As an entomologist with a focus on the habits of insects, I can tell you that flies do indeed defecate. Flies, like most insects, have an excretory system that allows them to eliminate waste. However, when a fly lands on you, the chances of it defecating on you at that exact moment are quite low. Flies tend to defecate when they are at rest for a longer period, and since flies are typically in constant motion, the likelihood of them depositing feces on you is minimal. Additionally, the small size of a fly's waste makes it nearly unnoticeable to the human eye.

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Flies' mouths are soft and spongy; they can't chew. In pretty short order, the food is metabolized, and they poop out the rest in what we usually call "fly specks." Fly poop is tiny black or brown dots. You might also find amber-colored spots, but that's excess SFS left over from the meal.read more >>