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AT A BISONS game one evening last week -- another win -- the air suddenly was saturated with mayflies. I could look up and see hundreds of them flying in the stadium lights, silhouetted against the dark sky. Fluttering slowly, they were easy to identify.

The more than 200 mayfly species in this region have minor variations, but most have narrow bodies about an inch long with two tails extending from the end of their abdomen for an additional inch. Their forelegs also are long and are held forward when they alight.

Later, en route home on Metro Rail, I noticed one perched like a stick pin on the blouse of a young woman standing next to me. After I pointed it out to her, she in turn showed me that one stood on my neck. It was so delicate that I had not even felt it.

Mayflies in mid-July? This was one more indication of how unseasonably late our spring has been. In some years, these insects even appear in April, when they are referred to by an alternative name, shad flies.

Despite those two names, these are not flies at all. True flies have a single pair of wings; mayflies have two pairs. Mayflies belong to an order all their own, Ephemeroptera, a name that means ephemeral wings. Indeed, the winged stages of this unusual insect are very brief. As adults, they survive only a day or two, some only hours. They can't even eat: They have no mouths. During this brief life, their sole function is to mate, and the females lay eggs. Nature has conditioned them to mature all at the same time so that these processes can be carried out effectively. They, therefore, arrive in huge swarms, or what entomologists call "blooms."

The appearance of those insects in this region is not at all unusual. I recall once driving across the Rainbow Bridge in Niagara Falls in a cloud of mayflies so dense that the bridge lights appeared dim. It was just like driving in a thick fog. Before I could slow down, my windshield was covered with squashed insects, and I could feel the car begin to lose traction on masses of their slippery bodies. At the end of the bridge, I had to stop to clear the car windows. Yet a short distance from the river, no more mayflies were to be seen.

Like many area cottagers, I also have witnessed windrows of their fetid dead bodies along Lake Erie beaches. Those eggs that are deposited in the water will hatch in a few weeks, but the nymphs or naiads they produce -- wingless but also with two tails -- will live on the bottom of the river or lake for a year or two before emerging.

During that time, they will eat underwater vegetation and sometimes other insects, and many will molt more than 20 times as they increase in size. Finally, in another spring, they will transform into subadults with wings and leave the water to dry. (Among all insects these are the only winged immatures.) After a day or two, they will molt a final time into adults, and another bloom will occur.

Mayflies are near the bottom of the food chain and form an important part of the fish diet. Those who tie flies for fishing know these insects well, and many of their lures mimic them.

Although they can be a brief nuisance in the spring, we should welcome these harmless insects. They are especially sensitive to pollution, and we can hope that their reappearance in such numbers signals continued water quality improvement.

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