, disease is most prevalent in the western and southeastern states. Cattle and sheep are especially susceptible to infection, especially in North America. Most sheep and cattle remain asymptomatic, however. Hemorrhagic virus is reported to cause clinical disease in such North American species as white - tailed deer, Odocoileus virginianus ; pronghorn, Antilocapra americana ; bighorn sheep, Ovis canadensis ; elk, Cervus elaphus ; mountain goat, Oreamnos americanus ; and American bison, Bison bison . Hemorrhagic disease is particularly lethal to white - tailed deer and pronghorn. Though much less of a problem thus far, many African species seem to be susceptible to infection, including greater kudu, Tragelaphus strepsiceros ; muntjac, Muntiacus reevsi ; Grant ’ s gazelle, Gazella granti ; gemsbok, Oryx gazella ; sable antelope, Hippostragus niger ; African buffalo, Syncerus caffer ; ibex, Capra ibex ; and others. Antibodies to bluetongue virus have been found in carnivores in Africa, including wild dog, Lycaon pictus ; lion, Panthera leo ; cheetah, Acinonyx jubatus ; spotted hyena, Crocuta crocuta ; and others. It is possible that these carnivorous animals were infected by preying on infected ungulates.

Yellow Fever

Yellow Fever is found in the Americas, Europe and Africa, but is strangely absent from Asia. Formerly, it was quite important in both North and South America, including some cool - weather cities. Currently, it seems to be increasing in importance in Africa, making it not an emerging pathogen, but a re - emerging pathogen. This disease originated in Africa, and likely was introduced to the western hemisphere by the slave trade. In the 1600s – 1800s it caused epidemics in North American cities, including New Orleans, Philadelphia, New York, and Boston in the USA, and Halifax in Canada. Epidemics also occurred in Ireland, Wales, Spain, Uruguay, and Chile. It decimated Napoleon’s army in Haiti in 1802, causing the French to scale back their territorial ambitions in the New World. Yellow Fever virus was the first virus that was shown to be transmitted by a mosquito.

Yellow Fever cycles in both urban and forested (sylvan) environments, though the important hosts and vectors vary among locations. Primates (including grivet, Chlorocebus aethiops ; mangabeys, family Cercopithecidae; bush babies, family Galagidae; baboons, Papio spp.; and chimpanzees, Pan spp.) are the important wild hosts, and yellow fever can be fatal to howler monkeys, Alouatta spp; squirrel monkeys, Saimiri spp.; spider monkeys, Ateles spp.; and owl monkeys, Aotus spp. Typically, infection of monkey populations causes reduction in their density and collapse of the yellow fever epidemic, so the disease moves slowly through the population over a large area, returning after the population is replenished. Mortality among monkeys is much less common in Africa than in the Americas. Other animals such as the anteaters Tamandua tetradactyla and Cyclopes didactylus ; kinkajou, Potos flavus ; and various rodents may be involved as hosts, but their importance is uncertain.


Affected animals display fever, vomiting, pain, dehydration, and prostration, and sometimes hemorrhage including renal failure and death. Human infection produces similar maladies, including liver damage (resulting in jaundice, hence the name ‘ yellow fever ’ ), and up to 75% of affected humans die.

The mosquito Aedes aegypti is often called the yellow fever mosquito due to its association with yellow fever, particularly in urban situations. Aedes aegypti is particularly dangerous due to its affinity for houses, where it enters and feeds while the inhabitants sleep. Although eliminated from South America in the 1930s and 1940s, A. aegypti has recovered and now poses a significant threat to South American urban areas due to its association with yellow fever. In sylvan (forested) sites of the Americas, other Aedes spp., Haemagogus spp., and Sabathes chloropterus are the important vectors. In Africa, other Aedes spp. and Dicermyia spp. transmit the virus among monkeys and to people. Antimosquito programs have brought yellow fever under control in many urban areas, but in rural areas it remains a threat. It is sometimes referred to as ‘ woodcutter ’ s disease ’ due to its association with forested environments and the people who work there.

