Re: What exactly happens during metamorphisis of a butterfly?

Well Heather, this is no small question. Not even the most knowledgeable of insect physiologists are certain about every aspect of the process. But we do know quite a bit.

Metamorphosis is quite distinct among various orders of insects. The Lepidoptera, or butterflies, are holometabolous insects--that is, their transformation is considered "complete." This is somewhat obvious, as a caterpillar looks nothing like an adult butterfly. However, other insects, such as roaches and the "true" bugs (stink bugs, for example), undergo a gradual process of metamorphosis that is not unlike mammalian growth, at least in appearance. These insects are said to be hemimetabolous. There are other forms of metamorphosis in addition to these two common forms.

Even among the holometabolous insects, metamorphosis is controlled via various methods from order to order, but in general, two neuropeptides control the process: Juvenile Hormone (JH) and Ecdysone. A number of environmental stimuli, particularly daylength, dictate the production of these hormones. Changes in daylength that occur in concert with the changing seasons reslut in changing concentrations and proportions of JH and Ecdysone.

Certain concentrations of Juvenile Hormone and Ecdysone act on two main structures: the epidermis and the imaginal disks. Imaginal disks are discrete tissues within the larval insect which give rise to specific adult structures such as eyes, legs, and antennae. The epidermis and the imaginal disks are controlled by JH and Ecdysone in three general ways:

• High amounts of JH in the presence of Ecdysone results in the maintainance of larval epidermal cells and elicits no change in the imaginal disks.
• Moderate amounts of JH in the presence of Ecdysone (ie, a drop in JH) results in pupation--the formation of a crysalis.
• If only Ecdysone is present (ie, JH production ends), the epidermal cells are programmed for death, the imaginal disk cells proliferate and create adult structures, and the pupa becomes an adult. (In the butterflies, the adult cuticle arises from the lysed tissue of the larval epidermal cells, unlike other insects in which the adult cuticle arises from imaginal disks).

When the adult butterfly is fully formed and ready to emerge, Eclosion Hormone allows the butterly to escape the pupal casing.

Please realize that this is an extremely brief description of metamorphosis. Many other horomes are involved. This description is meant merely to be an overview. For a more thourough treatment of this subject, the classic text The Insects: Structure and Function by R.F. Chapman is indespensable. Unfortunately, this volume is currently out of print because Chapman is revising it. The new edition should be available in early 1997. However, you should certainly be able to find a copy of the most recent edtion.

Hope this helps. Good luck.