The adult house fly has a life span of about 15 to 25 days, but may live as long as two months. That fly develops from an egg to an actual fly in no less than eight days. The female lays her eggs on or close to rotting organic matter — manure mostly, but rotting vegetation also makes an appealing breeding site. Those eggs hatch into larvae, which feed and pupate, forming a cocoon, inside which they turn into adult flies. There’s no such thing as a “baby” fly. Little flies don’t grow up to be big flies; once they have wings, they don’t get any bigger.