Postembryonic Development

FIGURE 21.15. Forms of the vetch aphid. Megoura viciae. The fundatrix (A), a parthenogenetic female that emerges from the overwintering egg, produces the fundatrigeniae (B) from which many generations of wingless (C) or winged (D) virginoparae develop. In late summer, wingless egg-laying oviparae (E) and males (F) are formed. [From A. O. Lees, 1961, Clonal polymorphism in aphids, Symp. R. Entomol. Soc. 1:68-79. By permission of the Royal Entomological Society.]

FIGURE 21.15. Forms of the vetch aphid. Megoura viciae. The fundatrix (A), a parthenogenetic female that emerges from the overwintering egg, produces the fundatrigeniae (B) from which many generations of wingless (C) or winged (D) virginoparae develop. In late summer, wingless egg-laying oviparae (E) and males (F) are formed. [From A. O. Lees, 1961, Clonal polymorphism in aphids, Symp. R. Entomol. Soc. 1:68-79. By permission of the Royal Entomological Society.]

polyphenism in which these structural and physiological features gradually change from generation to generation (Lees, 1966; Hardie and Lees, 1985). Aphids reproduce partheno-genetically (and in some species paedogenetically) for a large part of the year, giving rise to large numbers of wingless individuals that can exploit the rich food supplies available in spring and summer. However, to avoid starvation caused by overcrowding, to move to nutritionally more valuable food sources, and to reproduce sexually, it is necessary for winged individuals (alates) to develop.

The development of alates is influenced by many environmental factors, for example, photoperiod, temperature, population density, and water content of food plants, all of which ultimately appear to bring about changes in endocrine activity. Thus, treatment of presumptive gynoparae (a winged migrant form) of the bean aphid (Aphis fabae) with JH mimics the effects of long-day conditions, causing them to molt to wingless adults and to show a preference for tic bean (Vicia fabae),their normal summer host. Conversely, topical application of precocene (which destroys the corpus allatum) to winged adults of other species stimulates production of winged progeny. In some species these environmental factors act directly on an individual to modify its form, whereas in others the effect is not seen until the 647

following generation. For example, when first- or, to a lesser extent, second-instar larvae of

the green peach aphid (Myzus persicae) are kept under crowded conditions, they develop DEVELOPMENT

into winged forms. In contrast, in the vetch aphid (Megoura viciae) crowding young larvae does not induce wing development either in the larvae or in their progeny. In this species, sensitivity to crowding develops in the fourth (final) instar and is retained through adulthood. Thus, when older larvae or wingless adults are crowded, their progeny are winged. Experiments have indicated that the stimulus is not visual or chemical but due to repeated physical contact between individuals. Interestingly, ants that tend aphids for their honey-dew have a "tranquilizing" effect on the aphids. This effect apparently leads to reduction in the amount of physical contact between the aphids, which thus remain apterous, to the ants' obvious advantage. It is presumed that the crowding stimulus operates via the brain, which somehow reduces corpus allatum activity. The nature of the link between the brain and corpus allatum is not known but is likely to be hormonal because in species such as M. viciae the crowding stimulus is received by the maternal brain, but its effect is made apparent in the progeny. In other words, it is the corpora allata of the developing embryos within the mother whose activity is modified, yet there is no nervous connection between these glands and the mother's brain (Lees, 1966).

In colonies of social insects, individuals fall into a number of functionally and, usually, structurally distinct castes. In lower termites, for example, there is a pair of primary reproductives (king and queen) which found the colony, supplementary (replacement) reproductives which develop as the colony reaches a certain size and eventually take over the reproductive function, soldiers, nymphs (juveniles with wing buds from which primary reproductives develop), and larvae (juveniles that lack wing buds). Each caste contains members of both sexes. The number of individuals belonging to each caste is normally maintained as a fixed proportion of the total number of insects in the colony by means of inhibitory pheromones secreted by already differentiated individuals (Chapter 13, Sections 4.1.1 and 4.1.2). When the concentration of a pheromone falls below a certain level, resulting from, for example, growth of the colony or death of the pheromone-producing individuals, inhibition no longer occurs, and differentiation of new individuals restores the correct proportion. The relationship of the castes and the course of development in a lower termite, Zootermopsis angusticollis, are indicated in Figure 21.16. Young larvae pass through several progressive molts (i.e., grow and differentiate) until they become pseudergates (false workers) comparable with the true workers of higher termites. Pseudergates may undergo additional progressive molts to form specific castes, or may molt without differentiation (stationary molts) (Yin and Gillott, 1975).

