[robbiep]

I don't know exactly what happens but apparently they mostly turn to mush then reassemble. Interestingly their nervous system/memories have been shown to survive between the different stages.

Re genes turned off: as another poster mentioned huge numbers Of genes are switched off and on at different stages in our life cycle. It's kind of the same as saying 'but neurone don't need the same genes as skin cells so isn't it wasteful having all those extra genes doing nothing?' DNA is relatively cheap so while some 'skin' genes may never Be used in 'neuron' cells the cost is not prohibitive To the organism.

Http://bit.ly/VNYxx2 (a link to livescience.com - apologies am moving between different devices, blocked HN on my mac!)

[polyfractal]

A lot of insects have a fully intact, if immature, nervous system during their adolescence. For example, fruit flies (drosophilla) have brain features called "mushroom bodies". These grow during larval development then hit a state of hibernation until the larva changes into an adult fly, at which point growth continues and the mushroom body fully develops.

I'm not familiar with a lot of other insects, but I believe it is fairly common for the bulk of structures to form, hibernate, and then finish growing during metamorphosis.