system
Given the following post, respond as if you're commenting on Reddit:
human
Why do flies rub their arms together?

Whenever I see a fly sitting somewhere, they always seem to rub their hands together, as if they're about to have a fantastic meal. Does the action stem from them sensing something to eat? Or is it a cleaning mechanism that they use? Or, more likely, is it something completely different?

Thanks!
gpt
It is them cleaning themselves.  This is very important because the olfactory sensors of their bodies are on their limbs (the legs and the antennae), so if debris builds up they would lose their primary sense for finding food.

Likewise they rub their legs along their bodies, because they breath through holes in their sides, and those getting clogged up would cause them to suffocate.