An elegant Eocene fly


USA: Colorado, Garfield, Hwy 139, Douglas Pass.

Here is one of the more elegantly preserved specimens among the collection of Green River Formation fossil insects that I am photographing. It is obviously a fly (order Diptera), but I don’t agree with the preliminary identification of “Mosquito?” as indicated on its label. Rather, I think it is one of the fungus gnats—also members of the suborder Nematocera and, thus, closely related to mosquitos (family Culicidae), but with distinctly elongate coxae (bases of the legs) and lacking the elongated proboscis that mosquitos use for sucking blood. It’s hard to decide between Mycetophilidae (fungus gnats sensu stricto) or Sciaridae (dark-winged fungus gnats), which differ in whether the eyes meet above the antennae (Sciaridae) or not (Mycetophilidae). However, Borrer & White (1970) mention that species of the former are generally less than 5 mm in length, while the latter range from 5–10 mm. This specimen measures 4.15 mm from the front of the head to the tip of the abdomen, so  maybe that is evidence supporting Sciaridae (although perhaps there were smaller mycetophilids 50 mya than today).

Here is a view of the whole fossil, measuring approximately 50 mm on each side:



Borrer, D. J. & R. W. White. 1970. A Field Guide to the Insects of America North of Mexico. Houghton Miffton Company, Boston, 404 pp.

Copyright © Ted C. MacRae 2012

11 thoughts on “An elegant Eocene fly

    • These belong to a local collector of fossils here in St. Louis. He contacted a friend of mine, also an entomologist, looking for help with identifications, who then forwarded them to me. I have plenty more photos to share! 🙂

  1. Looks like a Mycetophilidae to me Ted. In fact it is reminiscent of Cordyla to me. Of course this is based mostly on the the general habitus. The more hunchbacked nature along with what might be large tibial spurs. The legs are also longer than typically seen in sciarids.

  2. I agree with Chris – looks more like a mycetophilid than a sciarid. The Manual of Nearctic Diptera says “2.2 13.3 mm long” for Mycetophilidae and I’ve certainly keyed out mycetophilids under 5 mm.

  3. The Mycetophilidae of tradition has been split into several families including Keroplatidae, Bolitophilidae, Diadociidae and Ditomyiidae. Family ID of these gnats is therefore usually left to experts these days…
    I am no expert (although a dipterist), I would call this a Mycetophilidae s.str.; not one of the newly split off families.

  4. From Vladimir Blagoderov in a comment on Facebook:

    This is Mycetophilidae, most probably Mycetophilinae. I doubt that mouthparts are elongated – these are palpi or parts of fore coxae detached. Alas, nothing more can be said. Contrast of your photo can be improved and more details probably revealed. Put a few drops of alcohol and a cover slip before photographing – you can do it safely with Green River material.


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