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.