June 2, 2016
In the entomological world, “scavenger” can be a dismissive term, hurled at animals that seem to feed indiscriminately on any available garbage or rotting material. The ultimate scavengers are indeed those insects that frequent trash bins and dumpsters: unsophisticated diners on our scraps and leftovers, annoying infesters of our cities and houses. The image of an unsavory “scavenger” can obscure some fascinating and extremely specific matters of lifestyle that defy the notion of a creature with wholly undiscerning habits. One example is our previously featured “coffin fly” (Conicera tibialis), a tiny phorid that burrows through the soil to reach its buried prize. This fly is perfectly capable of going through its life cycle in test tubes, feeding on meat, but in nature it is virtually never found in an unburied corpse. I was reminded about scavengers by the submission to the Museum of some fly pupae, found in the shells of dead snails by SLIME participant Cedric Lee. We reared the pupae to adulthood, and each one yielded an adult sarcophagid fly. The flies of this family (Sarcophagidae; often shortened to “sarcs” by dipterists—i.e., fly specialists) are commonly called “flesh flies” due to their breeding in dead bodies and attraction to nearly all types of noxious decaying material: carrion, dung, dead insects, etc. They are large flies, with gray and black striped bodies and red eyes. The fact that, at least externally to a non-expert, most sarcs look extremely similar leads to their often being labeled as “just” scavengers. In fact, sarcs are among the most diverse families of flies when it comes to the types of lifestyle they employ. The larvae of various species are: scavengers (often highly specialized), predators (that feed on and kill more than one host), parasitoids (that feed on and kill a single host), and true parasites (that feed on but do not kill a host). Unfortunately, for me, sarcs are also among the most disgusting flies. I know my colleagues who work on sarcs—and who are as fond of them as I am of phorid flies—might be dismayed by my contempt, but I can’t help being revolted by their reproductive process. Sarcs are ovoviviparous, meaning that the eggs hatch within the female abdomen and the female deposits larvae, rather than eggs, on the food source. When studying phorids associated with millipedes, I have frequently been repulsed by the arrival of female sarcs, who immediately spew several maggots on the scene, ruining my experiments. The larvae enthusiastically crawl into the millipede body and start feeding. Of course, the most gut-churning sarcs are the parasites, some of which infest incapacitated humans and cause noteworthy and alarming medical conditions. Most people interact with sarcs soon after noticing the smell of something dead under their house or in the walls. They’ll start to see large, clunky sarcs flying around their windows, trying to escape the house. These flies are the offspring of a female who somehow found a way to lay larvae on or close to whatever died, and who have helped to get rid of the body. I am often asked by homeowners how to eliminate these flies, and my answer is to let them do their scavenging, so that in a few days all the decaying material will be gone. Circling back to my original point, Cedric’s flies are not just scavengers—they are probably highly specialized feeders on dead molluscs (several sarcs are known to do this). I say “probably” because it takes a specialist to identify sarcs, and we will have to send ours out to one of the three or four people in the world qualified to tell us what they are. Such unusual natural history discoveries can depend on the thoughtful observance of Citizen Science participants like Cedric. One of our top citizen scientists, Cedric will likely contribute to one of the first records of a species breeding in dead snails!