Flies are around all year, hiding from the cold during the depths of winter which is why we do not see them very much....can you blame them. People often wonder why fly problems seem to lessen as we approach the winter months, this is because the drop in temperature affects their breeding, development and lifecycle, reducing their activity levels considerably. Flies go through complete metamorphosis, egg, larva, pupa and adult. During winter many species of fly remain simply inactive, hiding out on the warmer west or south sides of buildings in their immature state.
Years ago people thought that flies died off as winter approached, but if they all died, how can they reappear in the spring, they would have to somehow spontaneously regenerate themselves from nothing....not scientifically possible! What they actually do is seek sanctuary during our darkest and coldest months somewhere warm, inside a building or on its warmest side, preferring to remain inactive and avoid any activity during heavy rain, strong winds and temperatures below 15 degrees centigrade.
When spring comes and the temperature rises, they get active again. Quickly heading for the outside and into the fresh air to get going. Sometimes where they overwinter in large numbers, like cluster flies in a church bell tower, when they all wake up and head outside, the sheer numbers of them on the move can be quite a shock to many people who were not aware of their presence previously.