Among my many pet peeves are when people refer to Drosophila as fruit flies (they are not). Real fruit flies (Tephritids) feed, mate, and lay their eggs on live fruit — for this reason, many are agricultural pests (e.g., the medfly). Drosophila, on the other hand, feed on the micro-organisms found primarily on rotting fruit or other rotting plant parts. For this reason, I like to think of Drosophila as one of the most refined insects because they prefer fermented sugars (like beers and wines).
While the majority of Drosophila feed on rotting plant material, some have found an even more exotic host: crabs. Yes, just like you can have “crabs” living in your nether regions (well, hopefully not you, per se), crabs can have flies living on them. In this fascinating review Stensmyra and Hanssona discuss some of the classic work done to characterize the various species of flies found on land crabs in the Caribbean and Indian Ocean.
Flies from both the genus Drosophila and the genus Lisssocephala are known to live, feed, and oviposit on land crabs. The flies hang around near the mouth of crabs, eating the micro-organisms that feed on the food scraps stuck to the crabs’ various mouth appendages. After mating, the female lays her eggs on the crab. In one species of Drosophila, the larvae develop and pupate on one of the mouth parts of the crab (the maxillipeds, shown in the image on the left). In another species of Drosophila, the larvae hatch from eggs laid on the mouth of the crabs, travel to the gills, then return to the mouth, and finally fall off the crab and pupate on the ground. The Lisssocephala species are not very selective about the crab hosts, and the adults do not feed on the crabs. They do, however, oviposit on crabs of two genera, and, depending on which genus they lay their eggs on, the larvae either develop on the mouth or in the gills.
Interestingly (as if all of this isn’t interesting) the shift to the crab hosts has evolved multiple times — and in at least two genera. The microhabitat offered by land crabs provides all the necessary resources for some species of Drosophila and a convenient place for some Lisssocephala species to lay their eggs. There’s food, a safe place for the young to develop, and plenty of urine. Yes, urine. The urine that is excreted near the mouth of crabs helps maintain the healthy population of microorganisms in the flies’ crabitat (Stensmyra and Hanssona’s portmanteau, not mine).
Green MM. 2002. It really is not a fruit fly. Genetics 162: 1-3 [link]
Stensmyra MC and Hanssona BS. 2007. Flies’ lives on a crab. Current Biology 17: R743-R746 doi:10.1016/j.cub.2007.06.015