This time of year, you can find lots of interesting creatures in seasonal ponds and puddles, as well as under rocks in streams and rivers. This column looks at the larvae of a couple of different genera of mayfly. 

Adult mayflies are short-lived, as commemorated by the root word for “ephemeral” in their scientific order: Ephemeroptera. Some nymphs, however, can live for several years underwater, growing through a series of molts. Most nymphs feed on algae and vegetation, though some are predators of smaller larvae and other micro-creatures. 

Mayfly nymphs have up to seven pairs of leaf-like gills, which you can see here on the sides of the abdomen. I believe this mayfly is in the Baetidae family, a safe bet because it’s a species-rich family of very small mayflies. They’re also known as “minnow mayflies, because they are talented swimmers.

Another nymph I found has a notably wide head and isn’t known for its swimming skills. It’s in the Heptageniidae family of flat-headed mayflies. BugGuide says the nymphs are found under stones and logs in shallow, rapid streams or in lakes or ponds. 

Fly fishing websites are great places to find information about mayflies—because hatches of the insects are siren calls for fish. One such site, Troutnut, compares flat-headed mayflies to Formula One race cars, saying their ability to stick to rocks as water rushes over them is “a ‘miracle’ of nature’s hydrodynamic engineering.”

All mayflies have two or three “tails,” known as caudal filaments. Research papers refer to the filaments as mechanoreceptors, which give the nymphs (and presumably the adults) information about the movement of water and air. 

Stay curious!

See more of Karen’s work here.


Troutnut, on flat-headed mayflies: Accessed 2/22/24.

Good information on both genera: Accessed 2/22/24.