Moulting stages

Greenaway, P. Calcium balance at the past moult stage of the freshwater crayfish Austro-potamoblus pallipes. J. exp. Biol. 61, 35 (1974)... 

Administration of Tyv-specific monoclonal antibodies to rat pups already infected with intestinal larvae causes larvae in the epithelium to be expelled (Carlisle et al, 1990). Only the LI stage is susceptible to expulsion once the larva has moulted to L2 it resists the effects of the antibodies (Carlisle et al, 1990). Expulsive immunity is transferred by three IgG isotypes, F(ab )2 fragments, as well as IgM (Carlisle et al., 1991a). These findings argue against a role for Fc-mediated effector functions and imply that antibodies against Tyv can disturb the larva s niche in a direct fashion. 

The newly synthesized cuticle is highly folded and convoluted and thus allows rapid growth after the moult. Certain stages of some parasitic nematodes retain the old cuticle as a protective sheath. 

Juvenile hormone plays a critical role in maintaining the juvenile or larval stage of insects, and if its secretion is not controlled, normal development to the adult stage is prevented. Use of hormones or substances with hormonelike activity to control insect populations is an area of intense research interest and activity.2 The secretion of juvenile hormone is controlled by other hormones originating in the brain (brain hormone) and the phthoracic gland (moulting hormone, ecdysone see Table 30-2). 

Systemic, ingested insecticide active by root application. Interferes with moulting affecting larval stages of insects. Also reduces fecundity in treated adults and have some ovicidal properties... 

Nonneurotoxic insect growth regulator with contact and stomach action. Exhibits a strong juvenile hormone activey, inhibiting metamorphosis to the adult stage and interfere with the moulting of early instar larvae... 

The two hormones regulate each step of development from embryo through larva or nymph to adult, and via pupa, where this is formed. At each stage the moulting hormone is produced when the old cuticle is cast off and a new one is formed. The presence of JH ensures that the new stage is also juvenile. A more complex action of JH allows the larva to develop into a pupa, and the absence of JH permits the formation of the adult form. The function of neither hormone ends there, for both are produced again in the adult insect, with other effects as described in a later discussion. 

Ecdysone is a steroid that is formed and secreted by the prothoracic glands of immature insects but also by the ovaries of adult female insects. It was first isolated from the silk worm, Bombix mori. Bedstone is the prohormone of the moulting hormone eedysterone, but the prohormone has also effects on its own at other stages of insect development. 

Test insects should be of the same sex. at the same stage in the life cycle and same slate of nutrition. In stars likely to mature should be tested soon after moulting to avoid further changes before the assessment of modality, the timing of which should be chosen to be at ihe point of maximum effect. 

It is concluded that brassinosteroids applied to insects with food may be resorbed and may have effects. Apparently the effects depend on the species as well as the dose and type of brassinosteroid applied. Selective metabolism of brassinosteroids in the gut cannot be excluded. The overall effect of an active brassinosteroid is a lengthening of the larval stage by a delay of moulting. 

Our present meagre understanding of normal endocrine processes in invertebrates makes the assessment of chemical endocrine disruption in the field difficult (LeBlanc 1999). Steroid roles differ between species and sometimes sexes, and their influence may vary at different developmental stages. In most studies of invertebrates, endocrine disruption appears to involve androgenization rather than oestrogenic effects (see Box 7.2). Arthropods (crustaceans and insects), annelids and molluscs use ecdysteroids, terpenoids and vertebratelike sex steroids for endocrine control. For example, the ecdysteroid ecdysone is naturally converted to 20-hydroxyecdysone (Fig. 7.10), which induces moulting (ecdysis) in both insect larvae and crustaceans. 

In insects, ecdysone is secreted from the prothoracic glands (PG) at every moulting and pupal stage. Conversion of ecdysone to 20-hydroxyecdysone occurred in fat body, Malpighian tubules, gut and body wall tissues [189]. Although ecdysone may have direct hormonal effects, it is generally held that ecdysone serves as a prehormone that is converted to 20-hydroxyecdysone which functions as the active hormone. This is because in many bioassay systems, 20-hydroxyecdysone appears to be much more active than ecdysone. 

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