Insect structure and function

Abstract

The integument (Fig. 2). This consists of a cellular layer, the epidermis, with an outer non-cellular cuticle. The epidermis secretes the greater part of the cuticle and is responsible for dissolving and absorbing most of the old cuticle when the insect moults (p. 104) as well as repairing wounds and differentiating so as to determine the form and surface appearance of the insect. The cuticle forms the outer exoskeleton and is also present as a lining to the fore and hind intestine, to the tracheae and to other parts similarly formed by an ingrowth of the ectoderm or epidermis. Typically, it is composed of three layers:

• 1. The outermost layer, or epicuticle, less than 4 (μm thick, consists mostly of a hardened protein, but also contains the waxes which are largely responsible for reducing water loss through the cuticle, as well as an outer ‘cement layer’.

• 2. The exocuticle is a much thicker layer consisting mainly of chitin and proteins, the latter being ‘tanned’ by phenolic substances to produce a hard, brown material called sclerotin, which gives the cuticle its rigidity. The exocuticle is absent or reduced in the more flexible regions of the integument, and may be entirely absent from insects with a soft, thin cuticle.

• 3. The endocuticle, which is usually the thickest layer, also contains chitin and proteins, but the latter are not tanned and this part of the cuticle is therefore soft and flexible.