Postecdysis

Several processes are continued or initiated after ecdysis. As noted wax secretion continues, and the major portion of the endocuticle is deposited at this time. Indeed, endocuticle production in some insects appears to be a more or less continuous process throughout the intermolt period. It is also at this time that the dermal glands release the cement.

The most striking postecdysial event, however, is the differentiation of the exocuticle, that is, the hardening of the outer procuticle (Figure 11.5H). This results from a biochemical process known as tanning (sclerotization), in which proteins become covalently bound to each other (and hence stabilized) by means of quinones. Hardening is usually accompanied by darkening (melanization), though the two may be distinct processes; that is, some species have pale but very hard cuticles. Though tanning is discussed here in the context of the cuticle, it should be noted that it also an important process in the final structure of insect egg shells, egg cases (oothecae) and protective froths, cocoons, puparia and various silk structures. Indeed, much of the basic understanding of tanning came from studies using the cockroach ootheca and the fly puparium (Andersen, 1985; Hopkins and Kramer, 1992). More recently, the cuticles of the Manduca sexta pupa and oflocusts and grasshoppers have proved to be excellent models for study of this process. Though the details may differ, it is now possible to provide a basic scheme for the events that culminate in a tanned cuticle (Figure 11.6).

Before tanning begins, the level of the amino acid tyrosine in the hemolymph increases. The tyrosine is mostly bound to glucose, phosphate, or sulphate, forming water-soluble conjugates. This is thought to increase the amount of tyrosine that can be carried in the

FIGURE 11.6. Summary of the tanning process.

hemolymph, to protect it from oxidation, and to prevent its use in competing metabolic pathways. In a few species the tyrosine accumulates within fat-body vacuoles, particularly as the dipeptide N-p-alanyltyrosine. The tyrosine is converted to "dopa" (dihydroxypheny-lalanine) which is then decarboxylated forming dopamine, N-p-alanyldopamine, and other catecholamines (Hopkins and Kramer, 1992; Hopkins et al., 1999). Like tyrosine, the catecholamines occur as conjugates and accumulate in the hemolymph. Only when the catecholamines are taken up by the epidermal cells are the conjugates released. Within the epidermis, catecholamines are converted to the corresponding N-acetyl derivatives which then move via the pore canals to the epicuticle where they are oxidized to quinones under the influence of phenoloxidases. Two distinct phenoloxidases have been isolated from cuticle, tyrosinase, and laccase. Tyrosinase may be involved in tanning in some situations but is likely more important in wound healing (Chapter 17, Section 5.1). By contrast, the activity of laccase is closely correlated with the period of tanning, and this enzyme is therefore assumed to play the major role in the synthesis of quinone tanning agents. The quinones diffuse back into the procuticle and link protein molecules together. According to Hackman (1974), quinones may polymerize, a process that makes them larger, more effective tanning agents. The polymer will have more reactive sites and be capable of bridging larger distances, the result being that more protein molecules can be linked. The quinones combine covalently with the terminal amino group or a sulfhydryl group of the protein to form an N-catechol

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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