Just in case you’re a fan of obscure diptera, I wanted to share some images of an insect I’d never seen before: Anthrax georgicus.
I initially identified it as Ogcodocera analis, which looks exactly alike to the untrained eye (like mine) but isn’t found in Pennsylvania. Anthrax georgicus parasitizes tiger beetles, apparently. The females lay eggs near the entrances to tiger beetle burrows and then the larvae attach to the beetle larvae and suck hemolymph. If you live near a place with dense population of tiger beetles, keep your eyes peeled for this fly. Or read about them here, in a fantastic post by Matt Pelikan. I gather they lay eggs by dive bombing.
The next photograph shows what it did in response to my flash.
I searched around to see if all flies do this, but couldn’t find anything about light-induced wing spreading. I did find an article by Andrei Sourakov on the startle response of a long-legged fly, again caused by a flash — they exhibit a tumbling escape behavior (see paper for pics). In a separate paper, Sourakov discusses the startle response of skippers (Hesperiidae) — again, I highly recommend taking a look at the figures so you can see the insects during their escape tumbles. In hindsight, I should have played around with flash sync speed to see if I could measure how quickly it could spread its wings. The shutter speed I was using was 1/200 second.
Just in case I’m completely wrong about the ID (which I based on this page) and you’re an expert with a moment to spare, I put a better view of a wing at the bottom. The posterior margin of the alula looks convex. I don’t have a better view of the antennae, unfortunately, which I know would be helpful for Bombyllidae. As a side note, insects that are jet black but have shiny white parts are a complete pain to photograph.