Chitin lamellogenesis in locust cuticle

The organization and functions of cuticular lamellae, a structural feature common to all arthropod skeletons, have been studied by growth experiments analysed with the polarizing light microscope. Use has been made of the daily incremental growth of locust solid endocuticle, in which morphogenesis of chitin lamellae (lamellogenesis) can be experimentally altered by varying light and temperature at the time of deposi-tion. Locust endocuticle is all deposited after ecdysis and is composed of alternating night zones (lamellate) and day zones (non-lamellate), distinguishable morphologically, histochemically, mechanically, and morphogenetically. Chitosan tests, confirmed by chromatography, show that the lamellae contain chitin. Endocuticle grown in constant light at constant temperature is non-lamellate; it still contains the same proportion of chitin as normally lamellate cuticle, indicating that lamellogenesis is mainly a periodic process of permanent change in organization of chitin crystallites, occurring at deposition. Lamellogenesis in solid cuticle is independent of chitin synthesis and of chitin pore-canal fibril formation. Swelling, mechanical, and form-birefringence tests combined with histological observations show that lamellate cuticle is structurally more highly organized but less well oriented than non-lamellate cuticle. One possible function of lamellae is in stabilizing the three-dimensional chitin framework. Normally in night conditions each chitin lamella is individually determined within about 30 min, and lamellae are continuous over large areas of cuticle. The structure and formation of chitin lamellae in locust solid and rubber-like cuticles are compared. Three-dimensional birefringence analysis of the locust hind tibia has revealed three chitin components: (1) chitin lamellae with the chitin crystallites oriented predominantly parallel to the plane of the cuticle surface; (2) chitin pore-canal fibrils, running radially through the cuticle; (3) chitin fibrils oriented longitudinally along the leg axis. Of these, the lamellae and longitudinal fibrils can be morphogenetically altered by suitable techniques