Posting on behalf of a neighbor who asked:
Can anyone identify this insect? It's about 3/8" long. There are, it seems, hundreds flitting low over our lawn. There were none 2 days ago.
This is the Northern California Bay Area Peninsula.
Biology Stack Exchange is a question and answer site for biology researchers, academics, and students. It only takes a minute to sign up.Sign up to join this community
Great pictures. First, fly or wasp?
In the second picture, under the head you don't see jaws of a wasp, but do see a bit of a tongue-like organ, like a fly. Further down, you can see that there are only 2 wings. Just below where the left wing meets the thorax, you can see what looks like a tiny wing or paddle. That's a haltere, a reduced wing that vibrates to provide stability, like a gyroscope. Flys have halteres. Finally, in the first picture at the very end of the abdomen are 2 points. That ain't a stinger, it's an ovipositor. It's a fly, female, and from the size of the abdomen, gravid.
The closest I've gotten to identifying this one is Axymyia furcata http://bugguide.net/node/view/1216479. Everything matches, as well as we can see from a photo, except the coloration. Also, A furcata is associated with streams and is found in the north-east and north-central US, while, your bug was found on a lawn in CA. For more information about A. furcata than you need, there is a master's thesis available: http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1882&context=etd.
I've looked at hundreds of fly pics to find the one I linked to above. That's the only fly I found that matches the shape of yours. Maybe it's a different fly within that genus, maybe it's just a color variation.
Their maggots live in rotten wood and decaying plant material, and the adults are mostly interested in more maggots, so this isn't a pest, just a neighbor.
Edit: furicata --> furcata, oops.
My neighbor submitted the photo to http://bugguide.net and it was identified as a sugar cane soldier fly, "Inopus rubriceps"- Female
They are from Australia and were accidentally introduced to San Francisco 50 years ago. It feeds on grasses.