The structure and deposition of the cuticle in the adult mealworm, Tenebrio molitor L. (Coleoptera).

IN three earlier papers (Wigglesworth, 1933, 1945, 1947a) the structure and deposition of the cuticle of Rhodnius proltxus have been described. The main conclusions reached were as follows. The epicuticle is a composite structure made up of four layers: (i) a 'cuticulin' layer composed of a condensed lipoprotein subsequently tanned by quinones; (ii) a layer of material rich in dihydroxyphenols; (iii) a thin layer of crystalline, orientated wax molecules responsible for the waterproofing of the cuticle (cf. Beament, 1945); and (iv) an outermost 'cement' layer protecting the wax. The lipoproteins which form the foundation of the epicuticle are apparently synthesized by the oenocytes before being transferred to the epidermal cells. The oenocytes reach their maximum development immediately before the cuticulin layer is deposited and then diminish rapidly in size. The polyphenol layer is secreted from the epidermal cells by way of the pore canals which appear to penetrate the cuticulin layer. The wax is secreted in the same way immediately before moulting. The cement layer is poured out from the dermal glands over the surface of the wax immediately after moulting. The hardening which takes place shortly after moulting is due to the tanning of the lipoprotein of the epicuticle and the cuticular protein of the exocuticle by quinones produced by the oxidation of the dihydroxyphenols (Pryor, 19406; Pryor, Russell and Todd, 1947). The endocuticle is laid down by the epidermal cells during the next few days. In the deposition of the cuticle the epidermal cells first separate from the old cuticle and undergo active mitosis. Many more cells are produced than are required. Consequently, great numbers of them suffer autolysis with the formation of the 'chromatic droplets', until, finally, a regular epithelium with the nuclei evenly spaced is produced, which then proceeds to lay down the new cuticle (Wigglesworth, 1943a). If the cuticle of Rhodnius (and many other insects) is gently rubbed with alumina dust, the cement layer and the wax layer are abraded; the layer containing the polyphenols is then exposed and will stain a deep chestnutbrown if the insect is immersed in ammoniacal silver hydroxide. The polyphenol layer is likewise exposed more or less readily by extraction of the cuticle surface with chloroform and other wax solvents.