This group of flies contains some spectacular species including the stalk-eyed signal flies, belonging to the genus Achias. True to their name, the males have eyes on long stalks extending from either side of their heads.
Stalk-eyed signal flies occur mainly in New Guinea (with more than 90 recorded species) and in Queensland (with five recorded species). Male stalk-eyed signal flies spend much of their time in rainforests on shaded tree trunks which they use as courtship territory and avidly defend against rivals. When fighting face to face, rival males gauge their opponents' size and strength from their head width. Small males (which may have begun life as under-nourished or disadvantaged larvae) have very short or no eye-stalks. The clear difference in head-width allows the weaker fly to back off early and avoid possible injury.