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We were studying butterflies in class and I asked my 7th grade teacher, but she deemed the concept "too complicated." I'm really curious now, not only about butterflies but insects in general...

Thank you!

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Could you add some detail to your question? To start, what species(s) are you interested in? –  kmm Aug 21 '13 at 20:11

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It depends of what order and even what family of insects we are talking about. Insects may have complete or incomplete metamorphosis, being the complete more complicated. Even inside the first group (wich in fact contains most of the insects since coleoptera, diptera, lepidoptera and himenoptera have complete metamorphosis) there are important differences. However, I'll try to explain some basics.

First of all, metamorphosis is a molting-related process. It's regulated by the same hormones and they share many characteristics. In insects with uncomplete metamorphosis, this is very visible, since the "metamorphosis" is the last molt, wich usually involves growth, sexual maturity and the development of functional wings.

The choice between a regular larval molt is determined primarily by the levels of juvenil hormone, wich decays during the last larval instar (pupae and adult are instars, too). The molting is triggered by a peak of ecdysone, an steroid.

Once the insect has entered into the state of pupae, the organs start to suffer diverse changes. The formation of adult organs de novo arise from some special structures called imaginal discs, wich are clusters of stem cells that are more or less isolated in the larvae.

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Position of the imaginal discs in Drosophila. Source.

Note that diptera larvae lack legs, mandibles and even head, while coleptera larvae usually have true legs, a set of mandibles and many other adult features.

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A typical larva of a coleoptera of the genus Lucanus. Note the mandibles and the legs. Source.

Some other organs remain more or less the same, such as the hearts or many parts of the nervous system. On the other side, some larval organs needs to dissapear and suffer programmed cell death. The gut, many parts of the nervous system, between others, suffer modifications that may be more or less drastic. In many cases this happen by simple growth and differenciation of the tissue or degeneration by apoptosis, but in some others the whole organs suffers apoptosis and regenerates itself with adult characteristics. For example, the innervation of the abdominal parapods (false legs) in caterpillars suffer a progressive degeneration, while the opposite happens in toracic nerves, wich will have to control legs and wings. This whole process is pretty similar to embrionic development and it shares many traits with it.

Finally, metamorphosis in other animals, such as crustaceans, vertabrates (fishes and amphibians), echinodermae, annelidae, cnidaria and many others will probably vary widely. I know for sure that amphibians metamorphosis is regulated by tiroid hormones, but I'm not even sure that many of the other examples have even been studied at a phisiological level.

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