Beneath the Surface book coverA little excerpt about mayflies–

Many aquatic insects, especially mayflies, are indicators of good-quality water. Their larvae, which live at the lake bottom, need oxygen, so if the water is polluted, they will no longer be present. In fact, their reappearance in lakes and rivers is proof that pollution control is working. Around some lakes and large rivers, mayflies can be so numerous that they need to be removed with snow shovels. In 1996 an enormous mayfly hatch blew from Lake Erie into Toledo, Ohio, and almost paralyzed the city due to the masses of slippery creatures that covered the roads. I once parked a car under a light pole behind our cabin late in June, and the next morning it was covered with mayflies that had also completely packed the ventilation system. In addition, caddis flies had somehow gotten into the supposedly hermetically sealed instrument panel–and remained there for the duration of the car’s life. Although I spent several hours picking mayflies out of the vents with a long forceps, the car smelled of dead mayflies for several years when it rained.

Most mayflies are associated with clear streams and rivers, but a few species inhabit lakes as well. The most prominent mayfly in our northern lakes is one of the largest species, Hexagenia limbata, commonly called the fishfly or the giant Michigan mayfly. The adults, which can reach 1 1/2 to 2 inches from the top of the head to the tip of the tail, are among the largest of all the species of mayflies. To most people living near water and to lake anglers, mayflies are most noticeable in their final adult stage, when they lie spent on the water or collect below lights on land. But, like so many animals, the bulk of a mayfly’s existence is spent as a feeding larva hidden in a burrow in the lake bottom. This is all preparation for the final flurry of its life cycle–the brief period when it emerges as a mature winged mayfly whose entire brief existence is devoted to reproduction.

The life cycle of the giant mayfly begins when the female drops her 4,000 to 8,000 fertilized eggs onto the surface of the water, usually just after dark in the late spring or early summer. The eggs are heavier than water and sink to the bottom. There, their sticky surface coat attaches them to rocks or aquatic vegetation, where in two to four weeks, depending upon the water temperature, they hatch into larvae (nymphs), which are initially less than a millimeter long. Mayfly nymphs live in 6- to 12-inch U-shaped burrows under silt or sand bottoms in lakes. They spend much of the daytime in their burrows and come out at night or during periods of very low light, when they feed upon various particles of organic detritus that they run across. During the two to three years that they spend as nymphs, they both grow in size and develop new body parts through a succession of as many as 40 to 50 molts. With each molt they shed their exoskeleton and then grow a new covering as their overall structure becomes more mature. Biologists call each stage between molts an instar. Actual molting may take 5 to 10 minutes. The new instar is pale and somewhat larger than the former one and remains inactive for a few hours after molting to let the new exoskeleton harden. When in their final stages, the larvae are about 1 1/2 inches long. Alongside the abdominal segments are feathery gills, which in addition to a respiratory function, create water currents in the burrow through regular pulsations.

Responding to signals that are still incompletely understood, large numbers of nymphs in the same lake undergo a rapid series of major changes that bring them from their long period of larval life to their mature winged form. The most visible external change is the development of wingpads, which presage the ultimate emergence of the wings of the adult form. Internally, even more striking changes occur. The most sweeping change is the degeneration of the entire digestive system, including mouthparts, since the adult mayfly does not eat. While losing their digestive tract, the late instars gradually develop genitalia in anticipation of the reproductive functions of the adults. The body cavity remaining after degeneration of the digestive tract often becomes filled with gas, which facilitates the rise of the emerging larvae from the bottom sediments to the surface film, often tens of feet higher in the water column. These same gasses also enable more efficient flight by reducing the specific gravity of the adult mayflies. The final stages of mayflies’ emergence is the stuff of trout fishing lore because at these stages, especially in streams, mayflies are especially vulnerable to predation by fish, which often go on wild mayfly-feeding binges. Large hatches also occur in lakes, and in very infertile northern lakes many of the predatory fish, such as walleyes, move in on the mayfly hatch and absolutely gorge themselves on emerging mayfly larvae and adults to the virtual exclusion of other foods.

During their metamorphosis from larva to winged adult, the mature nymph, which has already undergone many of the changes leading to adulthood, rises to the surface of the water during the late evening. There it undergoes another major metamorphic change. Its hard outer casing splits longitudinally down the back (thorax), and out from the larval casing emerges a sexually immature winged mayfly called a subimago (or dun by the trout angler, because of its dull color). Through the split emerge first the wings, followed by the thorax, head, abdomen, and finally the three-pronged tail of the dull-colored, but otherwise adult-looking, subimago. The nymphal casing, which is left floating on the water and is very visible during mayfly hatches, is called the shuck.

Lacking in mouthparts and a digestive system, the newly hatched mayflies must be careful to preserve body moisture during the precarious few days of their adult existence. The newly emerged subimagos fly from the surface of the water to shaded vegetation near the water, where within two to four days they undergo a final molt to their sexually mature adult form (imago). A prominent change is the transformation of the dull-colored wings of the subimago to the glassy transparent wings of the imago. The sexually mature Hexagenia is yellow to tan in color with brownish markings. Cabin owners alongside lakes that have mayfly hatches may notice shed casings of the subimago skin on porch screens. Mayflies are the only insect forms to undergo a second molt once they have developed wings.

Once a full-fledged adult, a mayfly may have only hours of precious existence in which to perpetuate its lineage. At this stage its only remaining function is to reproduce. How they do so is fascinating. For most species of mayfly, the males fly around in swarms above the water, and then the females are somehow attracted to these swarms. How this attraction occurs is a mystery, however. Some biologists think that there is a behavioral element (a male dance), but I suspect the males secrete a pheromone (a chemical attractant that can be detected over great distances) that encourages females to enter the swarm. Once within the swarm, the female is seized by a male, and the two copulate in midair.

Mating is facilitated by huge eyes that take up much of the facial profile in the males. The eyes of females are of more usual dimensions. Upon fertilization the biological usefulness of the males is over, and they fall exhausted onto the water, where they are easy pickings for hungry fish. Spent mayflies may still flap their wings for a while and are called spinners by trout anglers. After copulation the females retreat to shaded vegetation for a short time while the fertilized eggs inside them begin to ripen. Then the females fly over the water and deposit their eggs, initiating the next cycle in the mayflies’ multimillion-year history.