The Integument

up between 25% and 60% of the dry weight of procuticle but is not found in the epicuticle 359

and cuticulin envelope. It is associated with the protein component, being linked to protein molecules by covalent bonds, forming a glycoprotein complex. Studies have shown that the epidermis secretes more than a dozen major proteins into the cuticle in a carefully timed sequence, probably under hormonal control (Suderman etal., 2003). Interestingly, cuticular proteins of similar molecular weights have been found in a range of insect species suggesting that the chemical nature of the cuticle has been strongly conserved through evolution. The amino acid composition of the proteins determines their properties. For example, endocu-ticular proteins are generally rich in hydrophobic amino acids with bulky side chains and are loosely packed (not compact) molecules. This provides the endocuticle with flexibility and will also facilitate "creep" (the ability of layers to slide over each other), hence intrastadial growth in soft-bodied insects such as caterpillars. Conversely, in hard, stiff exocuticle, it is small, compact amino acids that predominate (Hepburn, 1985).

In the exocuticle, adj acent protein molecules are linked together by a quinone molecule, and the cuticle is said to be tanned (Section 3.3). The tanned (sclerotized) protein, which is known as "sclerotin," comprises several different molecules. Resilin is a rubberlike material found in cuticular structures that undergo springlike movements, for example, wing hinges, the proboscis of Lepidoptera, the hind legs of fleas (Chapter 14, Section 3.1.2.), and the wing-hinge ligament that stretches between the pleural process and second axillary sclerite (Chapter 14, Sections 3.3.1 and 3.3.3) (Neville, 1975). Like rubber, resilin, when stretched, is able to store the energy involved. When the tension is released, the stored energy is used to return the protein to its original length.

In addition to these structural proteins, enzymes also exist in the cuticle, including phenoloxidases, which catalyze the oxidation of dihydric phenols used in the tanning process (Section 3.3). These enzymes appear to be located in or just beneath the epicuticle.

A variety of pigments have been found in the cuticle (or in the epidermis) which may give an insect its characteristic color (Section 4.3). Also, in a few beetles and larvae and pupae of some Diptera, mineralized calcium (as the carbonate) is deposited, presumably to increase rigidity (Leschen and Cutler, 1994).

Certain processes occur at the surface of the cuticle after it has been formed, for example, secretion and repair of the wax layer and tanning of the outer procuticle. Thus, a route of communication must remain open between the epidermis and cuticular surface. This route takes the form of pore canals which are formed as the new procuticle is deposited (Section 3.1), and which may or may not contain a cytoplasmic process. Most often, the canals do not contain an extension of the epidermal cell but have at least one "filament" produced by the cell. Locke (1974) suggested that the filament(s) might keep a channel open in the newly formed cuticle until the latter hardens, and anchor the cells to the cuticle. In some insects the pore canals become filled with cuticular material once epicuticle formation (including tanning) is complete. The pore canals terminate immediately below the epicuticle. Running from the tips of the pore canals to the outer surface of the epicuticle are lipid-filled channels known as wax canals.

The epicuticle is a composite structure produced partly by epidermal cells and partly by specialized glands. It ranges in thickness from a fraction of a micrometer to several micrometers and generally comprises three layers. The layers are, from outside to inside, cement, wax (these are secreted outside the cuticulin envelope), and the so-called protein epicuticle. The nature of cement varies, though it is likely to be approximately similar to shellac. The latter is a mixture of laccose and lipids. The cement is undoubtedly a hard, protective layer in some insects. In others it appears to be more important as a sponge that

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