Mating Minute Black Scavenger Flies

Subject: Flies that love “docking”
Location: Santa Barbara, CA
April 6, 2016 4:54 pm
In the past two weeks or so (since mid-February), my apartment in Santa Barbara, CA has become home to these little flies (~3mm long) that seem to like to spend all their time docked posterior-posterior. Solo, they’re pretty active and prefer windows, mirrors, or just flying around roughly at head height; docked, they like walls and (especially) ceilings, and seem to spend 8–10 hours totally stationary—though when they do move, they do so as one, rather than separating first. They must go somewhere to hide overnight, because I only see them during the day. Also, unlike the fruit flies that sometimes invade my apartment, I’ve never figured out what these eat.
I’ve just never seen anything quite like this—in fact, for the first week or so, I thought it was a single long, skinny insect, and was very surprised the first time I saw that it was actually a pair. Maybe you can shed some light on what these are?
Signature: Curious in California

Probably Mating March Flies
Mating Minute Black Scavenger Flies

Dear Curious in California,
We believe these Dipterans are mating March Flies, but we really wish your image had higher resolution allowing us to see the details better.  March Flies in the family Bibionidae are sexually dimorphic, meaning there is a distinct visual difference between the sexes.  Males have much bigger heads and eyes than females, and the head in the upper Fly in your image is difficult to discern.  According to BugGuide, the species is found in Santa Barbara and BugGuide has an excellent image of a mating pair.  Of the family BugGuide notes:  “Adults emerge synchronously in huge numbers and often form dense mating aggregations. Males form loose ‘swarms’ and copulate immediately with females as they emerge from the soil. After mating, female bibionines dig a small chamber in the soil with their fossorial fore tibiae, lay eggs, and die within the chamber (
Plecia lay eggs on the soil surface). Adults are short-lived (3-7 days).”  Perhaps the most notoriously famous March Flies are the Love Bugs in the genus Plecia from the southeast, including Florida, that emerge by the millions and seem to be perpetually in flagrante delecto.  While we were much amused at your “docking” euphemism, since the insects in your image represent opposite sexes, the term is really not accurate.  A much better visual representation can be found in these mating Big Poplar Sphinxes.

Possibly March Fly
Minute Black Scavenger Fly

Thank you for the reply! I had in fact wondered whether they were love bugs (since I was able to guess what they were really up to), but the pictures I found online looked different enough to what I was seeing—and the stated range on Bibionidae also being larger than the 2–3mm of my “guests”—that I wasn’t sure. Unfortunately, the only camera I have access to is the one on my outdated iPhone, so those pictures are probably as high-resolution as I can get. In person, even under low magnification, I can’t quite tell whether their heads (which are both under 0.5mm) are different sizes. However, on the basis of descriptions like “The male and female attach themselves at the rear of the abdomen and remain that way at all times, even in flight” (from Wikipedia), which comports exactly with what I’m seeing, I’m satisfied that these are in fact Bibionidae. So, thank you again for resolving the mystery.

Update:  April 9, 2016
Thanks to a comment from Cesar Crash, we agree that these are mating Minute Black Scavanger Flies in the family Scatopsidae, which is represented on BugGuide where it states:  “Larvae feed on decaying organic matter, such as detritus or excrement.”  Minute Black Scavenger Flies and March Flies are classified together in the infraorder Bibionomorpha.

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