Lately, I’ve been spotting a few types of dance flies in woodsy areas on sunny days. These flies in the Empididae family are small and seemingly unremarkable, but like nearly every insect I’ve looked into, they have some peculiar and provocative features.
Dance flies are also known as dagger flies, because of their pointy proboscises. Some species have much longer spears than those pictured here, which are in fact used to impale prey, as seen above, where a Tachydromiinae sub-family fly has shish-kebabbed a non-biting midge.
Dance flies have some unique mating behaviors, and I’ll point them out from least extraordinary to most. To start, they’re named “dance” flies because of the way swarms of them seem to choreograph synchronous, bobbing moves during the spring mating season. Also, some long-tailed dance fly females in the Rhamphomyia genus can inflate their abdomens, extending what look like thin red cushions out to their sides while they fly, to attract males. Those flies have only been spotted in the eastern U.S., however.
You might see the third oddity in our Oregon Empididae: nuptial gifts. That’s just what it sounds like. The males deliver what they view to be an attractive offering to a female, in the hopes she’ll fancy him. This might be a dead insect (darling, you shouldn’t have!) or a secreted silken balloon, that may or may not contain a dead insect (it matches your eyes!). There’s a link to some amazingly sharp photos of gifting flies from the Oregon Entomological Society in the sources list below.
It’s unclear from my reading what the yearly life cycle of these flies looks like. They’re obviously cold tolerant, as it’s been in the 20’s a few times this fall. But, as is becoming a theme, they could use more study.
General information, nicely presented, from the North American Dipterists Society: http://www.nadsdiptera.org/Doid/Empidchar/Empidchar.htm. Accessed 12/4/20.
Amazing pictures of nuptial gifts on last page of an Oregon Entomological Society newsletter: http://odonata.bogfoot.net/oes/OES_Bulletin_2013_Spring.pdf. Accessed 12/4/20.