Fine structure of the cuticle, epidermis, and fat body of larval Elateridae (Coleoptera) and changes associated with molting

Abstract

The intermolt cuticle of three species of soil-inhabiting wireworms consists of four layers: non-oriented lipid over an oriented lipid monolayer, outer epicuticle. membrane epicuticle, and an inner dense epicuticle. Sclerites lack the surface lipid layers, but have a thinly laminated transformation zone underlying the epicuticle. There are three layers of exocuticle in sclerites, but only the central one of these is continuous through adjacent membranes. The cuticular pore canal system has medial axial filaments and peripheral fibrils, which seem to serve as wicks in transporting, primarily, lipids to the surface of membranes, and sclerotins, cuticulin, and lipids into the upper layers of cuticle in sclerites. In exuvial histolysis there is an initial degradation of subcuticle by enzymes from the epidermis before the new epicuticle is formed, and a subsequent histolysis of procuticular lamellae by enzymes from ecdysial droplets released primarily by ecdysohemocytes in the exuvial space after the new epicuticle is formed. The ecdysial membrane originates by delamination of the procuticular lamellae of the exuvium, and seems to consist primarily of enzyme-resistant components that accumulate from successive lamellae, particularly of sclerites, for discard with the exuvium at ecdysis. The dynamics of the epidermal and fat cells during formation of cuticle and muscle attachments are described and discussed.