A quick follow-up to my last post, in which I showed a photo of an diapriid wasp on a puffball. Yesterday I photographed another tiny wasp on a mushroom, and whereas the puffball wasp flew away after the first shot, this one hung around for a long time, apparently laying eggs in the mushroom cap. When I looked at it from the side, I was struck by its similarity to a gall wasp (Cynipidae)–it had the same humpbacked appearance. Browsing through images on BugGuide.net, I determined that it is a member of the subfamily Eucoilinae of the family Figitidae, which belongs to the same superfamily (Cynipoidea) as Cynipidae. Eocoilines are all parasitoids of fly puparia, and some Ganaspis species are in particular associated with flies in mushrooms, according to Matt Buffington’s comment here. I believe my wasp is in this genus, but in any case it’s definitely a eucoiline. This is the first figitid wasp I’ve ever seen. Not very often I get to meet a whole new family of insects. [Added 9/13/11: Matt Buffington has confirmed that this wasp is Ganaspis.]
Ganaspis, a eucoiline figitid (1.2 mm), exploring a mushroom cap.
The mushroom. The wasp is visible as a black speck near the far right edge of the cap.
BugTracks 
Very interesting. I am getting more and more interested on mushroom spore’s dispersal. May be the flies that these wasps parasitize are among the spore carriers. Another entire ecosystem and one we know very little about. If you find something else, let me know.
This mushroom was also covered with drosophilids, none of which wanted to hang around to be photographed. I don’t know if their larvae were the wasp’s intended victims (the 1979 Hymenoptera catalog lists just three Ganaspis species, with no host information, so whatever Matt Buffington was referring to must me fairly new information). In a number of my close-ups of insects on mushrooms, I can see the spores rising up around them, even when the air is totally calm. With spores that tiny and numerous, I imagine that anything that lands on the mushroom is a potential disperser.
Thank you so much for providing fascinating information and observations! I’ll be on the lookout!
More great info — thank you again!
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I wonder if this is the kind of ‘mushroom wasp’ discussed in the beginning pages of
Our Ecological Footprint (I’ve been trying to track that insect down for some years)
No, this wasp is a parasitoid of fly larvae living within the mushroom. I would bet that the author is referring to a species of fly (fungus gnat?) rather than a wasp. I’m not aware of any fungus-feeding wasps, plus the word “maggot” normally is only used for fly larvae. I have a dim recollection of having read about flies that reproduce in that way, but I can’t summon any details at the moment…