ABSTRACT
The outer skin or integument of an insect is known as the cuticle. This forms a non-cellular, waterproof layer over the body and is composed of chitin and protein, the precise chemical composition and thickness determining its hardness and rigidity. The cuticle has three layers (epicuticle, exocuticle and endocuticle) and is secreted by an inner lining of cells that form the hypodermis or basement membrane. When first produced, the cuticle is elastic and flexible, but soon after deposition it usually undergoes a period of hardening or sclerotization and becomes more or less darkened by the addition of a chemical called melanin. The adult cuticle is not replaceable, except in certain primitive insects. However, at intervals during the growth of the immature stages (larvae and nymphs), the ‘old’ hardened cuticle becomes too tight and is replaced by a new, initially expandable, one secreted from below.
