Given the common names marsh fly and snail-killing fly, you can guess the habitat and peculiar diet of this splay-legged, long-faced fly. But if you read further, you’ll learn some other fascinating traits, and two new vocabulary words.

The larvae of every fly in the genus Sepedon live in ponds and streams and eat snails. Specifically they eat little snails that breathe air and don’t close off their openings with trap-door devices called operculum. That’s your first word of the day. Operculum is Latin for “little lid.” Each fly larva can eat anywhere from eight to 50 snails. There’s a lot of variety in size of snail and species of larvae in that broad statistic. The genus enjoys a medley of mollusks: Some species of snail-killing flies eat slugs and clams as well. 

You’ll notice the adult has quite a robust head adornment. That prominent crown is a feature of the genus, as are the forward-leaning antennae. See that extra antenna wisping out above the first? That’s the “arista,” your second vocab word. Yes, Arista is a record label and a computer networking company, but do their executives know that to entomologists, an arista is a bonus bristle attached to a fly’s antenna? 

Studies show that for Sepedon fuscipennis, an East Coast species, the arista has receptors for touch and chemical information that help them find food and communicate during mating. It’s likely that’s true for other Sepedon species, but they’ve not been experimented on. 

This fly appeared more red-purple in person. Look for them on grasses, often facing the ground. 

Stay curious!

See more of Karen’s work here.


BugEric’s post on Sepedon flies: Accessed 4/1/22. 

Research on Sepedon antennae and arista: Accessed 4/1/22.