Why fly in summer?

Question

Why do flies only come out when it's hot? Where do they go in the Winter?

Answer 1

Some flies spend the winter in a dormant state in some sheltered place where they hopefully won't freeze or get eaten. Unfortunately I can't tell you more than that, although many arthropods will either overwinter as adults (then die after laying eggs in the spring) or as eggs or larvae (and then grow up starting in the spring). I would expect flies to show those two main strategies as well.