362 apolysis, the detachment of the epidermis from the old cuticle. The epidermal cells, at this time, show signs of preparation for future synthetic activity. One or more nucleoli become prominent, the number of ribosomes increases, and the ribonucleic acid content of the cells is elevated. Two components of the epidermal cells are especially important, namely, the Golgi complexes and the plasma membrane plaques, whose activities alternate to create the new cuticle. Just prior to apolysis, Golgi complex activity increases, and the vesicles produced migrate to the apical plasma membrane where they release their contents—ecdysial droplets—between the epidermal microvilli (Figure 11.5A). The ecdysial droplets contain proteinases and chitinases for cuticle digestion, though the enzymes remain in an inactive form until after formation of the new cuticulin envelope when the epidermal cells secrete an "activation factor." In Calpodes ethlius large quantities of an amidase are generated by the epidermis and fat body during the intermolt. The amidase (in its inactive form) accumulates in the hemolymph until the molt cycle begins, when it moves into the molting fluid and is activated, enabling precise initiation of cuticle breakdown (Marcu and Locke, 1999). Between 80% and 90% of the old cuticle is digested and may be reused in the production of new cuticle. In earlier accounts it was assumed that the molting fluid, including the breakdown products, were resorbed across the body wall. However, recent studies have demonstrated that most of the molting fluid is recovered by both oral and anal drinking, reentering the body cavity by absorption across the midgut wall (Yarema et al., 2000). The exocuticle, muscle insertions, and sensory structures in the integument are not degraded by molting fluid. Thus, an insect is able to move and receive information from the environment more or less to the point of ecdysis.

After release of the ecdysial droplets, the microvilli are withdrawn and their plaques are pinocytosed and digested in multivesicular bodies (Figure 11.5B). New microvilli, with plaques at their tips, then differentiate. The first layer of new cuticle deposited is the cuticulin envelope. Minute convex patches of cuticulin appear above the plaques (Figure 11.5C), the patches eventually fusing together to form a continuous but buckled layer (Figure 11.5D). The buckling permits expansion of the cuticle after molting and is also important in the formation of annuli and taenidia in tracheae and tracheoles (Chapter 15, Section 2.1). Other buckling patterns determine the specific surface structure of scales, bristles, and microtrichia. Oenocytes are maximally active at this time, and it is possible that they are involved in cuticulin formation, perhaps by synthesizing a precursor for the epidermal cells. When the envelope is complete, it becomes tanned. The Golgi complexes then show renewed activity, their vesicles discharging their contents to form the inner (protein) epicuticle (Figure 11.5E).

Before the inner epicuticle is fully formed production and deposition of the new procu-ticle begin. In contrast to the epicuticle, whose layers are produced sequentially from inside to outside, the new procuticle is produced with the newest layers on the inside. Again, it is the plasma membrane plaques that are involved, new chitin fibers arising on their outer surface (Figure 11.5F,G). However, details of the mechanism by which new procuticle is produced remain sketchy. The epidermal cells contain the enzymes necessary for synthesis of acetylglucosamine from trehalose. Acetylglucosamine units perhaps are then secreted into the apolysial space, polymerization into chitin being promoted by the enzyme chitin synthetase attached to the plasma membrane plaques. Some procuticular proteins are synthesized by the epidermal cells while others are acquired from the hemolymph (Sass et al., 1993; Suderman et al., 2003). How the proteins become incorporated into the procuticle remains unclear.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

Get My Free Ebook

Post a comment