Last July, while wandering about the coastal forests of St. Lucia in eastern South Africa, I happened across an intriguing scene half-way up a spiny Acacia trunk. Some diminutive gray flies were pestering a trail of ants as they walked along the tree.
The flies’ exact activities were hard to observe with the naked eye, but it looked like nothing I’d ever seen. They seemed to be grabbing ants, pinning them to the trunk, and after a few seconds letting them go again.
The macro lens on my camera serves as a handy field microscope. Conveniently, the flies were so focused on attacking the ants I could place the lens nearly on top of them and observe the details of their activities without spooking them. On inspection, it turned out that the flies were stealing food.
Later, I sent my photos to milichiid expert Irina Brake. She identified the perpetrators as Milichia patrizii, a species whose behavior had not been previously recorded. So we wrote up a short article in African Invertebrates that came out today. As the paper is behind a subscription barrier I’ll share a few of the photos here.
Stalking the victims.
The flies stop the ants by grasping the ants’ antennae between their own. According to Irina, this use of antennae to subdue other insects is entirely new for flies.
The fly triggers an automatic regurgitation response by tapping the ant’s palps with her proboscis. Oddly enough, once the ant has acquiesced the fly releases her victim’s antenna and the ant just sits there. There may be a chemical communication going on between the tip of the ants antennae and the concave upper lip of the fly, but we don’t know. Might be a project of interest to a South African student. If any are reading this. Hint, hint, hint.
The ants leave themselves open to kleptoparasitism by having a highly stereotyped food exchange behavior. Here, two of the Crematogaster demonstrate how it is supposed to be done. The flies have apparently figured out how to game the ants’ own communication system.
Source: Wild, A.L. & Brake, I. 2009. Field observations on Milichia patrizii ant-mugging flies (Diptera: Milichiidae: Milichiinae) in KwaZulu-Natal, South Africa. African Invertebrates 50 (1): 205-212.