Why do some flea species have better-developed eyes than others?

The development of eyes in flea species is primarily determined by their environmental needs and the nature of their hosts.

Generally, fleas that parasitize diurnal (daytime-active) hosts possess well-developed eyes to assist in navigation and host detection. Conversely, species that have adapted to live on nocturnal animals or in subterranean environments—such as moles or bats—typically exhibit poorly developed eyes or lack them entirely, as vision provides little utility in these dark or hidden habitats.1