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578 hormone from the median neurosecretory cells of the brain regulated protein synthesis in the fat body (references in Highnam and Hill, 1977). Only later came the realization that the neurosecretion does not act directly on the fat body, but is allatotropic in effect; that is, it stimulates the corpora allata to produce JH, which then promotes vitellogenesis in the fat body in addition to its effects on the ovary (Figure 19.8A). In some insects, for example Rhodnius, neurosecretion also has other important functions in the reproductive process (see Section 7.2) (Figure 19.8B) (Davey, 1997).

A different arrangement occurs in mosquitoes in which neurosecretion [ovarian ecdys-teroidogenic hormone (OEH)] is released for only the first few hours after the blood meal (anautogenous species such as A. aegypti) or after eclosion (autogenous species, for example, A. atropalpus, that do not need a blood meal to mature eggs), and, as noted earlier, the corpora allata are not important in vitellogenesis (Klowden, 1997). In these insects OEH acts directly on the ovary, stimulating it to release ecdysone, which regulates vitellogenin synthesis in the fat body (Hagedorn et al., 1975) (Figure 19.8C). Of particular interest is the involvement of ecdysone, a hormone traditionally associated with molting and produced by glands that disappear in all but a very few adult insects. Ecdysone has now been detected in adults of many insect species and, in higher Diptera also is involved in vitellogenesis (Hagedorn et al., 1975; Hagedorn, 1985; Yin and Stoffolano, 1994). It is now clear that the follicle cells are the major source of the ecdysone. However, ovariectomy does not lead to the complete depletion of the hormone in the hemolymph, and other sources, notably oenocytes, have been suggested. Relatively huge (microgram) amounts of ecdysone occur in the ovaries of Locusta migratoria, 99% of which is in the terminal oocytes in the conjugated (inactive) form. During embryogenesis, free ecdysone is periodically released, the peaks of release corresponding with bouts of embryonic cuticle production and hatching (Chapter 20, Section 7.2).

Among the Lepidoptera, which show a wide range of reproductive strategies, the endocrine control of vitellogenesis and metamorphosis are often closely linked (Ramaswamy et al., 1997). Four patterns have been identified within the order. In those species in which egg development is completed in the larval or early pupal stage, vitellogenesis is triggered by the release of prothoracicotropic hormone. This stimulates the molt glands to produce ecdysone, which initiates vitellogenesis in the fat body and may also induce patency in the follicular epithelium. In some other Lepidoptera (Pyralidae) where vitellogenesis occurs later in the pupal stage, it is the declining level of ecdysone that apparently promotes the process. In a third group, of which Manduca sexta is an example, the dependence on ecdysone is lost: vitellogenesis occurs in the pharate adult in the absence of ecdysteroids, though what other hormonal factor(s) regulate the process are unclear. Finally, in the Lepidoptera that develop eggs only after eclosion and a period of sexual maturation (for example, butterflies and noctuid moths), JH serves both to promote vitellin production in the fat body and the development of patency in the follicular epithelium, as occurs in insects in general.

An aspect of egg maturation that requires much more study is the interrelationship of the endocrine glands, ovary, and fat body. That is, how is endocrine activity increased or decreased during each cycle of egg development so that the system operates most efficiently? For most insects, the neurosecretory system acts as the center for translating external stimuli into endocrine language. As a result of these stimuli, production and release of allatotropic, allatostatic, and other neurosecretory peptides will be altered, causing, in turn, changes in the level of production of JH, ecdysone, etc. However, endocrine gland activity can be modified in other ways, as is obviously the situation in insects whose neurosecretory system is not involved in egg development. For example, in the allatic nerves (Figure 13.4) there are non-neurosecretory axons that conceivably can carry neural information from the brain 579

to activate or inhibit the corpora allata. Another possibility is that the corpora allata respond directly to the nutritional milieu in which they are bathed. In an actively feeding insect whose hemolymph contains large quantities of nutrients, the glands could be expected to be active to facilitate production and accumulation of yolk. As oocytes mature, feeding activity declines, so that there are fewer nutrients in the hemolymph, which might lead to a reduction in corpus allatum activity. An important question is, therefore, "What causes feeding activity to decline as vitellogenesis nears completion?" To date, there is no clear answer to this question, though some authors have suggested that distension of the oviducal or abdominal wall by mature eggs might inhibit feeding behavior via the central nervous system.

A further possibility that may account for the cyclic nature of egg production involves a common endocrine principle, namely, feedback inhibition. There is evidence that a high level of circulating JH may inhibit synthesis and release of neurosecretion. In turn, this will result in a decline in corpus allatum activity, a drop in the level of JH and, eventually, a renewal of neurosecretory activity.

A relatively unexplored aspect of these interrelationships is whether the ovary may function as an endocrine structure whose secretion may competitively bind with a hormone receptor, for example, that for JH, or may inhibit production of hormone by a gland. As noted earlier in this chapter and in Chapter 13 (Section 3.4), oostatic and antigonadotropic hormones are produced in Musca, and Rhodnius and Locusta, respectively, and it will be interesting to see whether comparable situations are widespread among Insecta.

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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