by Joseph DeSisto
While tearing apart logs in Sumter National Forest, I came across this little spectacle:
The stuff that looks like a fried egg is a fungus, growing on the rotting wood. The worm-like animals are insect larvae, and I found them in a cluster with 20 or so others, all squelching about over the fungus.
The lack of legs and “maggot-like” appearance mark these as larvae that will one day become flies. The problem is, the fly order Diptera contains around 120,000 known species, and many undescribed. Since they were found on a fungus, I felt comfortable placing them in the Sciarioidea or “gnats,” which includes a lot of flies with fungus-feeding larvae. That’s a little more specific, but with seven families and nearly 12,000 described species, I still had a long way to go.
The two largest families of fungus-feeders in this group are the fungus gnats Mycetophilidae and Sciaridae, with 3,000 and 1,700 known species, respectively. Many more species have yet to be discovered, especially in the tropics which is pretty much a black box as far as fungus gnat diversity is concerned.
Below is an example of an adult fungus gnat. Most of these insects are less than a centimeter in length as adults, although the maggots can be a bit larger.
The name “fungus gnat” makes these creatures sound pretty wimpy. It’s true, the vast majority spend most of their lives as larvae burrowing into mushrooms, until they emerge as minute flies that live around a week. In general, people who don’t specialize in fungus gnat research regard them as pretty boring, at least compared to some of the larger, more spectacular flies out there.
Let’s take another look at our mystery maggots:
See the shiny, gluey stuff covering some of the fungus? That’s silk, and it’s hygroscopic which means it harvests moisture from the air. It’s our biggest clue in figuring out where these larvae belong, because silk is used not to eat fungus, but to ensnare prey. These innocent little fungus gnats are actually the maggot equivalents of spiders, ambush predators that wait for helpless insects to become trapped in a sticky web.
Predatory fungus gnats are not mycetophilids or sciarids, but belong in their own family, Keroplatidae. This family used to be considered a subgroup of Mycetophilidae, with a modest diversity of 1,000 or so known species.
Despite relatively low species diversity, keroplatids have a wide range of lifestyles, most of which involve predation on other invertebrates. The ones I found in South Carolina are relatively benign – other species weave drops of acid into their webs, to hasten the demise of their victims. Still others attract prey by bioluminescence, and are called glow worms.
The best-known glow worm is Arachnocampa luminosa, a New Zealand cave-dwelling species – you might remember it being featured in David Attenborough’s Life in the Undergrowth series. Arachnocampa larvae live in silk tubes on the ceilings of caves, and dangle silken threads with drops of acidic glue. When one gets hungry, it emits a light from its rear end, drawing insects up to the ceiling, where they become trapped. The gnat larva can then harvest and eat its prey whenever it wishes. For more about keroplatids, I recommend the introduction to the catalog by Evenhuis (2006), cited below.
When predatory fungus gnats finish their murderous childhood, they emerge as flies. At this point they look just like any other fungus gnat. Harmless and plain they may be, but boring? Ha!—
Evenhuis, N.L. 2006. Catalog of the Keroplatidae of the World (Insecta: Diptera). Bishop Museum Bulletin in Entomology 13: 1-178.