It’s November and the subject of this column is mayflies. The pictures above are new adults I saw not in May, but in late October, so what’s the story? In fact, they don’t always fly in May, and these delicate water-loving insects have some noteworthy adaptations and a few vocabulary words all their own.
Mayflies are in the insect order Ephemeroptera, which is Greek for “short-lived, winged” creature. While the common name is misleading, the scientific name is spot-on. They are ephemeral indeed. Imagine an animal that spends more than 99% of its life underwater, looking like a shell-less lobster, then matures to live above the water for just one or two days. The sole purpose of the adult mayfly is to mate and to lay eggs. They don’t have working mouth parts, and don’t eat.
Here’s the craziest adaptation I’ve learned: Some male mayflies have developed an extra set of eyes that detect UV light and are positioned above their compound eyes. The red eyes in the picture above are an example of these so-called “turban” eyes, which are sometimes even more exaggerated than on this Ephemerella genus individual. When mayflies swarm, the males use these upward-facing eyes to detect females flying above them.
Not all male mayflies have an extra set of eyes. The blue-eyed specimen above, Epeorus albertae, is also a male. Its front legs, here held up and pointing forward, are extra-long, so it can clasp on while mating. This individual is a full adult, which you can tell by the fact its wings are transparent, and that brings me to the next oddity.
Mayflies are the only insect that molts after it has its wings. When they first emerge from the nymph stage in the water, they’re known as a subimagoes, or duns to anglers. The wings are opaque and they’ll shed their skin one more time before they can reproduce. After the final molt, they become imagoes, or spinners in fishing lingo. I found several spent cases on the undersides of leaves near the Coast Fork of the Willamette River. In the photo of the exuvia (molted skin) above, you can even see the insect’s three thin tails.
We have many varieties of mayflies in Oregon, which is a good thing because it means our streams and rivers are clean enough to support them. Most local mayflies have three tails, but some have two. Some adults have one pair of wings and some have two, but the second pair is always much smaller, as you can see in both the adults above.
There will be larger swarms of mayflies in the spring. So if there aren’t any warm days later this month to search for them, you can start looking again in early 2021.
The Ephemerella above was a first Oregon sighting for this late in the year on BugGuide.net. Here’s a link to the page, including a few notes: https://bugguide.net/node/view/1912836. Accessed 11/5/20.
An article about mayfly swarms, with a link to a National Geographic video, from Entomology Today. Accessed 11/5/20.