Summary
The cuticle consists of a relatively soft and colourless endocuticle, hardened and darkened in its outer part in some places to form a rigid exocuticle, and a complex epicuticle made up of several layers.
The endocuticle consists of polyacetylglucosamine (chitin) intimately associated with a characteristic protein (arthropodin). Perhaps these are combined in the form of a mucoprotein in which the relative amounts of protein and polysaccharide vary with the type of animal and with the part of the body. The substance of the endocuticle separates, apparently spontaneously, into laminae of varying dimensions.
In most insects the submicroscopic crystallites of chitin, and perhaps protein, tend to lie at random with the long axis in the plane of the cuticle. They can be orientated by tension or compression. In the Coleoptera they are arranged in microscopic bundles (‘Balken’) lying parallel in a given lamina, but at an angle of 60o in successive laminae. The crystallites are orientated also in the long axis of tendons and bristles.
In the exocuticle the protein is tanned by quinones derived by oxidation from dihydroxy‐phenols. This tanned protein (sclerotin) impregnates the chitin framework to form a rigid fabric (like cellulose impregnated with a resin plastic) which is moderately impermeable to water. Hard cuticles in insects are always dark, largely because the quinonoid groups are chromophore, in part perhaps because during the oxidative hardening some true melanin is formed from tyrosine. In a few insects impregnation with lime takes the place of tanning.
The cuticular substance has a tendency to crystallize in the form of multiple thin plates; these are responsible for the iridescent colours of many insects.
The epicuticle is responsible for most of the impermeability to water. It consists of a thin layer of tanned lipoprotein (cuticulin), a layer of crystalline waxes about a quarter of a micron thick, and a layer of cement (likewise consisting, perhaps, of tanned protein containing some lipides) protecting the wax.
The wax varies in character from a soft grease to hard white crystalline materials. If the temperature is raised to a critical level, some 5–100 C. below the melting‐point of these waxes, the insect shows a sudden increase in the rate of transpiration. If the cement and wax layer are abraded by fine dusts or removed by lipid solvents, the loss of water increases enormously. The properties of the epicuticular layers control to some extent the entry of insecticides through the cuticle.
In the deposition of the cuticle the cuticulin layer is first laid down; the lipoproteins which compose it appear to come from the oenocytes. Formation of the endocuticle takes place around cytoplasmic filaments (the pore canals) which extend from the interior of the epidermal cells and appear to penetrate the cuticulin layer of the epicuticle. Droplets of material rich in polyphenols are exuded from the tips of the pore canals and fuse to form a continuous layer over the cuticulin. The wax is then secreted, durin...