St. Louis Encephalitis

This virus is common throughout the western hemisphere, and until West Nile Virus was introduced to New York in 1999 it was the mosquito - transmitted disease agent of greatest medical importance in North America. St. Louis Encephalitis is transmitted by Culex mosquitoes in North America. Birds are generally considered to be the most important hosts for amplification of the virus, with the disease transmission cycle typically described as mosquito - bird - mosquito. Nestling birds are considered to be particularly susceptible to infection, possibly because they are confined to the nest and less able to defend themselves from biting mosquitoes. They also display less immunity and display higher levels of viremia than do adults of the same species. Among the birds known to be suitable hosts of St. Louis Encephalitis are northern cardinal, Cardinalis cardinalis ; American robin, Turdus migratorius ; northern bobwhite, Colinus virginianus ; blue jay, Cyanocitta cristata ; northern mockingbird, Mimus polyglottos ; and mourning dove, Zenaidura macroura . In urban environments birds such as the house sparrow, Passer domesticus ; house fi nch, Carpodacus mexicanus ; and European starling, Sturnus vulgaris , are important in virus amplifi cation. Birds inoculated with the virus by mosquitoes quickly develop viremia, allowing other mosquitoes to contract the disease by feeding on the blood of the infected bird, although the virus disappears equally quickly as the birds recover from infection. Thus, the ‘ window of opportunity ’ for mosquitoes is quite short, and disease spread occurs mostly when mosquitoes are very abundant. Outbreak of the disease dissipates as the birds become resistant to the virus. Unlike some other mosquito - borne arboviruses such as Eastern Equine Encephalitis and West Nile Virus, St. Louis Encephalitis does not adversely affect the bird hosts.

Other vertebrates are suspected to be hosts in the southeastern USA, where antibodies were found in Virginia opossum, Didelphis virginiana ; northern raccoon, Procyon lotor ; cotton mouse, Peromyscus gossypinus ; and nine - banded armadillos, Dasypus novemcinctus. Elsewhere, antibodies were found in big brown bat, Eptisicus fuscus ; little brown bat, Myotis lucifugus ; coyote, Canis latrans ; red fox, Vulpes vulpes ; striped skunk, Mephitis mephitis ; jackrabbits, Lepus spp.; deer mouse, Peromyscus maniculatus ; yellow - bellied marmot, Marmota fl aviventris ; and many other vertebrates. The presence of antibodies, though suggestive, does not by itself prove that an animal is an important host because the concentration of virus may be inadequate for successful infection of mosquitoes, or the host may not be attractive to mosquitoes that are important in the spread of disease to humans. A few mammals have been shown experimentally to develop viremia adequate for spread of the virus, including wood rat, Neotoma mexicana ; Audobon ’ s cottontail rabbit, Sylvilagus audubonii ; young (but not old) cotton rat, Sigmodon hispidus ; and least chipmunk, Eutamias minimus .

In Central and South America, the disease is transmitted by at least seven genera of mosquitoes. As in North America, birds seem to be the most important hosts, but the St. Louis Encephalitis virus has also been found in other vertebrates such as vesper mouse, Calomys musculinus ; grass mouse, Akodon arviculoides ; a rice rat, Oryzomys nigripes ; southern opossum, Didelphis marsupialis ; howler monkey, Alouatta nigerrina ; a spider monkey, Atles panisicus ; a three - toed sloth, Bradypus tridactylus ; various bats, and others. Whether or not these non - bird vertebrates are important in disease cycling is not evident, but at the very least these animals provide mosquitoes with blood meals, enhancing mosquito survival and reproduction.

Many vertebrates have become viremic, some consistently, but disease is not apparent in wildlife. In humans, the virus ranges from unapparent infection to coma and fatal encephalitis. However, only about 1% of infections are clinically apparent and most remain undiagnosed. Different strains of the virus exist, and they differ in their ability to cause disease. The young and elderly are more often affected, but typically it is a more severe ailment in the elderly than in the young. St. Louis Encephalitis inflicts neurological damage such as memory loss, paralysis, and deterioration of fine motor skills in humans. The damage is limited to the brain and spinal cord. 

