Endocrine Regulation of Development

Postembryonic development is controlled by three endocrine centers: the brain-corpora cardiaca complex, corpora allata, and molt glands (see Chapter 13, Section 3, for a description of their structure and products). A molt cycle is initiated when, as a result of appropriate signals (see Section 6.2), the median neurosecretory cells of the brain release ecdysiotropin [prothoracicotropic hormone (PTTH)], which stimulates molting hormone (ecdysone) (MH) production by the molt glands. In all insects studied there is a major peak of MH in the hemolymph during the second half of each molt cycle, and it is this MH surge [in reality, the MH is first converted to the biologically active form, 20-hydroxyecdysone (20-HE), probably at its target site] that initiates the various physiological events constituting a molt cycle (Figure 21.13). In addition, in the final larval instar of holometabolous species one or more smaller hemolymph MH peaks precede the major peak and are thought to be responsible for reprogramming tissues for pupal rather than larval syntheses.

The corpora allata produce juvenile hormone (JH). The regulation of corpus allatum activity is complex (Tobe and Stay, 1985). Both allatotropic (corpus allatum-stimulating) and allatostatic (corpus allatum-inhibiting) neurosecretory factors have been reported, while in some insects the brain exerts direct neural control over the gland. It should also be noted that the hemolymph contains highly active esterases with the potential for degrading free JH. Thus, the concentration of circulating JH is determined not only by the secretory activity of the corpora allata but also by these hemolymph esterases.

JH can exert an influence on development only in the presence of MH, that is, after a molting cycle has begun. It is the concentration of circulating JH during one or more critical periods of the stadium that determines the nature of the succeeding molt. If the concentration of JH is above a threshold value* during the critical period, the next molt will be larval-larval (for this reason, JH has been described as the "status quo" hormone). When there is little or no circulating JH, an adult will appear at the next molt (Figure 21.13).

This scenario applies to hemimetabolous insects, which have a single critical period during each stadium and, of course, lack a pupal instar. In holometabolous insects there are two critical periods in the last larval stadium. In the first the absence of JH programs the development of pupal characters. In the second, which is just before the larval-pupal molt, a sharp increase in JH concentration occurs that prevents premature differentiation of imaginal discs. In the pupal stadium the absence of JH in the critical period permits the expression of adult characters. In species that show phenotypic polymorphism (Section 7) there may be several extra JH-sensitive critical periods for expression of the various forms (Nijhout, 1994).

* The absolute concentration appears unimportant, provided that it is above or below a threshold range. However, the threshold range will vary among species.

FIGURE 21.13. Schematic comparison of endocrine control of development in hemimetabolous and holometabolous insects. Pulses of prothoracicotropic hormone (PTTH) trigger synthesis and release of MH. Levels of JH determine the nature of the molt: when JH is present during a critical period, a larval-larval molt occurs; if no JH is present, the next molt will be larval-adult (Hemimetabola), or larval-pupal or pupal-adult (Holometabola). In the final larval stage of Holometabola there are two critical periods: in the first (no JH present) the switch to pupal development occurs; in the second, there is a pulse of JH that prevents premature differentiation of imaginal discs. Just prior to, or immediately after, each molt, release of eclosion hormone (EH) or bursicon (B), respectively, occurs (see also Figure 21.14). JH and MH levels are not drawn to the same scale. Numbers on the horizontal axes indicate the percent duration of each stadium. Other abbreviations: F-1, penultimate larval stadium; F-2, antepenultimate larval stadium. [Original, based on data from several sources.]

FIGURE 21.13. Schematic comparison of endocrine control of development in hemimetabolous and holometabolous insects. Pulses of prothoracicotropic hormone (PTTH) trigger synthesis and release of MH. Levels of JH determine the nature of the molt: when JH is present during a critical period, a larval-larval molt occurs; if no JH is present, the next molt will be larval-adult (Hemimetabola), or larval-pupal or pupal-adult (Holometabola). In the final larval stage of Holometabola there are two critical periods: in the first (no JH present) the switch to pupal development occurs; in the second, there is a pulse of JH that prevents premature differentiation of imaginal discs. Just prior to, or immediately after, each molt, release of eclosion hormone (EH) or bursicon (B), respectively, occurs (see also Figure 21.14). JH and MH levels are not drawn to the same scale. Numbers on the horizontal axes indicate the percent duration of each stadium. Other abbreviations: F-1, penultimate larval stadium; F-2, antepenultimate larval stadium. [Original, based on data from several sources.]

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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