Wild Side: Fungus gnats

There are approximately 150,000 fly species known in the world.


Nearly everybody has at least a vague notion of what a fly is. To a biologist, “fly” means a member of the insect order Diptera, unique among insects in possessing only one pair of wings instead of the usual two. For the rest of us, the word “fly” calls to mind a relatively small number of well-known species: house flies, blow flies, and horse flies, for example, and the ever-popular mosquito.

This general knowledge, while representing a good start, is woefully deficient for characterizing this important insect order. The fact is that the Diptera is vast and varied, comprising a bewildering array of appearances and life histories. Some flies are large by insect standard; many others are so small as to be barely visible. Some benefit humans while others are pests or the vectors of horrible diseases. Many have evolved amazing, distorted body forms, and many others engage in bizarre parasitic relationships with hosts in other insect orders. In all, something around 150,000 fly species are known in the world, a number that steadily grows as research advances and may reflect only a smallish fraction of the diversity that is really out there.

The cool, damp weather of late autumn is challenging for most insects, but it showcases some of the odder examples to be found among the Diptera. A case in point are the fungus gnats, a group of closely related families that, as their collective name suggests, focus their lives largely around association with fungi. Primitive flies in evolutionary terms, most fungus gnats are on the small side, even for flies. And many are challenging to detect: adults may be drab and inactive, spending most of their time hiding in moist, dark areas, while the larvae often live underground or actually inside fungi such as mushrooms. But these insects are widespread and often abundant: An alert observer can readily find them even as temperatures dip near freezing.

Regarding their relationship with fungi, some fungus gnats are generalists, their larvae cheerfully feeding on whatever fungi are available. But as is typical of flies in general, there are many instances of close, obligatory associations between one fly species and one fungus species. Fungus gnat larvae are generally small, elongated maggots, often featureless except for a black head, which seems to be very close to a universal trait in this group. Identification with any measure of precision is very difficult: While fungus gnats as a group are pretty easy to recognize, you often must resort to dissection or examination under a microscope to identify one of these gnats to species or even genus.

Perhaps the easiest members of this group to get a look at are the dark-winged fungus gnats. These are the members of the family Sciaridae, representing something on the order of 200 species in North America. Based solely on their appearance, you’d expect dark-winged fungus gnats to be difficult to find; they’re tiny, mostly just three or four millimeters long and, with very few exceptions, featureless black in color. Moreover, many members of this family have evolved to specialize in inaccessible settings, including high elevations, high latitudes, deserts, and even caves.

But a quirk of their habitat preferences makes some species of Sciaridae surprisingly easy to see: Potted plants, such as the ones on your windowsill, offer exactly the conditions Sciaridae seeks out and often support a robust population of these flies. If you notice tiny, black insects flying weakly up out of the potting soil and around on your plants, odds are good you’re seeing adults of this family. Their presence is often a sign that you’re overwatering your plants: Soggy conditions promote fungal growth, which favors these flies since their larvae feed mainly on fungi. Adults feed little if at all and pose no direct threat to plants, though some sources indicate they can carry plant diseases on their feet.

Many other types of fungus gnats occur in more natural settings and can be found, if you look carefully, in association with damp, decaying organic matter. For example, members of the family Mycetophilidae are relatively easy to scare up out of wet leaf litter (though even when they take flight, these gnats are small, usually drab in coloration, and slow-moving, making them very hard to get a decent look at). Like many insects, Mycetophilidae (and other types of fungus gnats) are sometimes attracted to lights, a habit which sometimes brings them indoors. For example, I recently found a rather colorful example on the inside of our kitchen window and was able to identify it, with a fair level of confidence, as a member of the genus Mycetophila.

Tiny flies with a penchant for damp, moldy settings are unlikely to do much to rehabilitate the order Diptera in the popular mind. People, let’s face it, have a low opinion of flies. But from my perspective, the fungus gnats are a nifty group, biologically interesting and active at times and places that feature few other insects. They may lack charisma, but fungus gnats represent an important portion of insect diversity.