West Nile Virus

West Nile Virus has long been present in Africa, the Middle East, southern Europe, India and southern Asia. It was introduced to the western hemisphere in 1999, in New York, USA, and quickly spread through much of North America, attaining the Pacific coast in 2002. The source of the virus in North America is unknown, but several species of Eurasian birds occasionally migrate to North America, especially to the eastern seaboard of North America, and some may have been infected while in Europe. Among the occasional visitors to North America are Eurasian wigeon, Anas penelope ; green - winged teal, Anas crecca ; ruff, Philomachus pugnax ; little gull, Larus minutus ; and black - headed gull, Larus ridibundus . Alternatively, some seabirds are carried by tropical storms annually from Africa to North America, so this is a possible route of introduction. Birds such as cattle egret, Bubulcus ibis ; black - headed gull, Larus ridibundus ; yellow - legged gull, Larus cachinnans ; little egret, Egretta garzetta ; and gray heron, Ardea cinerea are possibilities. Finally, pet, zoo and domestic birds routinely pass through commercial airports such as J.F. Kennedy International Airport in New York, and could be a source of the virus. Epidemics in humans have occurred in Africa and Europe in addition to North America, and infection can cause fever, myalgia, rash, and encephalitis in some victims. In recent years it has become the most important arbovirus in North America. Its importance is likely to diminish as wildlife and humans develop resistance following exposure to the virus.

West Nile Virus is transmitted to birds principally by ornithophilic (bird - feeding) mosquitoes, although it has been found in other animals, and of course in humans. It also can be transmitted by organ transplant, blood transfusion, transplacental, and transmammary. Wild birds are the primary hosts, with ulex spp. being especially important vectors, and with humans and horses being accidental or ‘ dead - end ’ hosts that do not contribute to continued transmission. Domestic birds, except for geese, generally do not develop sufficient viremia to allow transmission. Domestic geese develop the necessary viremia to amplify transmission, and also suffer mortality. Migratory birds are important in the dissemination of West Nile Virus in temperate regions, and in North America the virus can persist during the winter in southern climates from Florida to California and then be reintroduced to northern areas annually.

In North America, West Nile Virus has become a major mortality factor of corvid birds. American crows, Corvus brachyrhynchos , are especially likely to perish when infected. Crows, blue jays, Cyanocitta cristata , black - billed magpie, Pica pica , and other species of Corvidae account for about 80% – 90% of the infected birds in most dead - bird surveys. Other birds that seem to have suffered signifi cant declines in abundance include American robin, Turdus migratorius ; tufted titmouse, Baeolophus bicolor ; house wren, Troglodytes aedon; chickadee, Poecile spp.; and eastern bluebird, Sialia sialis . However, some other birds seem to be quite susceptible, including various owls and raptors. The mortality of birds in the western hemisphere is much higher than typically occurs in the eastern hemisphere, likely due to the increased virulence in the native birds of the western hemisphere. It appears that most of the birds adversely affected are peridomestic (species that are often found in association with human habitations).

Although corvid birds regularly test positive for West Nile Virus, and often die, this does not mean that they are the most important species in maintaining the virus or enhancing dispersal in North America. American crows, for example, though very susceptible, typically comprise a small proportion of the bird community in an area, rarely exceeding 10%. Also, birds differ greatly in their attractiveness to mosquitoes, which affects the transmission potential. Field studies in the eastern USA have shown that American robin, Turdus migratorius , is highly preferred, being fed upon 16 times more often than if mosquitoes displayed no preference, and that virus antibodies are present in over 40% of the robins, documenting exposure to the virus. Importantly, it was estimated that nearly 60% of the infected mosquitoes became infected by feeding on virus - infected robins. In contrast, house sparrow, Passer domesticus , is avoided by mosquitoes and less than 20% showed evidence of exposure. Fish crow, Corvus ossifragus , also is highly selected by mosquitoes and fed upon more often than would be expected. However, their low level of abundance, like that of American crow, precludes them from being important in the epidemiology of the virus. Thus, variability in host - feeding behavior by mosquitoes, host suitability for virus amplification, and bird host abundance all interact to affect disease potential.

