Bat flies (Nycteribiidæ) are true files (Diptera, superfamily Hippoboscoidea) which are obligate parasites that live in the fur of bats. Both males and females live by sucking blood from their hosts. They have no wings at all (so shouldn’t we call 'em “walks” instead of “flies”?), no visible eyes (or at most eye spots), and only a vestigial head (a phrase you don’t often hear in biology)—that’s evolution: use it or lose it.