Fungus Gnat

2025 January 12

We have a variety of potted plants around the house, and they are infested at a low level [1] with some kind of fungus gnat. They are extremely tiny, if you are reading this at a normal font size, they are approximately the length of this dash (-) and honestly they are almost unnoticeable unless you look for them. Normally they flit around and are hard to get pictures of, but this one was on the wall of our bathroom on December 29, 2024 and held still long enough to run my macro lens up to high magnification and get a close look.

“Fungus Gnat” is an extremely generic term, it generally gets applied to all of the tiny gnats that live in soil and eat fungus [2]. Generally they are in the superfamily Sciaroidea, which also includes gall formers and some predatory gnats.

My pictures are actually good enough that we should be able to identify it a bit more than that. To start with, we can confirm its gnat-hood by noting that it only has one pair of wings, with the second pair reduced to the “halteres” that I circled in red. This shows us that it is certainly one of the flies, and not, say, a tiny wasp or barklouse or any other non-Diptera.

Second, we can look at the antennae. You can see that there are bulges, or “flagellomeres” along the length of the antennae. You can also see that each flagellomere is “binodal”, or consisting of two almost-separate bulges. There are twelve of these binodal flagellomeres, which is characteristic of male gnats in the supertribe Cecidomyiidi (the females have standard, nondescript antennae).

Of course, this supertribe contains about 64 known genera, and who knows how many individual species, so I will stop on the identification attempt at this point

[1] They would probably be infesting the house plants at a high level, except that whenever we find a spider wandering around the house we dump them into one of the houseplant pots. And at this point, most of our houseplant pots have at least one resident cobweb spider that is feeding on the fungus gnats as they mature and try to fly up. So it’s all good.

[2] This is the big reason we aren’t too concerned about them. They don’t bother the houseplants at all as far as we can see, so if they want to eat the fungus filaments growing in the soil, I don’t begruge it to them. Plus, I kind of like having some essentially tame spiders around the house, and they are good maintenance food for them so that if, say, mosquitos get in the house the spiders can kind of take care of them.

One Response leave one →
  1. Tim permalink
    January 13, 2025

    Plant pots are also the best place for spiders, as their detritus just falls into the soil. The tough part is to convince them to build a web there in the first place.
    I place a couple of sundew plants near my infestations, which seem to bring down the population.

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