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The skin or integument of insects consists, primarily, as in worms and all arthropods, of an epithelial layer of cells called the hypodermis. This layer secretes the cuticle, which is of varying thickness and flexibility, and is usually very dense, impermeable, and light, compared with the crust of the Crustacea, where the cuticle becomes heavy and solid by the deposition of the carbonate and phosphate of lime. This is due to the presence of a substance called by Odier chitin.^[9] The cuticle is thin, delicate, and flexible between the joints; it is likewise so in such diaphanous aquatic larvæ as that of Corethra, and in the gills of aquatic insects, also in the walls of the tracheæ and of the salivary ducts. The cuticle thus forms a more or less solid crust which is broken into joints and pieces (sclerites), forming supports for the attachments of the muscles and serving to protect the soft parts within.

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“Chitin forms less than one-half by weight of the integument, but it is so coherent and uniformly distributed that when isolated by chemical reagents, and even when cautiously calcined, it retains its original organized form. The color which it frequently exhibits is not due to any essential ingredient; it may be diminished or even destroyed by various bleaching processes.” (Miall and Denny.)