There’s a few reasons, others here are mostly focusing on how the fly survives a big force. I’ll focus on why the force isn’t that big to begin with.
You’re not transferring that much momentum to the insect if you think about it (or calculate it).
If you slap a bowling ball, your hand slows down by a lot, i.e. you transferres much momentum to the bowling ball. But a fly doesn’t really slow down your hand at all.
Momentum is mass times velocity. Force is mass times acceleration, i.e. how long it takes the fly to reach it’s end velocity. Your hand can deform, and the air acts as a cushion aswell. If you imagine falling onto a brick wall that hurts way more than falling onto a soft matress. You have more time to slow down gently on the matress. So even though your whole momentum gets down to zero, it takes a longer time so the maximum force is smaller (but acts on you for longer)
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