From Creatures Wikia
The Cave Fly is an annoying flying insect that likes dark, damp locations - they can carry disease and infection so keep an eye on any wandering norns! If you can not cope with the added danger from the Cave Fly you can swat them out of the air - or encourage your norns to. Note that Forest Norns, Ron Norns, and Purple Mountain Norns are very susceptible to the chemical Geddonase released by these flies!
Pressing INJECT will put 6 Cave Flies into the world near the submarine bay.