I thought about this. The word ‘fly’ can apply to a lot of insects with wings but she was obviously referring to the common house fly. They are one of the most widely distributed animals in the world, found practically everywhere there are higher forms of life. But it’s believed they evolved in the middle east and spread with humans as our species colonised the globe. This sub-tropical origin means they are not equipped to cope with an Irish winter.
The life-cycle of the house fly is quite typical for an insect. Eggs are laid in decaying organic matter. This is most commonly of vegetable origin, unlike the eggs of blue bottles which are normally laid in decaying animal matter. The eggs hatch into larvae, which are commonly called maggots. The maggots go through three instars, or moults, getting bigger each time. They then pupate inside a hard, reddish brown shell before hatching into adult flies with wings.
These don’t have a great life expectancy — 20 to 30 days if they’re not eaten by a spider or a bird, though they will live longer in controlled laboratory conditions. A female, after a single mating with a male, can lay up to 9,000 eggs in batches of 100 to 150. They literally breed like flies.
They have evolved two separate strategies for surviving through a winter. The first is connected to the fact that the maggots live in decaying organic matter. When we say that it’s decaying we mean that it’s in the process of being broken down into simpler compounds by a number of organisms, of which various species of fungi are usually the most important. The chemical changes that take place produce heat as a by-product. That’s why compost heaps steam and why if you brew beer or ferment wine the container gets warm. So maggots live, feed and eventually pupate in an environment that’s several degrees warmer than the ambient temperature.
The second strategy is called diapause. It’s very similar to hibernation, though not exactly the same. House flies at any stage in their life cycle can go into a state of suspended animation which allows them to survive for several weeks during cold weather and then wake up and pick up where they left off. Adult house flies do this in the attic of my house. So that’s where flies go in the winter.