These large red glowworm-like flies, the size of the average large eye, live in dark caves and their buzzing sounds like the growling of a bear (or alternately, like the sound of the creature/monster of the GM's choice. They feed on even smaller insects that their light attracts, and on the carrion eaten by larger predators where they also lay their eggs. They are not eaten by many things, partly because they taste terrible and partly because they fly around in pairs, the male and the female, and resemble glowing eyes when in the dark. This scares away many predators and so makes life safer for whatever life forms might be in the cave or tunnel, and so it is in the self-interest of the other life forms not to eat the Eye flies. They themselves are more or less harmless if annoying to humans and humanoid races, as they have no stings and their jaws are too small for biting humans. The real danger they possess from a human point of view is lulling the humans into a state of complacency so that when they come up against something really dangerous they are unprepared for it and get mauled.
(For GMs with a bit of a nasty streak...you can have your players encounter these a few times until they start to take them for granted and then have the next set of *eyes* they find really be the eyes of a Cave Bear/Giant Spider/Monster of your choice, meaning they are unprepared for the attack and the monster gets a free attack in on them before they can properly respond.)