To get to these, you need to take the tunnel from the "main room" of Crawlers' Nest that leads you to the Labyrinth Lizards. Take the route that leads back to the Rumble Crawlers, then travel north past a few Wespe's and you come to a room with these fly's, and there is occasionally an elemental in the same room with these.
Be careful though, because just about everything on that 1/2 of the nest aggros.