Why do Bats hang upside down?

Bats leg arrangements are not the same as birds. The thin membrane of a bats wing stretches from its elongated fingers to its legs, and is then attached to the sides of its body. Most bats also have a large membrane connecting both legs, which acts like a pouch and is used to capture insects. This means the legs of a bat are not free to do their own thing, but are tangled in wing and tail membrane.

This has meant that bats have not been able to develop the many different types of legs we see in birds – long legs, short legs etc. Anyhow the evolutionary upshot of all this is that for bats it appears the easiest way for them to stow away the membrane is to hang upside down.