Crawlies with Cri: Marsh fly

Marsh fly (Family Sciomyzidae – pictured is Genus Limnia)

This week’s crawly is one fly you do not want to shoo away. Meet the marsh fly (Family Sciomyzidae – pictured is Genus Limnia). While flies are cool in general, this one is extra cool – and not just because of those fly shades (see what I did there?)
What makes marsh flies so cool? First, there is their other common name: “Snail-killing flies.” We’ll get back to that, but the name calls it like it is.
Second, there aren’t a lot of them – not in our area at least. There are about 200 species of marsh flies in North America, but only a handful live here in Oregon.
Marsh flies are easy to tell apart from other flies in several ways. Most species of marsh fly are slender, yellowish or brownish in color with “never miss a leg day” thick hind legs. They range from one-quarter to one-half inch in length. Many also have fancy patterned wings like our pictured fly. Only flies in our pictured fly’s genus have the oh-so-cool striped eyes though.
The most distinctive feature of marsh flies is their forward-facing antennae. Our Limnia sports clean, feathery, white extensions on theirs.
Adult marsh flies are generally easy to spot because they favor perching over flying. They’ll most often sit as our pictured pretty is sitting, with their front half slightly elevated. Adults can live the chill life because they feed on nectar and dew, foods easily found and foods that don’t run away and need to be chased.
If they feed on nectar, why are they called “snail-killing” flies? The moniker makes them sound like fierce (if slow-moving) hunters. It’s the marsh fly larvae who are the snail assassins. They are obligate feeders on a wide variety of terrestrial and freshwater pulmonate snails and slugs.
Let’s unpack that a bit. “Obligate feeder” means the marsh fly larvae have to dine on snails – not in a snobby “I prefer escargot” way – but in a literal “they’ll die without it” way.
“Pulmonate” snails (and other pulmonated mollusks) are those with the ability to breathe air, because they have a pallial lung instead of a gill, or gills.
Marsh fly moms lay their eggs either directly on a snail or slug, or in their habitat. Some marsh fly larvae feed on very small snails (like the teeny ones you see in the river) so they are mobile and move around from meal to meal.
Other larvae complete their entire larval life by chowing down on one larger snail, then holing up in their shell to morph into adulthood.
Other species prefer different mollusks such as small, freshwater “pea clams.”
There are some species of snail who can live a life worry-free from marsh flies, such as Hawaii’s arboreal snail.
How do we know this? Some snails carry diseases, and some are intermediate hosts for liver flukes, which harm livestock. So, marsh flies have been imported into some areas where those snails need to be controlled. Before importing marsh flies to Hawaii, they determined that valued native snails would not be on the menu.
If you’re looking for an out-of-the-box job, there are labs where scientists raise marsh flies for study as biological controllers and feed them chunks of cut up snail meat.