K.M. Rudall

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’.

Abstract The amount of chitin and of protein in the blowfly larval cuticle remains essentially unchanged during the process of hardening and darkening of the puparium. In the formation of the puparium there is a gain of weight of about 6 % over the larval cuticle weight, and this can be accounted for by the incorporation of phenolic substances. These are derived from the free tyrosine in the blood which decreases by an amount sufficient to account for the weight increase of the cuticle. An insoluble and highly resistant fraction has been isolated from the puparium, and this consists of about equal quantities of protein and pigment and possibly represents the natural association of these two substances in parts of the puparium. All the evidence suggests that the tyrosine is deaminated before giving the derivatives which combine with the cuticle substance, hardening and darkening it. Further properties of arthropodin, the water-soluble protein of soft arthropodal cuticles, are described. X-ray and related studies give a picture of the polysaccharide/protein complex in the cuticle which implies a model with alternating monolayers of protein and chitin. This appears to be based on a ratio of 3 amino acid residues to 1 chitobiose residue, or for equal lengths of protein and chitin chains a weight ratio of 45:55. This basic weight-ratio of protein and chitin appears to occur in the soft cuticles of blowfly larvae and other arthropods. A review is given of the composition and properties of various cuticles, and it is evident that hardening may be achieved without darkening of the cuticle. The chemical basis of this type of cuticle stabilization merits further study.

Abstract The morphological, physical and chemical changes in th e cuticle during the formation of the puparium in cyclorrhaphous flies are described. The larval cuticle contains about 60% chitin, the puparium about 47%. Almost all the non-chitinous substance is protein. An explanation is given of the hardening of the puparium.