Cuticle structure

Cuticle
The cuticle is secreted by the epidermis and covers the whole of the outside of the body as well as lining the foregut and hindgut and the tracheal system, which are formed as in- vaginations of the epidermis. Most of the cuticle is composed of a mixture of proteins and the polysaccharide chitin. Out- side this chitinous cuticle is a chemically complex epicuticle that does not contain chitin. It is only a few microns thick.

Chitinous cuticle
Chitin occurs as long molecules that are bound together to form microfibrils. These microfibrils lie parallel to the plane of the surface and, at any depth below the surface, to each other. In successive layers the orientation changes, usu- ally giving rise to a helicoid (spiral) arrangement through the thickness of the cuticle. This gives strength to the cuticle in all directions. Sometimes layers of helicoidally arranged mi- crofibrils alternate with layers in which the microfibrils have a consistent orientation. These layers differ in their refractive indexes, and the metallic colors of insects typically are the re- sult of differences in the optical properties of the successive layers, so that only specific wavelengths of light are reflected.

The helicoid arrangement of microfibrils provides strength to the cuticle, but it does not impart hardness or rigidity. Hard- ness in insect cuticle derives from the linking together of pro- teins. The process of linking the proteins is called sclerotization, and the hardened cuticle that results is said to be sclerotized or tanned. Hardening is restricted to the outer parts of chitinous cuticle, so that the cuticle becomes differentiated into the outer sclerotized exocuticle and an inner endocuticle that remains un- sclerotized. Sclerotization does not take place until the cuticle is expanded fully after a molt and depends on the transport of chemicals from the epidermis. This is achieved via a series of slender processes of the epidermal cells that extend through the chitinous cuticle, creating canals in the cuticle that run at right angles to the surface. These are called pore canals.

Sclerotization affords some rigidity in addition to hard- ness, but in many areas of the cuticle this rigidity is enhanced by shallow folds in the cuticle. Their effect is comparable to that of a T-girder. The folds are seen as grooves, called sulci (singular: “sulcus”), on the outside of the cuticle. Sulci are most common on the head and thorax, where they define ar- eas of cuticle that are given specific names. Additional rigid- ity is achieved where fingerlike inpushings of the cuticle, called apodemes, meet internally, forming an endophragmal skeleton. This occurs in the head of all insects, where two pairs of apodemes, originating anteriorly and posteriorly on the head, join beneath the brain to form the tentorium, which provides the head with great rigidity in the horizontal plane. In winged insects lateral and ventral apodemes in the thorax may join or be held together by muscles forming a strut that holds the sides (pleura) of the thorax rigid with respect to the ventral surface (sternum). This is essential for the movement of the wings in flight. The tubular form of the legs and other appendages makes them rigid.

Flexibility in the cuticle, which allows different parts of the body to move with respect to each other, depends on regions of movable cuticle between the hardened plates (sclerites). Sclerotization does not occur in this flexible cuticle, which is referred to as “membranous.” It is most extensive in the re- gion of the neck, between the abdominal segments, and between segments of the appendages. Membranous cuticle also is found where the wings join the thorax and at the bases of the antennae, mouthparts, and other appendages, giving them freedom to move. Precision of movement is achieved by points of articulation at which there is only a very small re- gion of membrane between adjacent sclerites.

A rubberlike protein, called resilin, also is known to be pre- sent in some insects and may occur more widely. When it is distorted, it retains the energy imparted to it and, like a rubber ball, returns to its original shape when the tension is re- leased. There is a pad of resilin in the hind wing hinge of the locust and also in the side of the thorax of the flea, where the release of stored energy gives rise to the jump. Small amounts also are present in the hinge of the labrum in the locust and in the abdomen of some beetles.
The strength, rigidity and articulations of the cuticle pro- vide the insect with support, protection, and precision of movement. In larval forms, such as caterpillars and fly larvae, most of the cuticle remains unsclerotized. In these cases, the hemolymph (insects’ blood) functions as a hydrostatic (held by water pressure) skeleton, and movements are much less precise.
Epicuticle
Three or, in some species, four chemically distinct layers are present in the epicuticle. The innermost layer (inner epi- cuticle) contains lipoproteins but is chemically complex. Its functions are unknown. The next layer, the outer epicuticle, is made of polymerized lipid, though it probably also contains some protein. It is believed to be inextensible, such that it can unfold but not stretch. It defines the details of patterns on the surface of the cuticle. Outside the outer epicuticle is a layer of wax. This comprises a mixture of chemical compounds whose composition varies considerably between insect taxa. The wax limits water loss through the cuticle and so is a major feature contributing to the success of insects as terrestrial organisms, for whom water is at a premium. Because this layer becomes abraded (worn away) during normal activities, it has to be renewed continually. New compounds are synthesized in the epidermis and are thought to be transported to the sur- face via wax canal filaments that run through the pore canals and the inner and outer epicuticles. A fourth layer sometimes occurs outside the wax, but its functions are unknown.

Epidermis
The epidermis is a single layer of cells. In addition to pro- ducing the cuticle, it contains many glands that secrete chem- icals to the outside of the insect. These chemicals include many pheromones, involved in communication with other members of the same species, and defensive compounds that often are repellent to potential enemies. In the latter case, the glands frequently include a reservoir in which the noxious substances are accumulated until they are needed.