An assessment of birds important in introducing West Nile Virus to Europe from Africa, amplifying the virus, and enhancing its spread, suggested that perhaps 100 of the 300 bird species considered may play a role in West Nile epidemiology. Among the potentially important species are cattle egret, Bubulcus ibis ; gulls, Larus spp.; barn swallow, Hirundo rustica ; house martin, Delichon urbica ; common swift, Apus apus; common nightingale, Luscinia megarhynchos ; crows, Corvus spp.; blackcap, Sylvia atricapilla ; and European starling, Sturnus vulgaris . Further research will certainly show that not all are equally important. As in North America, Culex spp. mosquitoes seem most important in disease transmission.

West Nile Virus is not limited to birds, though it is best known as a pathogen of birds. Among other animals infected are eastern fox squirrel, Sciurus niger ; black bear, Ursus americanus ; white - tailed deer, Odocoileus virginianus ; and American alligator, Alligator mississippiensis . It is likely that the list of non - bird hosts will grow as the disease becomes further established in North America.

Avian Pox

This widespread disease, also known as fowl pox and bird pox, affects a large number of bird families and is found throughout the world. Infection causes localized proliferation of epithelial cells and production of lesions. In many cases it is only a mild affliction, and does not result in death. However, when the eyelids or mucous membranes of the oral or respiratory cavities are affected, mortality among infected birds can be high. It is a greater problem when population densities of birds are high, as among flocking birds and in zoos. There are a number of strains involved in this disease. It is most abundant in warm and moist regions, and occurs most commonly when mosquitoes are numerous, as they vector the disease.

Distribution and prevalence of this disease are determined by a number of factors, including weather, vector abundance, host abundance, and the strains present. It is a slow - developing disease that is manifested in the presence of pox lesions (wart - like growths) on the unfeathered portions (legs, feet, and skin around the eyes and beak) of the bird. Affected birds also display weakness, vision and breathing problems, difficulty in swallowing, emaciation, and reduced egg production. Secondary infections are common. Among wild birds, it occurs most commonly in upland game birds, songbirds, marine birds, occasionally in raptors and rarely in waterfowl. Avian pox is transmitted mechanically by mosquitoes and other biting arthropods, by direct contact among birds, and by inhalation of virus particles. In remote island locations where exposure to avian pox is limited, exposure of native indigenous fauna has resulted in high levels of bird mortality. Also, among wild populations of northern bobwhite, Colinus virginianus , and wild turkey, Meleagris gallopavo , in the southeastern USA, avian pox is regarded as a serious problem. For many bird species, it is not a major issue. Management involves eliminatanding mosquitoes and their breeding sites, eliminating
infected birds, and in the case of domestic birds, by disinfecting feeders, watering devices, and cages.

Myxomatosis

This viral disease of New World rabbits is transmitted by mosquitoes, black flies, fleas, lice, and mites. The virus adheres to the mouthparts of the arthropods as they probe the epidermis. Once the mouthparts are contaminated, the virus can be inoculated into another host during a later probe. The virus does not replicate in the vector. The natural hosts of the myxoma virus are the jungle rabbit, Sylvilagus brasiliensis , in South and Central America and S. bachmani in California, USA. It has only mild effects on New World rabbits, causing cutaneous fi broma (relatively harmless wart - like skin blemishes) but not a systemic disease. However, the myxoma virus is quite deadly to Old World rabbits, Oryctolagus spp. Infected European rabbits display a bedraggled appearance, with partially or completely swollen eyelids, discharge from the conjunctiva and nose, and swollen ears and head. Nodules measuring up to 1 cm in diameter may be found anywhere on the body. In addition to being almost blind, affected animals frequently are in respiratory distress.

