Moulting control

Bryant Is that moulting controlled by an ecdysone-like hormone ... 

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). 

A stock solution was prepared by dissolving 1.25 mg of each extract in 1 ml of solvent (5 % DMSO in acetone for the hexane, chloroform and ethyl acetate extracts, and water for the methanol extract) to obtain concentrations of 1250 ppm. The stock solution was diluted to 25, 12.5, 6.25, 2.5 and 1.25 ppm. Freshly moulted late 3" -instar Anopheles gambiae larvae (20) were released into each test beaker containing 100 ml of distilled water treated with the respective sample concentration. In the control experiments, the larvae were exposed to 2.0, 1.0, 0.5, 0.2 and 0.1 ml acetone diluted to 100 ml with distilled water. Fish food was provided for the larvae, and the test beakers were placed in a room maintained at 26°C and with a photoperiod of 12 hours light and 12 hours darkness. Mortality of the larvae was monitored after every 24 hours until the death of the last larva or pupa or their emergence into adults. The experiment was replicated 5 times for each concentration. Mortality was calculated according to the formula % mortality = lOOY/Z, where Z = initial number of larvae introduced into each test beaker and Y = mean death defined by the difference between the mean test deaths and the mean control deaths. 

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 1917 the Polish biologist Kopec tied up the middle part of a caterpillar with a thin thread. The head section of the caterpillar moulted and pupated normally, but its hind end remained an unaltered caterpillar (Kopec, 1922). He concluded from this that the moulting and pupation of insects is regulated by a brain-controlled hormone. 

For many years, scientists were aware that peptide factors from the eyestalks of decapod crustaceans regulated glucose metabolism, moulting and female reproduction. It was only much later when these peptides were isolated from the XO-SG complex and structurally elucidated, that it became apparent that the peptides controlling these diverse processes were structurally homologous to each other and, hence, warranted inclusion as members of one peptide family. 

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