entomology

The Morphing Mayfly

By Arlen Thomason

Mayflies are the only insect that has two winged stages. Why? And how do they do it? Stunning photos from Arlen Thomason.

Note: To fully appreciate this article, click on each photograph to get an enlargement.

wo hundred million years ago a Jurassic fish rose to the water surface to gulp down an emerging mayfly. The fish wasn't a trout. It would take millions more years for nature to come up with the salmonids, but mayflies were on the piscatorial menu long before trout--or even dinosaurs--were around.

Although the ancient assailant may have had a momentary advantage, mayflies would have the last laugh. This fish and many piscean families of the period would eventually become extinct, but the descendants of those early mayflies would survive little changed into modern times. Of all the orders of winged insects living today, mayflies are among the oldest.

And in some ways, they are among the strangest. Mayflies live in two worlds--water and air--at different stages of their lives and change radically to meet the demands of the two environments. Their nymphal youth is a time of gills, a time to eat, grow and avoid being eaten. Adulthood, on the other hand, is a time of breathing air, of winged flight, and of brief but frenzied sex.

They share these traits with some other aquatic insects, but one peculiarity is theirs alone: among all living insects, they are the only ones with two winged stages separated by a molt. Entomologists refer to them as subimagos and imagos. Anglers call them duns and spinners.

Six-legged Adolescents

Since duns are young adults that have not quite reached maturity, they may be thought of as adolescents. Their bodies have taken on the basic appearance of adults but have still to develop their full capabilities. Once they leave the water, mayfly duns sit around in a languid state, doing little. (Not unlike, some cynics might say, many adolescents of all stripes.)

This state doesn't last long--one to two days at most. For the March Brown, Rhithrogena morrisoni, shown in the illustrations it lasted about 30 hours. Then a most remarkable thing takes place-the molt to spinner.

In the example pictured, the molt took about 5 minutes from beginning to end. So if you are watching a dun and hoping to see the molt, you have to pay attention or you will miss it. Trust me; I learned this the hard way!

To begin the molt, the dun slowly lowers its wings to the sides, then back along the body. The top of the thorax splits open and the new thorax emerges, followed by the head. It's amazing to see even the covering of the old "eyeballs" left behind during the process. A steady pull and the front legs are free, followed by the hind legs. The last step, extracting the tails, seems to be the hardest part, and many molted spinners drag around the shed casing on their tails for quite some time. One or both tails are often broken off during the process.

Wired for Sex

For mayfly spinners, it's all about sex. They don't stop to eat or drink; in fact, they have lost the nymphal mouthparts and digestive system that would allow them to do so. Reproductive activities are pursued with such vigor that at the end they have nothing left, and the spent spinners drop dead. The whole affair is usually over within a few days. These bugs are not called Ephemeroptera--think "ephemeral"--for nothing.

Back in the days before dinosaurs, adults of early mayfly-like insects were not so single-minded. The fossil record suggests that the primitive ancestors (Paleopterans) of today's mayflies had functioning mouthparts, and presumably digestive systems. Somewhere along the evolutionary line adult mayflies apparently "decided" that eating was a childish activity.

Why Make the Change?

So why two winged stages? Entomologists have been deliberating that issue for some time. No one knows for sure, but there are clues provided by differences in the structure and function of duns and spinners.

Some of the differences between the two stages are most apparent in the males of many mayfly species. Compared to duns, the forelegs and tails of male spinners are usually much longer and the eyes often much bigger, especially in the upper regions.

Mayfly mating typically occurs within groups of insects in the air, and the longer front legs are used by the males to reach up and grab females from below. The larger eyes on top of the head are probably useful for spotting females above them, in a sea of males.

Long tails are thought to provide additional stability to spinners in flight, acting as rudders that allow tricky aerial maneuvers.

Duns don't fly much after leaving the water, usually undertaking a short flight to the nearest tree or bush and settling in for a long rest. Spinner males on the other hand need all the flying help they can get as they often hover in groups waiting for a female to fly through, darting to grab her before another suitor does. It's mostly the conspicuous, amorous males that you see dancing in clouds over the water, but after mating in mid-air they usually fly off to expire on land. The females are the ones that end up on the surface and are eaten by trout when they come back to lay eggs and die.

Staying High and Dry

Well and good, you may say, but why not just go directly from nymph to spinner? The answer may lie in the requirements for transitioning from water to air. Most mayfly duns emerge from the nymphal exoskeleton in the water. Yet they do not get wet! That is, water does not adhere to them; they are "hydrofuge," in science-speak.

Being hydrofuge is important to an emerging dun because a wet mayfly may not fly (sorry; pun intended). Place a spinner, which does get wet, in water and it is usually stuck there for good. Drop in a dun and it promptly extracts itself and flies away. Once a dun pulls itself above the surface film, it floats high and dry (a possible clue to those tying imposters to imitate them).

The key to the dun's relative water resistance is in its outer covering. The wings, body and legs are covered with tiny hairs called microtrichia that repel water and prevent it from accumulating on the surface. (See illustration.)

While useful for water-proofing, microtrichia are considered to be a liability when it comes to flying. Witness the lumbering flight of most duns compared to the fast and agile aerial moves of a spinner. Hence spinners shed the "wetsuit" of the dun to reveal a slick wing and smooth body more suitable for flying than for exiting the water.

Making Sense of It All

So now it all begins to make sense. A winged insect emerging from water to air needs to stay dry-and we see the necessary equipment in the dun stage. An adult insect trying to beat out competing suitors for the most enticing mate needs to be adroit on its wings, in addition to possessing all the tools for interaction with the opposite sex-features of the spinner stage.

This explanation may not be exactly correct, but it represents a reasonable scientific hypothesis to fit the evidence. If the evidence changes, so may the explanation. In the meantime, it's all great fun delving into the intricacies of a life form so favored by one of our own favorites: hard fighting trout.

 
Arlen Thomason, Ph.D., is a "regular" in the Westfly Oregon forum and from time to time posts photos and descriptions of insects there. He was trained in biochemistry and molecular biology and worked in biomedical research for many years. He is now engaged as a freelance biologist and photographer and is working on a project to characterize the aquatic insects of his home water, Oregon's McKenzie River, and nearby areas.

Arlen Thomason lives on the banks of Oregon's McKenzie River

Uploaded 06/21/2005.

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Mayfly Dun (March brown). Click on photo to get an enlargement.

Becoming a spinner. Click on photo to get an enlargement.

Wing detail: Dun (left) and spinner (right). Click on photo to get an enlargement.