My RV dump into the city sewer system at my house has flies in the Summer. I didn't see flies when I dumped a few weeks ago. I grew upon a farm. I'm pretty sure those flying critters are not sophisticated enough to distinguish between cow poop, horse poop, pig poop, or human poop. They just go for it. Ohhh... I take that back. We had pigs... lots of them. I think those flies particularly love pig poop better. lol And remember, we humans are much like pigs, especially so in the digestive system. And pigs are as smart as many humans too.