Incorporation of lipid into the epicuticle of Rhodnius (Hemiptera).

Abstract

The incorporation of lipid into both the outer and inner epicuticle during deposition is described. Waterproofing of the epicuticle by secretion of the wax layer, and sclerotization with or without melanization, are controlled from a distance by the epidermal cells by way of the pore canals. The pore canals gradually narrow as they approach the epicuticle. On reaching the inner epicuticle the canal ends in a conical projection from the apex of which a permeable lipophilic channel about 20-25 nm in diameter runs vertically to the apex of which a permeable lipophilic channel about 20-25 nm in diameter runs vertically to the surface. Shortly before ecdysis, silver-binding material (perhaps protein rich in tyrosine, or other precursors concerned in sclerotization) spreads radially from a point in the cuticular channels just below the outer epicuticle, and gradually impregnates the outer two thirds or more of the inner epicuticle. The precise pattern varies in different cuticular structures. Argentaffin materials (polyphenols) first appear in these same sites at the time of ecdysis and increase rapidly during the next 24 h. Lipid appears in the lumen of the distal parts of the pore canals (with a patchy distribution) shortly before ecdysis. When digestion and absorption of the old endocuticle are almost complete, minute lipid droplets appear on the surface of the epicuticle, apparently exuded from the epicuticular channels, and spread to make a uniform layer. When first formed this layer strains readily with Sudan B, but the lipid becomes incorporated in a delicate non-lipid silver-binding membrane (also exuded from the epicuticular channels) and hardens just before ecdysis, to form the so-called 'wax layer' which then no longer stains with Sudan B. Within half an hour after ecdysis the alcian blue-staining cement layer is poured out by the dermal glands, and forms a continuous but somewhat irregular covering over the 'wax layer'. Changes in the epicticle that accompany the repair of abrasions are described.