In 1950, myxoma virus was introduced to Australia to reduce the population of European rabbits that had been deliberately but regrettably introduced, and where they had few natural enemies. The rabbits in Australia have caused the extinction of numerous animals and plants, and caused extensive soil erosion due to overgrazing. The virus quickly reduced the population of rabbits from 600 million to 100 million, but the rabbit populations have recovered partly as disease resistance has developed. Recently another virus, rabbit calicivirus, has been introduced to attempt population suppression in Australia. Myxomatosis was also introduced to France, where it spread rapidly throughout Europe, causing significant decreases in rabbit populations, and populations of some of the predators that fed upon them. Overall, the reduction in rabbit abundance is estimated at 30% – 40% in Australia to 50% – 70% in Great Britain. Vaccine is used in Europe to provide protection of rabbits from the virus; both live attenuated virus vaccines and inoculation with rabbit fibroma virus (which is innocuous to European rabbits) are used. In many respects, rabbit fibroma virus can generally be considered the eastern North American equivalent of myxoma virus, and is often seen in eastern cottontails, Sylvilagus floridanus , but with few serious effects.

The Nature of Parasitism

Organisms that live in association with another animal are usually referred to as a host and symbiont, and the relationship is referred to as symbiosis . In the case of wildlife disease, the microparasites (e.g., viruses, bacteria, and fungi) or macroparasites (e.g., protozoa, helminths, and arthropods) are the symbionts , and the wildlife (but in some cases livestock and humans) are the fi nal or defi nitive hosts. In the examples here, arthropods are often intermediate hosts. The nature of symbiosis varies, with the most common types:

• Mutualism . The host and symbiont are dependent on one another, and the relationship is mutually probeneficial. The microbial fl ora found in the digestive system of ruminants is a good example of mutual dependency.

• Commensalism . The host provides the habitat and food for the symbionts, which live without benefit or harm. In this case, the symbionts are dependent on the host, but the host does not depend on the symbiont. Many microbial parasites of wildlife have co - evolved with their normal hosts and cause no measurable harm, so under normal conditions they qualify as commensals. However, this relationship can shift when the host is stressed, and under these conditions may cause injury.

• Parasitism . The host supplies physiological support for the symbiont by providing habitat and sustenance, and often transport, and the symbiont is harmful to the host. The symbionts discussed here are parasites of wildlife to some degree, and sometimes to livestock, pets or humans. Their effect on wildlife may be severe or mild, and sometimes nearly imperceptable. There are several forms of parasitism, with parasitism classifi ed according to where the parasites are found, their temporal occurrence, and their relationship with their host. Some major forms of parasitism include:

• Ectoparasites . These organisms live on the external surface of the host, or cavities that open directly onto the surface. Foremost among the ectoparasites are the arthropods, such as lice, mites, and ticks.
• Endoparasites . These live within the body of the host, including the digestive system, lungs, liver, tissues, cells and freely in the body cavity. Viruses, bacteria, fungi, protozoa, tapeworms, and nematodes are good examples of endoparasites.
• Temporary parasites. These visit the host only briefl y, usually for food. Bloodsucking arthropods are a good example of such parasites.
• Stationary parasites . These spend a definite period of development in association with the host, either on or in it. Some are periodic parasites , leaving to spend another portion of their life in a non - parasitic mode. Examples of this include ticks and botflies. Others are permanent parasites , spending all of their life on a host except for a brief period when transferring from host to host. The viruses, bacteria, protozoa and helminths are examples of this (although in some cases they may persist for long periods, in the absence of a suitable host, in a resting stage).
• Incidental or aberrant parasites . These are parasites that occur only occasionally in a host, usually because there are behavioral or ecological barriers that keep the parasite from the prospective host.
• Obligate parasites . These parasites cannot continue with their life cycle unless they have access to a certain host (or hosts). Most of the parasites discussed here have a certain host or a restricted range of hosts to which they must gain access.
• Facultative parasites . These are parasites that do not require a certain host, but sometimes are found in association with it.

