The sole purpose of a fully grown mayfly is procreation. It literally doesn’t have a mouth, only vestigal mandibles. The final stage of their lives after metamorphosis and emerging from their cocoon is literally fucking around until they die - Usually within a day.

I was trying to allude to the possibility that alien beings wouldn’t necessarily need a waste disposal facility as you so charmingly call it. They could be custom made for particular missions and wouldn’t need a digestive system and thus no anus between them alien cheeks.