Insects - Hormones in Growth and Reproduction
In insects hormones regulate moulting and metamorphosis. The larvae or nymphs which hatch out of the eggs undergo regular moulting that is followed by growth, as in crustaceans and ultimately they become adults. The change in form from larva to adult is termed as metamorphosis. In insects such as cockroach, grasshopper etc., (hemimetabolous insects), the change in form is gradual. Though in some other insects like moths, butterflies, houseflies etc., (holometabolous insects), the change is much more conspicuous during the later period of the life history and the adult that emerges from the pupa is quite different. As in crustaceans, the hormone which brings about moulting in these animals is as well ecdysone but in insects it is secreted through prothoracic glands. How is metamorphosis in insects brought about? In insects the hormone that is accountable for preventing the animal from metamorphosing is juvenile hormone secreted by the corpus allatum. Actually, as the name implies, juvenile hormone keeps the insect juvenile. Thus in effect it inhibits metamorphosis. We have previously seen that the prothoracic glands in immature insects secrete ecdysone. This hormone causes the insect to moult. As long as the larva moults in the existence of high titres of juvenile hormone, it moults into another larva. If the moult occurs when the titres of JH are low in the blood, it results in a pupa. Finally when there is no JH circulating in the blood, the pupa moults into adult. So the concentration of JH in blood determines the type of resulting individual. It is as well known that JH causes the repression of those genes that are accountable for adult differentiation.