THE CHITIN SYSTEM

Abstract

SUMMARY The view is supported that chitin is not found in Deuterostomia because of the absence of chitin synthetase, and is not found in higher plants because of the absence of glucosamine. In Fungi, control mechanisms are present affecting the synthesis of glucosamine; chitin is often present, but when it is absent this probably results from a failure to synthesize glucosamine. A review of conformation maps for cellulose and chitin indicates the possibility of a slightly right‐handed twist in small groups of chitin chains. The occurrence of α, β and γ‐forms of chitin in the peritrophic membranes of various insects is described. Gamma chitin seems to be the commonest form. In several beetles, optical and electron‐microscope studies trace the formation of chitinous cocoon fibres from larval peritrophic membrane and define the discrete ribbon‐like nature of the, β chitin produced in the mid‐gut. By studying apodemes it is found that orthopteroid insects are most varied, different molecular structures being present in levator, depressor and pretarsal tendons. By contrast, Hymenoptera and Coleoptera show very similar structures in all three apodemes as well as in other parts of the cuticle. Apodemes are regarded as sampling the cuticle at their varying points of origin; they provide especially favour able material for diffraction studies. In arthropod cuticles there is evidence for the widespread occurrence of α chitin micelles which are three chains thick in the direction of the c axis. This is compared with the structure of γ chitin where the chains repeat in groups of three along the c axis. Changes in the diffraction pattern are related to the series of proteins defined by Hackman. The chitin‐protein complex is not affected by water or neutral salt extrac tion, but is disrupted by treatment in urea. Electron microscopy defines the unit of structure as a composite microfibril: a core of chitin surrounded by adsorbed proteins. This consists of ‘primary’ protein (often repeating as regular units along the fibrils) and a quantity of ‘satellite’ protein which obscures the imaging of the regularly arranged ‘primary’ protein. There are apparent ‘bridges' between the microfibrils. New diffraction data give information about the size and arrangement of micro‐fibrils. These fibrils may be arranged in layers of ‘rods’, or as an hexagonal arrange ment of ‘rods’.