An Underexplored Microhabitat

Although most of what used to be my lawn is now an untamed meadow interspersed with gardens, fruit trees, and berry bushes, once a month or so I do break out the ol’ battery-powered lawnmower to maintain a network of trails through it all. On June 11, which was one of those rare occasions, I paused my mowing and flipped over the mower to scrape out the wads of vegetation that were starting to choke up the blades. I was surprised to find a fly puparium, which I at first interpreted to have been something I had run over with the mower that had miraculously made it past the spinning blades intact.

When I found a second one just like it, I realized that these were puparia from larvae that had developed in the decomposing vegetation that I had failed to scrape off the mower after I had finished my previous mowing session. They looked similar to Anthomyiidae puparia—some anthomyiids are leafminers, but others feed on a variety of other things including decomposing vegetation—but they could have belonged to some other related family.

As I continued to dig through that crud, I found this bristly thing, which I believe to be the puparium of a lesser house fly (Fanniidae: Fannia canicularis)…

…as well as a bunch of larvae like this one:

The fact that this larva has a distinct little head capsule tells us that it’s not one of the so-called “higher” flies, like anthomyiids and fanniids, that form puparia (i.e., they pupate inside the hardened skin of the larva, which the adult ultimately escapes by inflating a big balloon from its face). It’s something more along the lines of a midge or crane fly that forms a naked, exposed pupa.

Naturally, I stuffed a chunk of this rotting plant matter, including these larvae and puparia, into a vial to see what they would all turn into. When I checked the vial on June 23 I found about a zillion of these inside:

This is Coboldia fuscipes, a member of the family Scatopsidae, which are known as the “black scavenger flies.” It belongs to the same group of “lower” flies (Bibionomorpha) as the gall midges, fungus gnats, March flies, and wood gnats, and it is the adult that goes with the larva in the previous photo. Nothing ever emerged from the puparia in the first two photos, but it turned out there was a third species of puparium-forming fly in the mix. Two females of Drosophila busckii (Drosophilidae) emerged, also on June 23, and I was able to find one of their empty puparia.

So, not quite the 30+ species of arthropods I found in a cubic foot of my lawn, but I think four species of flies in a handful of decomposing lawn isn’t too shabby either!

Thanks to Brad Sinclair for identifying the adult flies.