Photo from the Marietta College Department of Biology and Envrironmental Science

Water Penny Beetles are in a family called Psephenidae, under the genus Psephenus, which comes either from psephenos (ψεφηνος), meaning ‘dark, obscure’, or psephos (ψηφος), ‘a small round worn stone, pebble.’ Our money’s on the pebble, because that’s what the larvae look like. Or a penny. A tiny penny for dolls.

In case you’re thinking of taking a water penny beetle out on the town, here’s what you should know:

Favorite food: Microscopic organisms and algae salad.

Where they hang out: Swift currents in streams worldwide.

Personality: Kind of fussy. Sensitive to light, inorganic sediment, and too much algae. They cling to grassy clumps and the underside of rocks in the daytime. At night, they eat on top of the rocks: think, “outdoor balcony, after-hours in a posh restaurant.” The adults don’t live long, so after metamorphosis they mostly let the good times roll and mate a lot: scientists have observed females laying 400-600 eggs in a single patch. They really love oxygen and can’t tolerate pollution.

Which means: Water penny larvae have good taste and live in high-quality streams.

Which is too bad for them because: They make yummy treats for fish living in those high-dollar neighborhoods.

But good for us because: Their presence tells us when a stream is healthy. And they make free fish food.

For more information, visit UW-Milwaukee: Field Station Bug of the Week, the Tree of Life Web project entry on Psephenidae and the 1971 classic, “Ecology of the Water Penny Beetle (Psephenus Herricki)” by Chad M. Murvosh.