Reproduction

and food for the juvenile stages. Likewise, hibernation in newly eclosed adult Colorado potato beetles (Leptinotarsa decemlineata) is induced by the short day lengths of fall (and also by a lack of food at this time). At the onset of diapause, the beetles become negatively phototactic and bury themselves under several inches of soil. Termination of diapause in L. decemlineata is unlikely to be induced by increasing day length (given the beetles' position); rather, diapause may have terminated simply as a result of the passage of time or by a temperature cue as soil warms in the spring (Hodek, 2002, and personal communication).

Estivation is seen in Schistocerca gregaria and Hypera postica, the alfalfa weevil. Sexual maturation in S. gregaria is retarded by long and promoted by short day lengths. This observation can be correlated with the availability of food and oviposition sites in its natural habitat, arid areas of Africa and Asia, where in the summer the weather is hot and dry, but rain falls intermittently during the winter. H. postica adults emerge in late spring and undergo reproductive diapause before laying eggs in late summer and fall. Low winter temperatures prevent the eggs from hatching until the following spring when new alfalfa foliage on which the larvae feed has begun to appear.

The effects of photoperiod on egg maturation are mediated via the endocrine system, though for many species the evidence for this statement is largely circumstantial, for example, differences in the histological appearance of the endocrine glands in diapausing and non-diapausing insects. In diapausing adult insects the corpora allata are small, and the neurosecretory system is typically full of stainable material, which are taken to indicate inactivity of these glands. When diapause is terminated and egg development begins, the corpora allata increase in volume and the amount of stainable material in the neurosecretory system decreases. In the beetle Galeruca tanaceti autoradiographic studies have shown that in postdiapause beetles the rate of incorporation oflabeled cystine into neurosecretory cells (taken to be a measure of their synthetic activity) is high compared to that of estivating insects. A limited amount of experimental work supports these histological correlations. For example, in L. decemlineata maintained under long-day (non-diapause) conditions, allatectomy, treatment with precocene (which destroys the corpora allata), or cautery of the brain neurosecretory cells mimics the diapause-inducing effects of short days, that is, stimulates digging behavior, arrests yolk deposition, and causes oxygen consumption to decrease. Conversely, treatment of diapausing beetles with juvenile hormone or its mimics causes the beetles to leave the soil, and stimulates feeding activity and egg maturation (Denlinger, 1985).

Endocrine. The endocrine control of sexual maturation in female insects, especially the control of oocyte development, continues to be among the most intensely studied areas of insect physiology. Yet, despite the wealth of literature that has resulted, some major aspects of hormonal control remain unclear. It is apparent, however, that among the Insecta the relative importance of the various endocrine centers in reproduction may differ, as might be anticipated in a group of such diverse habits (Belles, 1998). The following account is therefore generalized, though the major points of contention and differences among insects will also be outlined (see also Figure 19.8).

The two principal hormonal components involved are juvenile hormone (JH) and cerebral neurosecretory factors, though in some insects ecdysone, oostatic hormone, or antigo-nadotropic hormone also are important.

The importance of the corpora allata in egg development first became apparent in 1936 when Wigglesworth and Weed-Pfeiffer (cited in de Wilde and de Loof, 1974b) demonstrated independently that in Rhodnius and Melanoplus, respectively, allatectomy (removal of the

FIGURE 19.8. Endocrine control of egg development. (A) Schistocerca gregaria and other locusts; (B) Rhodnius prolixus; (C) Aedes aegypti and other mosquitoes; and (D) Sarcophaga bullata. [After K. C. Highnam and L. Hill, 1977, The Comparative Endocrinology of the Invertebrates, 2nd ed.. By permission of Edward Arnold Publishers Ltd.]

FIGURE 19.8. Endocrine control of egg development. (A) Schistocerca gregaria and other locusts; (B) Rhodnius prolixus; (C) Aedes aegypti and other mosquitoes; and (D) Sarcophaga bullata. [After K. C. Highnam and L. Hill, 1977, The Comparative Endocrinology of the Invertebrates, 2nd ed.. By permission of Edward Arnold Publishers Ltd.]

corpora allata) prevented vitellogenesis. Since this date, many authors, using allatectomy, followed by replacement therapy (implantation of "active" glands from other insects, or treatment with JH or its mimics), have confirmed the importance of JH as a gonadotropic factor in most insects. However, in the flesh fly, Sarcophaga bullata, vitellogenesis occurs only when the median neurosecretory cells are present and is, apparently, independent of JH (Figure 19.8D). In mosquitoes, JH controls only the previtellogenic growth of the primary follicles and is not required for vitellogenesis (Figure 19.8C) (Dhadialla and Raikhel, 1994; Klowden, 1997).

Originally, it was believed that JH probably triggered the synthesis of yolk precursors in the follicle cells, which then passed these materials on to developing oocytes. However,

C Aedes and other mosquitoes

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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