If all the descendants of one housefly were allowed to lives in a single summer we might have many, many tons of flies. Of courses this isn’t possible, for almost all of a fly's eggs either fail to hatch and find enough food, or are soon eaten by birds and other small creatures. Also, a good number of them are destroyed by human beings. A mother housefly lays about 500 eggs at a time. If the eggs are in a fairly warm spot and the young have plenty to eat, they would be grown up and rowdy to lay their eggs in about a week. If half of this batch were females, then that would be about 125000 new flies all grown up by the end of another week
At this rates if all the flies lived that one mother fly that escaped the swatter in April could have 191 billion billion descendants by August 1st. So, since we all know that the housefly is rated as a pests then the thing to do is to swat as many as we can before they get a chance to swamp us with their children.