Flies

I think they die out after they've laid their eggs, then the eggs hatch when it's warmer, I'm not sure though.

Some die out, others find nooks and crannies in houses or warm places and hibernate the winter away. There are maggots at all times of the year usually unless it is cold enough to freeze rubbish in which case the flies are sleeping.

I think Dory is probably right. They would die out when it gets cooler because they don't like the cold, and they have already buried their eggs, or whatever it is that they do with them. But the eggs are probably hidden, hang on, what are baby flies? Are they maggots? Because they come out in summer too so that must be right. The eggs get laid in the summer and then the mummy and daddy flies die out and then the maggots turn into grown ups in the summer.