Parasite Induced Changes In Host Behavior

Parasites often have an intimate, highly developed relationship with their host that formed over evolutionary time. Both definitive and intermediate hosts display an array of defense mechanisms allowing probenefispective hosts to avoid, eliminate or tolerate parasites, while parasites display ingenious ways to find and exploit their host. Among the most intriguing aspects of parasitism is the modification by parasites of host behavior when the parasite has successfully infected the host. This is usually presented as an adaptation on the part of the parasite to enhance its fitness.

An adaptation is a genetically determined feature that becomes prevalent in a population because it confers a selective advantage to the organisms bearing the feature. In the case of symbiotic organisms, one species may benefit, but not the other. In the case of parasitic organisms, the changes in host behavior are believed to enhance parasite transmission from host to host. Arthropod behavior seems to be readily manipulated by diseases and parasites. For example, caterpillars that are infected with insect - pathogenic viruses often climb to the top of their host plant before they die, where they attach with their prolegs. After they die, their integument ruptures, allowing their liquefying body contents to drip down. Thus, foliage at lower levels of the plant is contaminated with virus, and other caterpillars ingest the virus as they feed. This assists in the spread of the virus from insect to insect. Similarly, flies and grasshoppers infected with fungal disease climb to the top of vegetation and attach, the fungus grows out of the dead insects and produces pores, and the elevated position of the cadaver facilitates the spread of fungal spores by wind or rain. Birds infected with arboviruses often display reduced activities, including reduced preening and other antimosquito behavior, allowing other bird - feeding mosquitoes better opportunity to feed on the infected host, and to acquire the virus for further spread. A classic example of modified host behavior involves the Lancet fluke, Dicrocoelium dendriticum , and Formica ants. When ants are infected, instead of returning to their nest during the evening and cool periods like uninfected ants, they attach to foliage where they can be consumed by grazing animals. Once ingested, the flukes can infect their vertebrate host, completing the parasite’s life cycle.

It has become commonplace to infer that modified behavior in parasitized hosts is an adaptation induced by parasites, but often the evidence is lacking. Rarely is an improvement in the functioning of the parasite actually demonstrated. Usually we can observe only the present state of a character, not its evolutionary history, so caution is needed before drawing conclusions about the adaptive value of a trait. For example, although the tendency of insects that are infected with viruses and fungi to perish in elevated locations is well documented, and logically this behavior will enhance dispersal of the parasites, there is another reason for insects to ascend vegetation. Infected insects will climb vegetation and bask in the sun, elevating their body temperatures to levels that are unsuitable for the parasites that infect them, and optimizing the temperature at which the insect ’ s immune system functions. This phenomenon is called behavioral fever , a form of self - medication that can result in elimination of the parasites due to exposure of ectotherms such as insects to high temperature. Thus, there are multiple explanations for a single behavior, and it pays to be cautious about concluding adaptive significance to behavior without adequate experimental evidence.

Integument

One of the unusual features of insects, and of arthropods in general, is the rigid integument (Fig. 2.1 ). The body wall, or integument , is hardened over most of the body to form a series of plates, called sclerites . The plates on the upper (dorsal) surface are called terga or tergites). Plates on the lower (ventral) surface are called sterna or sternites. Plates found on the sides (laterally) are called pleura or pleurites. The plates are useful because they provide a degree of protection, but such rigid structures can work against insects by limiting movement.

The sclerites are connected by areas of integument where a layer called the exocuticle is absent, decreasing the rigidity of the integument and allowing the integument to remain somewhat fl exible. These soft,  flexible areas are usually called intersegmental membranes . The outer area of the integument is secreted by the epidermal cells , the innermost living portion of the integument. The nonliving external area of the integument is called the cuticle . The principal regions of the cuticle are the thin waxy epicuticle externally, and a thick, rigid interior region that initially is called the procuticle . The procuticle differentiates into two layers: the outer region of the procuticle is called the exocuticle and the inner region is called the endocuticle . These two layers look slightly different, but their chemical composition is about the same, consisting mostly of various proteins and a polysaccharide called chitin . One of the most important proteins is called resilin , a rubberlike material that provides elasticity. The cuticle is covered by a thin waxy layer that provides waterproofi ng, and sometimes is topped by a cement layer. The integument is separated from the internal organs of the insect by a membrane called the basement membrane.

The functions of the integument are several, but perhaps most important is that it functions as a skeleton for these animals, providing support for the muscles and organs. Hence, the integument is often referred to as an exoskeleton . Admittedly, the skeleton is unusual because it is external, unlike the internal skeleton of mammals, but it is not very different from the external skeleton of lobsters, crabs, and shrimps. The integument sometimes has infoldings called apodemes , and these internal ridges serve to strengthen the integument, and also may serve as points of anchor for muscles. These infoldings usually are marked externally by narrow linear depressions called sutures . Sutures also sometimes mark the boundaries of plates, thus delimiting areas of the integument. In addition to supporting the body, the integument provides protection from injury, and reduces water loss to a very low level. The integument is not uniform, as some areas are differentiated. Not only are some areas thickened, but there are thin areas where sensory structures occur or where secretions are released from the epidermal cells via pore canals. The integument also imparts color to the animal. The integument typically has various rigid, pointed outgrowths called spines , and sometimes has movable ones called spurs.