Why do houseflies always end up dying on their backs?

Answer 1

Houseflies, along with other insects, generally end up dead on their backs because they have an exoskeleton that is rather heavy in comparison to the legs. Unlike an "animal" that will go find a comfortable place to lie down and die, insects generally go out fighting in mad fits of scampering, running, climbing or flying. This behaviour coupled with a typically heavy exoskeleton generally allows physics to place them on their backs. You may witness this the next time you use an insecticide on a roach, wasp, bee, grasshopper, etc.

Answer 2

I am not certain of this, but I believe the legs were never very strong, and they weaken badly just before death, and as coordination is also lost, the fly falls and rolls over.

The wings are far stronger and they often beat their wings, perhaps as a reflex action, but at that point the effort only spins them around, and the wings eventually weaken as well, and stop.

Answer 3

Because they are top heavy