The Integument

The gland cells again exhibit cyclical activity associated with new cuticle production, and 357

it has been proposed that they secrete the cement layer of epicuticle.

The cuticle, which is mainly produced by the epidermal cells, usually includes three primary layers, the inner procuticle, middle epicuticle, and outer cuticulin envelope (Locke, 2001) (Figure 11.2). In older accounts of the integument the cuticulin envelope is treated as part of the epicuticle. However, Locke (1998, 2001) has argued that, because of its distinct origin, structure and functions, the cuticulin envelope should be considered separate from the epicuticle. All three primary layers are present over most of the body surface and in the cuticle that lines major invaginations such as the foregut, hindgut, and tracheae. However, the procuticle is very thin or absent, and certain components of the epicuticle may be missing, where flexibility or sensitivity is needed, for example, over sensory structures and the lining of tracheoles. Only the cuticulin envelope is universally present, except for the pores over chemosensilla (Chapter 12, Section 4.1).

The procuticle (= fibrous cuticle) forms the bulk of the cuticle and in most species is differentiated into two zones, endocuticle and exocuticle, which differ markedly in their

FIGURE 11.2. Electron micrographs showing deposition of the three primary layers of cuticle in Calpodes ethlius. [From M. Locke, 2001, The Wigglesworth lecture: Insects for studying fundamental problems in biology, J. Insect Physiol. 47:495-507. With permission from Elsevier.]
FIGURE 11.3. Diagram showing orientation of microfibers in lamellae of endocuticle. [From A.C. Neville and S. Caveney, 1969, Scarabaeid beetle exocuticle as an optical analogue of cholesteric liquid crystals, Biol. Rev. 44: 531-562. By permission of Cambridge University Press, London.]

physical properties but only slightly in their chemical composition. In some cuticles the border between the two is not clear and an intermediate area, the mesocuticle, is visible. Adjacent to the epidermal cells a narrow amorphous layer, the assembly zone, may be seen where chitin microfibers are deposited and oriented.

The endocuticle is composed of lamellae (Figure 11.3). Electron microscopy reveals that each lamella is made up of a mass of microfibers arranged in a succession of planes, all fibers in a plane being parallel to each other. The orientation changes slightly from plane to plane making cuticle like plywood with hundreds of layers. The exocuticle is the region of procuticle adjacent to the epicuticle that is so stabilized that it is not attacked by the molting fluid and is left behind with the exuvium at molting (Locke, 1974). Not only is the exocuticle chemically inert, it is hard and extremely strong. It is, in fact, procuticle that has been "tanned" (Section 3.3). Exocuticle is absent from areas of the integument where flexibility is required, for example, at joints and intersegmental membranes, and along the ecdysial line. In many soft-bodied endopterygote larvae the exocuticle is extremely thin and frequently cannot be distinguished from the epicuticle and cuticulin envelope.

Procuticle is composed almost entirely of protein and chitin. The latter is a nitrogenous polysaccharide consisting primarily of N-acetyl-D-glucosamine residues together with a small amount of glucosamine linked in a (31,4 configuration (Figure 11.4). In other words, chitin is very similar to cellulose, another polysaccharide of great structural significance, except that the hydroxyl group of carbon atom 2 of each residue is replaced by an acetamide group. Because of this configuration, extensive hydrogen bonding is possible between adjacent chitin molecules which link together (like cellulose) to form microfibers. Chitin makes n-r-rw n-r-rw

Acety ¬°glucosamine Acetylglucosamine Glucosamine FIGURE 11.4. The chemical structure of chitin.

Acety ¬°glucosamine Acetylglucosamine Glucosamine FIGURE 11.4. The chemical structure of chitin.

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.

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