Parasites display many adaptations that make it possible to have a symbiotic relationship with their host. Although they have the same basic nutritional requirements as the host, the symbiont must have adaptations that allow them to enter or attach to a new host after they have escaped from their old host. The tarsal adaptations of lice that allow them to cling to the body hairs of their host are a readily visible adaptation, as are the hooks and suckers of helminths living in association with the host ’ s digestive tract. The symbiont must also escape the host, or at least its progeny must escape, in order to fi nd new hosts. The avenue of escape may be direct , as when eggs, immature stages of nematodes, or body segments (proglottids) of tapeworms are released from the host’s anus. Escape may also be indirect , as when an arthropod feeds on the blood of a host, takes up the symbiont, and transports it. Symbionts escaping both directly and indirectly may have one or more additional hosts that provide transport, maintenance, or an opportunity for propagation.

Organisms with direct escape from the old host and direct transmission to the new host can be said to have a direct life cycle . Those with indirect escape and indirect transmission can be said to have an indirect life cycle . The latter condition is more complicated, as host, symbiont, vector, and environment must all coincide for infection and development to occur. Here we are concerned mostly with disease organisms with an indirect life cycle, and specifically those that involve arthropods.

Cause Of Disease

It can be quite diffi cult to determine the cause of a disease. Factors that are thought to be causative factors of disease are called putative factors because their occurrence is often correlated with presence of the disease, but eventually evidence of their association is necessary before a cause and effect relationship is confirmed. The cause of a disease is often called the etiologic agent . It is also common to discuss the sign or clinical sign to indicate objective evidence that a disease is present, such as hair loss. It is unusual to use the term symptom with respect to diseases of animals, as this is a subjective assessment such as pain or lack of energy, and is something that cannot be communicated to humans by animals. It is interesting to note that entomologists have developed slightly different terminology than have wildlife biologists when describing diseases; entomologists use ‘ sign ’ to indicate physical manifestations of disease, and ‘ symptom ’ to indicate changes in behavior or function attributed to disease.

In some cases, criteria have been established to defi ne whether or not a cause and effect relationship exists between a disease and a putative cause. Often, healthy animals are exposed to a putative cause; if the disease develops, causation is considered to be proved. For infectious disease agents, it is necessary to prove Koch ’ s Postulates to establish the relationship between a disease and its putative cause. The rules of Koch ’ s postulates are:

• the putative agent must be found in every case of the occurrence of disease;
• the putative agent must not be associated with the absence of disease;
• the putative agent must multiply in the host when provided the opportunity;
• the putative agent must be capable of being re - isolated from experimental inoculated individuals.

Though Koch ’ s postulates are generally acceptable in ascribing cause and effect relationships, in many cases disease results from many stressors, including some abiotic factors. Thus, disease may be expressed only when a combination of events or factors occurs. Commonly, disease has primary and secondary causes, or situations that predispose an animal to disease. For example, it is easy to imagine how poor diet or adverse weather could interact with microbial pathogens by weakening the host ’ s ability to fend off infection. Some diseases are latent , which means that the potential for expression of the disease is present, but expression is suppressed and the animal is not contagious until triggered by something in the animal ’ s biology or environment. Indeed, it is not uncommon for an animal to be infected (to have the disease - causing organism within its body), but for there to be no apparent dysfunction (lacking disease). The lack of detectable dysfunction can be due to inherent resistance (the same pathogen in another population or species induces expression of disease) or by the condition or vigor of the animal, which suppresses disease expression.

