Mayfly Gender Identification

I am often asked, after being shown one, how I know that a mayfly is male or female. Some may say that this is unimportant knowledge for anyone who isn't interested in bugs; they just want to catch some fish. But knowing how to properly identify males versus females does have a fishing application.

At the beginning of most mayfly hatches, the first flies you see will probably be males. The males emerge, molt into spinners, and then await the females. It's not until the females are ready, that mating takes place...sound familiar? But there is a practical, natural reason for this in the mayfly world.

By the time a mayfly nymph reaches hatching maturity, it has "learned" how, or more likely been lucky enough, to survive under water. It knows where to hide from the underwater predators that would eat it. It has lived in the water world since emerging from its protected egg cocoon. But when a mayfly feels air for the first time, it has to start all over again. It doesn't know that birds, windshields, and crazy entomologists with butterfly nets can grab it and end its chance of reproducing. Or that thermal current could grab it and launch it miles from its home stream, reducing its chances of ever finding a mate. The air world is a dangerous place for mayflies, and the less time they spend in it, the more likely they are to pass on their genes.

Sure, female mayflies will ultimately have to face the air world too. But they'll spend less time in it before mating because the males will already be ready for them. This time difference can be anywhere from several hours for Tricos to a couple days for Green Drakes or even more for March Browns.

Why does this happen? It's simple–the females are more important. They carry the eggs that will continue the species. If an unexpected weather event such as a heavy frost or severe storm kills many of the mayflies before they can mate, it will most likely reduce the male population, but still leave enough to fertilize the females and continue the species. So, nature has designed that the males do the most waiting out in the big dangerous world above the water.

But how does this relate to fishing? You can get a pretty good idea where any hatch is in its progression by looking at the male to female ratio. For example, if you arrive streamside and find only male Brown Drakes, you know that the hatch is just beginning and that the fishing should only get better. If you find mostly females, then the hatch may be nearing its end and you may not want to travel to that stream next week to try to catch the Brown Drake hatch.

So how do you tell the difference between males and females? If you look at this first picture of a male Hexagenia orlando spinner from my Jet Lag blog, you can clearly see male characteristics.

Look at the size of the eyes. Males will generally have larger, more pronounced eyes than females. But the biggest indicator is the clasper. The clasper is the two appendages that look like pinches just under the male's tail. These are used to hold the females while mating in flight and only the males will have them.

---

It's a little more complicated to identify a female, but you can still do it.

First, and most obvious, it won't have a clasper and its eyes will usually be small. Second, if you look at the second body segment from the tail, you'll see that the body curves slightly upwards. This is where the female delivers her egg.

I apologize if I sounded a little too much like a seventh grade health teacher this week. But the best fly fisherman I know are the ones who pay the most attention to the small things. Maybe next week we'll discuss our changing bodies and why boys get facial hair.