Is it really true that ... ... flies always lay on their backs, when they die?

Dead flies in window sills are almost always lying on their backs. Why? The flies do not consciously lie down on their backs, when their lives are about to end. Instead, this common death position of flies is due to the fact that they and several other insects weigh relatively little, and their centre of gravity is at the top of their bodies.

When the insects die, their legs relax and fold under their bodies in the same way as the human hand will clench slightly, when it is relaxed. The folded legs influence the fly’s sense of balance, so it will easily fall over, rotating its horizontal axis.

The over representation of dead flies lying on their backs might also be due to the fact that it easily becomes fatal for a fly to lie on its back, when it is still alive. Normally, flies can turn themselves over by using their wings or a few legs on the one side to push, but injuries, old age, poisoning, thirst, or overheating impede the flies’ motions and coordination. So, they cannot turn over, if they have ended up on their backs – such as in a warm, sunny window sill – and then the fly will soon die of dehydration.