Molting

The problem with being encased in a fairly rigid integument is that growth is severely limited. A certain amount of growth can occur because of the elastic nature of the intersegmental membranes. However, to allow signifi cant increase in size, the insect must shed its old cuticle (the nonliving part of the integument) and produce a new, larger body covering. Insects accomplish this by producing a new, larger but soft integument beneath their old rigid body covering, then shedding the old one and expanding to accommodate the new larger integument. This process is regulated by hormones, particularly ecdysone (this is discussed further under glandular systems, below).

The epithelial cells produce the cuticle, so this area of the integument is central to the entire molting process. The fi rst important step in molting is called apolysis , which is the separation of the epidermis from the old cuticle. The space that is created between the epidermis and the old cuticle during apolysis is called the exuvial space , a region where molting fl uid is secreted by the epidermal cells. After apolysis, the epidermal cells begin to secrete the new cuticle. First deposited is the outer layer of the epicuticle, then the inner epicuticle is formed. This is followed by secretion of the procuticle. Thus, the new cuticle is produced starting with the outer layers and working inward. Now the molting fl uid is activated, which digests the old endocuticle. Up to 90% of the cuticle is digested by protease and chitinase enzymes, and recycled to help construct a new procuticle. After all the layers are in place, the insect produces a layer of wax that is secreted through pore canals onto the surface of the new epicuticle. This protects the insect from desiccation. Finally, the insect sheds its old cuticle, called the exuviae , in a process called ecdysis . Ecdysis is a tricky process, as the insect must escape from its old covering. Often it anchors the old integument and crawls out, and may use gravity to aid in its escape by hanging from a branch (Fig. 2.2 ). However, it must split the old cuticle somewhere that will allow escape, often in the head region. After it fi rst escapes the old integument it is white or pale in color, and soft - bodied; such insects are said to be teneral . Finally, the molting insect must expand its body size while the new cuticle dries and hardens, because once it hardens not much more growth is possible until the next molt. So insects swallow air or water, and expand their body to swell to its maximum size while their body cover hardens. Slowly the insect cuticle hardens and darkens during a chemical process called sclerotization , which cross - links the proteins to create a new rigid exoskeleton. After this physical expansion and sclerotization, insects have some opportunity to add body tissue before they need to molt again.