The source of lipids and polyphenols for the insect cuticle: The role of fat body, oenocytes and oenocytoids.

The newly fed fourth instar larva of Rhodnius lays down the outer epicuticle at 8-9 days, the inner epicuticle at 9 days, and it moults at 12 days. The oenocytes, which supply the lipid precursors, reach their maximum size at 7 days when lipid spheres and lipid-coated vesicles appear in their cytoplasm. The epidermal cells extend cytoplasmic strands to the contracting oenocytes and receive abundant lipid, which they transfer to the plasma membrane for construction of the outer and inner epicuticle. The oenocytes also transfer lipid to the epidermis attached to the basal lamina. This lipid is discharged through the lamina and taken up by ocnocytoids. which apply themselves to the basal lamina and liberate this copious absorbed material into the haemolymph before disintegrating. The synthesis of polyphenols for sclerotization takes place in the fat body, reaching a peak at day 10. After discharge into the haemolymph it is taken (presumably by a carrier protein) to the epidermis, where its uptake and transfer can be monitored by argentaffin staining. The tubular system of pore canals and tubular filaments is formed by invagination of the plasma membrane immediately after the inner epicuticle is complete, and is filled with lipid precursors and polyphenols. There is evidence that these metabolites are carried separately: the lipid in multiple tubular filaments; the polyphenol through the substance of the axial filament. Lipid and polyphenols are still supplied to the epidermis during days 10-12. Both are most richly supplied to the sites forming exocuticle-which illustrates the importance of lipid as well as polyphenol in cuticle hardening.