Fungus Gnats from the Woods

2011 May 21

In October of 2010, Sam and Rosie won a giant pumpkin in a raffle, which we carved and put out on Halloween. Afterwards, Sandy wheeled it out back to the edge of the woods for the deer to eat. Which they did over a period of several days, starting with ripping its face off:

Sandy then noticed that it was covered with small, slender flies, and let me know so that I could get pictures. It turned out not to be all one species of fly: “Species A” was black with smooth legs and unpatterned wings:

“Species B” had spikes on its legs, and a pattern of spots on the wings:

And “Species C” had some resemblance to a mosquito, with a longer body, longer legs, paler color, no wing spots, and a generally more fragile appearance:

They were all getting pretty plump on pumpkin juice, and in the “Species C” photo you can see a drop of liquid at the tip of its abdomen. It was probably drinking lots of low-nutrient fluid, extracting the nutrients that it could, and then dumping the excess water.

All three of these are evidently Fungus Gnats in the family Mycetophilidae. There are about 600 species of fungus gnats, and I’m not confident about identifying these much further. Although Bjoern Rulik on BugGuide thinks that “Species A” is probably in the genus Boletina, “Species B” is most likely in the genus Mycetophila, and that Species C is probably in the genus Mycomya

The small fungus gnats that infest houseplants are in the same superfamily as these (Sciaroidea, which also includes gall midges), but a different family (Sciaridae rather than Mycetophilidae). These larger ones are much less likely to get established indoors. For one thing, they are large enough to be mistaken for mosquitos, and so will get swatted much more aggressively. The big ones are much more likely to be encountered out in the woods.

The adults “don’t live long”, and the fact that these were out in November implies that they are (a) cold-tolerant, and (b) most likely going into a mating frenzy before the frost and snow set in. As for why they were so attracted to the pumpkin, well, the juices contain salts, sugars, and proteins that they probably needed pretty badly to produce eggs. At any rate, they sure liked it.

Depending on the species, the larvae either eat fungus (ranging from mold and mildew, to full-up mushrooms), or are predatory on the other fungus gnat larvae. Yes, it’s a gnat-eat-gnat world out there.

6 Responses
  1. May 21, 2011

    I’m trying to figure out how you shot those photos. Mini tripod, probably. And I see highlights that make me think you used a flash, but it’s not diffuse enough for bounce flash. So how did you damp down the flash so it wouldn’t overwhelm the interior?

  2. May 21, 2011

    Yes, I did use a flash, but it was equipped with a time machine flash concentrator of my own invention. It preferentially puts the light from the camera’s built-in flash right on the bug I’m photographing, without spreading a lot of stray light around elsewhere. This let me use a fast enough shutter speed that a tripod wasn’t necessary, and also a small enough aperature (f22) for good depth-of-field.

  3. May 21, 2011

    I love the bottom photo where the gnat is passing the water and filtering out the nutrients. It reminds me of baleen whale filtering tiny sea creatures out of the water.

  4. May 21, 2011

    Molecular baleen action? 🙂

  5. May 23, 2011

    KT: I think that these flies were probably doing about the same thing as “puddling” butterflies. The males of a number of butterfly species will sit and drink from mud puddles, extract the salts from the water, and then expel the water residue. Sometimes they squirt the water several inches, and may pass water amounting to many times their body weight.

    As to why they do this, animals living inland away from the oceans are chronically short of sodium. Animals require sodium, it is used to make nerve impulses fire properly. And since plants don’t have nerves, they mostly don’t need it – and, in fact, more than a small amount of sodium is toxic to most plants. So most plants don’t take up sodium, and animals that eat these plants need to get their sodium from other sources. To get sodium, certain male butterflies (and, probably, these fungus gnats) go prospecting for salt to pass on to the females that they mate with, to increase the fertility of her eggs.

  6. Della3 permalink
    July 31, 2011

    Tim, thanks for the interesting observations about salt intake. It’s food for thought.

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