For noninfectious diseases (those lacking an infectious agent that can be spread from host to host) such as exposure to pesticides, it is not possible to apply Koch ’ s postulates. In these cases, we must be content with exposing healthy animals to the stressor and observing the animal ’ s response, or by searching for chemical residues in animals suffering from disease. This is not always satisfactory, as it is difficult to know the appropriate dose to test. Also, though it is easy to assess acute toxicity caused by high doses because the animal ’ s response is usually rapid, it can be diffi cult to assess the effects of chronic toxicity caused by low doses applied over a long period. Low doses can interact with host metabolism, such as the hormonal system of the animal, or with other agents such as microbial pathogens, and it can be difficult to identify the true factor responsible for poor animal performance.

Disease Transmission

Wildlife disease agents can be transmitted vertically or horizontally. Vertical transmission refers to transmission of a disease agent from parent to offspring. Horizontal transmission refers to transmission of a disease agent from animal to animal, independent of their parental relationship. Horizontal transmission is the most common method of disease spread. It can result from several actions, including:

• skin to skin contact between members of the same species;
• airborne transmission of droplets containing disease agents;
• contact with secretions and excretions, including residual fecal materials;
• contact with genital and sexual materials;
• discharge from lesions;
• contact with infected carcasses;
• ingestion of contaminated water and food;
• transmission by other species.

Here we discuss only the latter means of disease transmission, transmission by other species, and specifically by arthropods. Because another species is involved in the transfer from host to host, it is considered to be a form of indirect transmission . Not surprisingly, direct transmission is defi ned as the transfer of a disease agent (pathogen) from one host to another without the involvement of another species. Transmission of an infective stage of a disease agent to a host can occur in various ways. Passive transmission occurs when the host is contaminated or infected accidentally through ingestion of food, water, or an infected arthropod; this occurs with many nematodes. Active transmission occurs when the disease agent actively penetrates the bodies of their host after gaining contact with them; this occurs with hookworms. Finally, inoculative transmission occurs when a vector such as a mosquito injects the disease agent into the new host during the process of blood feeding, as occurs with the protozoa causing malaria.

The importance of arthropod transmission as a means of disease spread varies greatly among diseases. For some wildlife diseases it is the only means or most important form of transmission, but for other diseases it is less important or arthropod transmission does not occur. It is useful to note that transmission of disease agents by arthropods is not completely independent of the other routes of horizontal transmission. Arthropods can be associated with food and carrion, for example, and wildlife could contract a disease from an insect or by feeding on contaminated food. Nevertheless, because an arthropod (serving as a ‘ vector ’ ) is involved in the transmission or transport process, such diseases are called vector - borne.

Vectors differ in their ability to acquire disease agents and to infect hosts ( vector competence ). Insects usually are short - lived, and acquire and transmit disease agents quickly, with the incubation period in the vector lasting perhaps a few days. Insects may be quite mobile, and typically take many small blood meals. In contrast, ticks are more long - lived, and incubation may require months. Ticks lack wings, so they are much less mobile, and normally take few but large blood meals.

Differences in vectorial capacity or vector competence are due to several factors. There are genetic variants among a single species of disease agent, for example, that differ in their ability to be acquired or transmitted. The vectors also differ in susceptibility to oral infection and effi ciency of transmission, population structure (density, longevity, etc.), host preference, and geographic distribution. Also, the vertebrate hosts differ in susceptibility, which may be manifested in the ability of the host to develop concentrations adequate to infect the vector, population structure (availability of susceptible stages), immune status (prior exposure may confer immunity), and overlap in space and time with the vectors. Often, the suitability of a host animal to produce adequate concentration of the disease agent, or the ability of the disease agent to replicate in the vector, determines the ability of the vector to acquire and transmit the agent. Disease concentration thresholds seemingly exist, especially with mosquitoes, below which transmission does not occur. Thus, certain hosts or vectors are more important in disease transmission cycles. With tick vectors, however, the concentration of disease agent within the host seems to be less important, as ticks can infect one another while feeding together on the same host. Such ‘ co - feeding ’ infection occurs with only minimal or incomplete systemic infection, and usually the ‘ donor ’ and ‘ recipient ’ ticks must be feeding in proximity.