Postembryonic Development

brain-corpora cardiaca complex via the stomatogastric nervous system (Clarke and Langley, 643

In continuous feeders it is often necessary for an insect to achieve a minimal nutritional status or body weight before a new molt cycle is initiated. This may be especially important for species whose diet is variable. In other words, during each stadium there is an initial period of obligate feeding, which results in acquisition of the minimal nutritive requirements (and release of sufficient PTTH to trigger a molt cycle), followed by a phase of facultative feeding, the nutritive contribution from which gives rise to larger larvae.

Photoperiod is another important environmental factor in the regulation of growth and molting, particularly in relation to diapause, a more or less prolonged condition of arrested development, which enables insects to survive periods of adverse conditions (Chapter 22, Section 3.2). Members of most species studied enter diapause when the daily amount of light to which they are exposed falls below a certain value (usually 14-16 hours). In diapause, an insect is physiologically "turned-off"; generally, it does not feed or move actively, and its metabolic rate is abnormally low. These effects result from inactivity of the endocrine system. In a manner that is not clear, short day lengths lead to reduced neurosecretory activity that, in turn, results in inactivity of molt glands and corpora allata. Conversely, diapause is terminated as the day length increases beyond a certain point in spring, because of renewed endocrine activity. In members of some species, however, the neurosecretory system must be exposed to low temperatures for a critical length of time during diapause before it can respond to increasing day length (Chippendale, 1977).

In some insects, the "feel" of the surroundings is important for continued normal development. For example, larvae of the wheatstem sawfly, Cephus cinctus, will not pupate if removed from the cavity at the base of the stem. Larvae of the squash fly, Zeugoducus depressus, live in the cavity of squash where the carbon dioxide concentration is initially about 4% to 6%. Pupation is delayed by this concentration of gas and will not occur until the level falls to about 1%, some 6 months later. This delay serves to synchronize the emergence of adult flies with the opening of the squash flowers (in which eggs are laid) the following season.

Still relatively unexplored are the changes of endocrine activity and other events that bring a molting cycle to a close with the shedding of the old cuticle. Certainly negative feedback pathways exist so that when the concentration of circulating hormone reaches a critical level, the activity of the gland producing it is depressed. The pathway may be direct, that is, the hormone itself depresses glandular activity. Alternatively, circulating ecdysone and JH may inhibit the activity of the PTTH- and allatotropic hormone-producing cells, respectively. A third possibility is that hormone levels are monitored by chemoreceptors, which send the information via sensory neurons to the brain. Reduction in activity of the molt glands and/or corpora allata might then be brought about via the nerves to the glands.

The complex behaviors that enable an insect to escape from the old exuvium are coordinated by the interplay of several hormones, including 20-HE, eclosion hormone (EH), pre-ecdysis-triggering hormone (PETH), ecdysis-triggering hormone (ETH), crustacean cardioactive peptide (CCAP), and bursicon (Truman, 1985, 1990, 1992; Reynolds, 1986; Horodyski, 1996; Myers, 2003). Only when the 20-HE level falls below a threshold value can molting occur, for two reasons. First, the target tissues for EH only acquire their competence to respond and, second, EH release only occurs at very low 20-HE concentrations. EH is produced in the Chinese oak silk moth, Antheraea pernyi, and other saturniid moths by neurosecretory cells in the ventral part of the brain. In larval instars these cells release EH at neurohemal organs on the hindgut; however, in the pharate adult the neurosecretory cells

FIGURE 21.14. Scheme for the hormonal control of molting. Abbreviations: CCAP, crustacean cardioactive peptide; EH, eclosion hormone; ETH, ecdysis-triggering hormone; PETH, pre-ecdysis-triggering hormone. [After R. F. Chapman, 1998, The Insects: Structure and Function (4th ed.). Reprinted with the permission of Cambridge University Press.]

FIGURE 21.14. Scheme for the hormonal control of molting. Abbreviations: CCAP, crustacean cardioactive peptide; EH, eclosion hormone; ETH, ecdysis-triggering hormone; PETH, pre-ecdysis-triggering hormone. [After R. F. Chapman, 1998, The Insects: Structure and Function (4th ed.). Reprinted with the permission of Cambridge University Press.]

are restructured and terminate in the corpora cardiaca. For about 25 years after its discovery in the early 1970s, EH was thought to be the only hormone involved in the initiation of the various behaviors and physiological events that encompass the shedding of the exuvium. However, it is now apparent that release of EH is but one step in a complex hormonal cascade (ZMan et al., 1996; ZMan and Adams, 2000).

ZMan and Adams (2000) have proposed the following model for the control of molting, which includes three phases: pre-ecdysis I, pre-ecdysis II, and ecdysis (Figure 21.14). The behavioral sequence is initiated when low levels of EH are released. The EH induces PETH and ETH secretion from the Inka cells (Chapter 13, Section 3.4). PETH acts on the abdominal ganglia to trigger pre-ecdysis I; that is, it causes motor neurons to fire, bringing about strong dorsoventral muscle contractions in the body wall. ETH has two effects. First, by positive feedback it causes further, massive release of EH (and, in turn, more ETH), levels of which peak about 1 hour before molting occurs. Second, it induces (also by acting on the abdominal ganglia) pre-ecdysis II (strong posterioventral and proleg muscle contractions). The high level of EH has both direct and indirect effects related to molting. First, EH released from the tips of neurosecretory axons within the abdominal ganglia stimulates other neurons to release CCAP, which controls the initiation of the third phase, ecdysis. CCAP switches off pre-ecdysis I and II, then activates motor neurons that control the swallowing of air, heartbeat, and skeletal muscle functions. Together, these actions lead to the splitting of the old cuticle, wriggling free, and expansion of the new cuticle and wings. In addition, EH released into the hemolymph causes parts of the cuticle to plasticize, stimulates the cement-producing (Verson's) glands to discharge, and is a signal for release of bursicon. As described in Chapter 11 (Section 3.4), bursicon has important roles in tanning of the cuticle. It also appears to be involved in cuticle plasticization and the degeneration of specific muscles and neurons. Other processes in which bursicon may play a role include postecdysial cuticle deposition, postecdysial diuresis, and tracheal air-filling (Reynolds, 1986). In some insects that must wriggle to the surface of the substrate in which they pupated, proprioceptive stimuli are also important. These inhibit early release of bursicon, so that premature tanning of the adult cuticle is avoided.

The above model is based largely on data from experiments with Lepidoptera, especially Manduca sexta. However, both EH and ETH have been shown to occur in larvae from a wide range of insect orders (Truman et al., 1981; ZMan et al., 2003), suggesting that this mechanism for hormonal control of ecdysis has been conserved across the Class.

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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