This is the kind of stuff I love to find in my inbox in the morning. I work on flies, and I’ve never seen or heard of these ones until Matthew Cobb posted about them in his email Z-letter this morning. Look at the creature in the top photo. Looks like a larva, right? In the bottom photo you can see the same beast with some larvae of the army ant Aenictus.
It’s not a larva at all: it’s an adult fly that mimics ant larvae. To be precise, it’s a phorid fly, Vestigipoda longiseta, from southeast Asia. (“Vestigipoda” means “vestigial legs,” which these guys have. That, of course, is evidence that these things evolved rather than being created.)
The long “grubworm” part of the body is simply the enormously elongated and unpigmented abdomen of the adult. The head and thorax are the dwarf structures at the left-hand side of the top photo. This has all evolved from an ancestor that looks pretty much like the flies you know.
You can imagine why natural selection would favor this resemblance: the ants tend and feed the larvae, and mistake the flies for their own brood. It’s a lifetime of free lunches! The ants also protect the flies and carry them (like they carry their own larvae) when a colony is on the move. Here’s an adult of V. longiseta being carefully carried by an Aenictus ant:
Why can’t the ants detect these intruders? Well, they’re not terribly harmful, getting just a bit of food from the colony, so there’s probably not strong selection to weed them out. Ants, of course, have pretty bad vision, so they probably can’t see the intruders as different from their own brood. Matthew Cobb hypothesizes, and I agree, that there’s probably chemical mimicry going on here as well: the hydrocarbon molecules on the fly’s cuticule may well resemble the compounds on ant larvae, so that the ants, who “taste” these hydrocarbons, are fooled by chemical mimicry. This could easily be settled with a bit of gas chromatography.
There are several species of Vestigipoda in southeast Asia. Here’s a shot of the head (right), thorax and first abdominal segment of another species, a Vestigipoda maschwitzi female (from Disney et al, 1998). You can clearly see the features of a fly.
This is not, as Christopher Taylor at Catalogue of Organisms points out, a case of neoteny—that is, of juvenile flies becoming sexually mature. (An example of that is the axolotl salamander, Ambystoma mexicanum, in which gilled juveniles are able to reproduce.) No, these phorids have the regular larval stages of flies, but then go through normal metamorphosis, winding up as an adult with ant-larval features.
Curiously, all known larval mimics of Vestigipoda are females. Where are the males? Taylor suggests that they may be “normal” flying flies, in which case they’d have to somehow sneak into the ant colonies to mate.
Phorids are just plain weird: they’re among the smallest flies, many are wingless, and there are some vicious parasites among them. Have a look at this one, which is about the most gruesome fly I know of. Its larvae live inside ants, crawl into their heads, decapitate them, and then use the empty head for protection during pupation:
There’s simply no end to the wonderful stuff evolution comes up with. There are tons of weird flies out there; I’m planning “Fly Week” when I return.
Disney, R. H. L., A. Weissflog & U. Maschwitz. 1998. A second species of legless scuttle fly (Diptera: Phoridae) associated with ants (Hymenoptera: Formicidae). Journal of Zoology 246 (3): 269-274.
Maruyama, M., R. H. L. Disney & R. Hashim. 2008. Three new species of legless, wingless scuttle flies (Diptera: Phoridae) associated with army ants (Hymenoptera: Formicidae) in Malaysia. Sociobiology 52 (3): 485-496.