The inhibitory pheromones influence caste differentiation by modifying the activity of the endocrine system, especially the corpora allata. The extensive studies of Luscher and his associates in the 1950s led to the development of an elaborate hypothesis for hormonal control (see Hardie and Lees, 1985). This included the suggestion that the corpora al1ata produced three distinct hormones: JH, for control of juvenile ^ juvenile and juvenile ^ adult differentiation (as in non-po1ymorphic insects); soldier-inducing hormone that promoted soldier formation; and gonadotropic hormone, which was produced in adult females to control oocyte development.

As a result of subsequent work by Luscher's group and other authors, especially experiments involving application of JH analogues, it was realized that the corpora allata do not produce more than one hormone. Rather, all developmental possibilities result from variations in the concentration of circulating JH at critical times during a stadium. Nijhout

FIGURE 21.16. Course of development in Zooter-mopsis angusticollis (Isoptera). Broken arrow indicates the potential for several molts. Abbreviations: L1-L5, first to fifthinstar larvae; 1N, 2N, first-and second-stage nymphs; PR, primary reproductive; SR, supplementary reproductive; PSol, presoldier; Sol, soldier. [After C.-M. Yin and C. Gillott, 1975, Endocrine activity during caste differentiation in Zootermopsis angusticollis Hagen (Isoptera): A morphometric and autoradiographic study, Can. J. Zool. 53:1690-1700. By permission of the National Research Council of Canada.]

FIGURE 21.16. Course of development in Zooter-mopsis angusticollis (Isoptera). Broken arrow indicates the potential for several molts. Abbreviations: L1-L5, first to fifthinstar larvae; 1N, 2N, first-and second-stage nymphs; PR, primary reproductive; SR, supplementary reproductive; PSol, presoldier; Sol, soldier. [After C.-M. Yin and C. Gillott, 1975, Endocrine activity during caste differentiation in Zootermopsis angusticollis Hagen (Isoptera): A morphometric and autoradiographic study, Can. J. Zool. 53:1690-1700. By permission of the National Research Council of Canada.]

and Wheeler (1982) presented a model for JH-mediated control of caste differentiation in termites (Figure 21.17). Their proposal includes three critical periods in which JH is influential: the first of these is when sexual characters are determined; in the second period the JH level determines whether or not adult non-sexual characters differentiate; and in the third soldier characters are regulated. It must be stressed that the JH levels presented in the model are hypothetical; that is, they have never been measured experimentally.

In contrast to the situation in aphids and termites, whose different forms are clearly distinct (discontinuous polymorphism), in some locusts a spectrum of slightly different forms (continuous polymorphism) can be obtained by varying the population density over a number of generations. At opposite ends of the spectrum are the solitary form (phase),

FIGURE 21.17. Proposed changes in hemolymph juvenile hormone level during caste differentiation in lower termites. In the larval stadium there are three critical periods when hormone level determines the path of differentiation to be followed. [From Nijhout, H. F., and Wheeler, D. E., 1982, Juvenile hormone and the physiological basis of insect polymorphisms, Quart. Rev. Biol. 57:109-133. By permission of University of Chicago Press.]

FIGURE 21.17. Proposed changes in hemolymph juvenile hormone level during caste differentiation in lower termites. In the larval stadium there are three critical periods when hormone level determines the path of differentiation to be followed. [From Nijhout, H. F., and Wheeler, D. E., 1982, Juvenile hormone and the physiological basis of insect polymorphisms, Quart. Rev. Biol. 57:109-133. By permission of University of Chicago Press.]

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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