Corpora Allata

Typically the corpora allata are seen as a pair of spherical bodies lying one on each side of the gut, behind the brain (Figure 13.4A,B). However, in some species, the glands may be fused in a middorsal position above the aorta, or each gland may fuse with the corpus cardiacum on the same side. In larvae of cyclorrhaph Diptera the corpora allata, corpora cardiaca, and molt glands fuse to form a composite structure, Weismann's ring, which surrounds the aorta. Each gland receives a nerve (NCA I) from the corpus cardiacum on its own side, though the axons that form this nerve are probably those of mNSC, and also a nerve from the subesophageal ganglion (NCA II).

The corpora allata produce a hormone known variously as juvenile hormone, metamorphosis-inhibiting hormone, or neotenin, with reference to its function in juvenile insects (Chapter 21, Section 6.1), and gonadotropic hormone to indicate its function in adults (Chapter 19, Sections 3.1.3 and 3.2). Juvenile hormone is a terpenoid compound (Figure 13.6A) and, to date, six naturally occurring forms (JH-O, JH-I, 4-methyl-JH-I, JH-II, JH-III, and JHB3) have been identified. In all insects investigated, except Hemiptera, Lepidoptera, and higher Diptera, only JH-III has been obtained. Though JH-III is reputedly synthesized in some Hemiptera, Numata et al. (1992) report that JH-I is the only form produced in the bean bug, Riptortus clavatus. In Lepidoptera the first five forms of JH listed

* In many Lepidoptera, including Bombyx, egg development begins in the pupa.

above occur, with one or two forms predominating at specific stages in the life history. In cyclorrhaph Diptera JHB3 is the principal or sole juvenile hormone, with JH-III occurring as a minor component in some species (Lefevere et al., 1993). In addition, a large number of related compounds have been shown to exert juvenilizing and/or gonadotropic effects. Currently, there is much interest in such compounds in view of their potential use in pest control (Chapter 24, Section 4.2).

3.3. Molt Glands

The paired molt glands generally comprise two strips of tissue, frequently branched, which are interwoven among the tracheae, fat body, muscles, and connective tissue of the head and anterior thorax. In accord with their varied position, they have been called prothoracic glands, ventral head glands, and tentorial glands, though these structures are homologous. Except in primitive apterygotes, solitary locusts (Chapter 21, Section 7), and, apparently, worker and soldier termites, the glands are found only in juvenile insects and degenerate shortly after the molt to the adult. The molt glands show distinct cycles of activity correlated with new cuticle formation and ecdysis. Their product, a-ecdysone, is a prohormone that when activated initiates several important events in this regard (see Chapter 11, Section 3.4 and Chapter 21, Section 6.1). Typically the active form is 20-hydroxyecdysone (= p-ecdysone = ecdysterone) though a large number of similar molecules with biological activity have been isolated. Ecdysones are steroids (Figure 13.6B) having the same carbon skeleton as cholesterol, which is almost certainly the natural precursor in insects.

3.4. Other Endocrine Structures

A variety of structures in insects have been proposed as endocrine glands at one time or another.

The oenocytes, which become active early in the molt cycle and again at the onset of sexual maturity in adult females, show ultrastructural similarities with steroid-producing cells in vertebrates (Locke, 1969; Romer, 1991). This has led to the suggestion that these cells may be a site for ecdysone synthesis, and a few biochemical studies support this idea (Romer, 1991; Romer and Bressel, 1994). However, their primary roles appear to be the synthesis of certain cuticular lipids and the lipoprotein layer of the epicuticle (Chapter 11, Section 2).

In contrast to those of vertebrates, the gonads of insects do not produce sex hormones that influence the development of secondary sexual characters. A large number of experiments in which insects were castrated could be cited to support this statement. The only exception to this generalization is found in the firefly Lampyris noctiluca (Coleoptera) where an androgenic hormone produced by the testes induces the development of male sexual characters. Thus, implantation of these organs into a female larva causes sex reversal. Ovarian tissue, however, does not produce a hormone for induction of femaleness (Naisse, 1969).

The ovaries of many insects do, however, produce hormones that affect reproductive development. Adams and others (see Adams, 1980, 1981) have demonstrated that the maturing ovaries of the house fly, Musca domestica, and other Diptera produce an oostatic hormone that regulates the pattern of egg maturation by inhibiting the release of OEH from the mNSC.In the absence of OEH, ovarian ecdysone release (see below and Chapter 19, Section 3.1.1) does not occur. After oviposition, the ovaries no longer produce oostatic hormone, and a new cycle of egg maturation begins. In contrast, the antigonadotropin produced by the abdominal perisympathetic neurosecretory organs in the bug Rhodnius prolixus does not act on other endocrine centers. Rather, it appears to act at the level of the follicle cells, blocking the action of juvenile hormone (Chapter 19, Section 3.1.1) (Davey and Kuster, 1981).

Reports of the existence of ecdysones in adult female insects, published in the 1950s, were largely ignored (after all, the molt glands that produce them were known to degenerate at the end of larval life), and it was not until the 1970s that the ovary was shown to be a major producer of these hormones in insects from a variety of orders (e.g., locusts, crickets, termites, mosquitoes and other Diptera, and several Lepidoptera). In some Diptera a clear role for ecdysone in vitellogenesis has been established (Chapter 19, Section 3.1.3) while in locusts the ecdysone largely accumulates in maturing eggs to be used later in the regulation of embryonic molting (Chapter 20, Section 7.2). For other species, its function remains unclear.

Evidence from a number of species indicates that the testes may also produce ecdys-teroids. In the moths Lymantria dispar and Heliothis virescens testis ecdysiotropin stimulates the testis sheaths to produce ecdysteroids. In turn these trigger release of testicular factors that promote growth and development of the reproductive tract (Loeb et al., 1996). Ecdysteroid production by testes, as well as by the male accessory glands and abdominal integument has also been reported in other moths, crickets, and grasshoppers (Gillott and Ismail, 1995, and references therein). However, except for a possible role in spermatogenesis (Chapter 19, Section 3.2), the function of ecdysteroids in these insects remains uncertain.

An endocrine role for the Inka cells within the epitracheal glands was originally described in Manduca sexta (Zitnan et al., 1996). However, it has recently been confirmed that these cells occur in representatives of all major insect orders, producing pre-ecdysis-and ecdysis-triggering hormones (PETH and ETH) at the end of each developmental stage (Zitnan et al., 2003) (Chapter 21, Section 6.2). InM. sexta there are nine pairs of segmentally arranged epitracheal glands, each attached to a large trachea immediately adjacent to a spiracle and containing a single Inka cell. However, in most insects, the Inka cells are numerous and scattered throughout the tracheal system. Both PETH and ETH are peptides, the latter composed of 26 amino acids in M. sexta.

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