Corpus Allatum and Juvenile Hormone
The corpora allata are rounded glands attached to the posterior side of every corpus cardiacum making a compact body just behind the brain. In a few (Hemiptera, higher Diptera) they are fused to make a single median structure. Experiments by Wigglesworth have shown that the CA secretes a hormone that determines the character of each larval instar by limiting the degree of differentiation in the direction of the development of the adult. He named this hormone like juvenile hormone (JH). This is also known as neotenic (youth substance), (Wigglesworth 1954) and gonadotropic hormone (Engelmann 1957). The juvenile hormone is identical in structure to terpenes. A large number of compounds with juvenile hormone activity have been isolated and several have been synthesized. Some synthetic ones appear much more potent than the naturally occurring ones. The juvenile hormones make sure the occurrence of larval moults and inhibit metamorphosis. Its existence or nonexistence determines whether the larva will moult into a larval/pupal stage or into adult stage. The main natural hormone isolated from the adult male Hyalophora cecropia moth has the following structure Figure.
Figure: Juvenile hormone isolated from Cecropia moth
The role of the corpus allatum and the juvenile hormones in metamorphosis has been proved via various experiments. It has been observed that if an extra corpora allata from a second stage larva is transplanted to the fourth instar last larval instar as compared to the moult does not generate a pupa or adult, instead an oversized larva develops. Removal of corpora allata early in larval life results in premature metamorphosis.