TRANSFORMATION

FOLLOWING WHAT appears to be an endless caravan of mayfly larvae swimming against the flow, a teeming migration departing from a scroll pond in the floodplain, I track my way up the little brook they are ascending. The streamlet has cut a narrow draw, down to stones, in a steep, forested slope that rises from the floodplain. Like most of the feeds to this serpentine coastal-plain river, this little brook appears to be seasonal. Two curiosities lead me on: where does the water come from, and where are these mayflies-to-be going? It is always intriguing to see where such a streaming issues from the earth, where it appears from subterranean runs to sparkle in the shifting lights of the forest and run silver and black in its shadows, enlivening the landscape with movement and, in spills here and there along its rocky descent, with murmurings and music. I am also drawn by the possibility, which rarely proves to be the case, that this slender springtime run emanates from a wetland large enough to support spotted or Blanding's turtles in the upland woods.

I am familiar with the migrations of mayfly larvae making their way against the drift of water throughout the gradually sloping wetlands, where they are emblematic of my first followings of water in a new season. Ascending from this farther floodplain later in the season, these larvae are quite likely a different species from the one I am so familiar with—it is not easy to make a definite identification of any one species of the 676 known to inhabit North America. These larval insects move with a determination and intimation of destiny that would seem more the province of spawning salmon. Minute bits of driven life, a living seasonal tide that is timed to move against a temporal streaming of water, they press on, swimming with their characteristic undulations, ceaseless, untiring. They are one of the inexhaustible living energies powered by an exceedingly minimal energy base, in this case bits of algae and decaying leaves. They will not be deterred by surges or tumbles of water in the uneven terrain or by debris dams of twigs and sodden leaves. In some level terraces along the run there are pools just deep and broad enough to harbor little green frogs and bullfrogs; they keep to outlying areas or headwaters in order to avoid larger members of their kind, who would make no distinction between them and anything else in their diet. How long is this column of larvae? It has lined the entire length of the draw along which I have walked, a distance of nearly half a mile. I did not join the procession at its beginning, but perhaps I can follow it to its end.

At length I do come to the end of my search, that is to say, to the beginning of the water. And here, too, the journey of the mayfly larvae has its end. The water's origin here is not a sphagnum-moss seep or rocky spring but a shallow shrub-swamp pool, thick with emergent winterberry. At times of abundant snowmelt or rain, groundwater wells to the surface at this interface, recharging a wetland basin high in the hills, filling it to the brim and over the brim, from which it flows on to follow the cut it has been carving in the landscape since the melting of the glaciers, down to the floodplain, for a final meander to join the river.

The pool teems with larvae. The air is filled with mayflies rising, pale-winged insects trailing long filaments, like tiny kites with three tails. I arrive at the site, and the moment, of a stunning transformation. As larvae continue to stream into the pool, subimagoes, as the mayflies transitional between larvae and adults are known, depart from it. They take to the air and fly short distances to alight on twigs and branches, where they will wait one more day for their final transition. At its culmination they shed one last skin, this time with delicate wing coverings, to go forth as fully formed adults, or imagoes.

Following this, one act of life remains: mating. They will not be feeding; in fact, they have no mouth parts. The males, great numbers of them, will unite in a swarm, filling the air above the pool and beyond, to perform a dance that may not last a day. The males of each species choreograph a unique aerial ballet, a synchronization of vertical and horizontal movements, cued to a certain time of day. The females of their kind recognize the visual and temporal pattern and fly into the swarm, where they are quickly seized by males. Immediately after mating, the females descend to the water to lay their eggs. All, males and females alike, die following this coupling in the air. A life of perhaps a year (two or three years in some species) as egg and larva ends in as little as several hours or one day of adulthood. The temporality that marks their order has led scientists to name it Ephemeroptera. They have their "day of wings," one might say, as the term is derived from the Greek ephemeros, lasting a day, and ptera, wings.

I have seen the migrations of larvae and the winged dances of adults throughout my familiar wetlands over the course of many years, but I have never before witnessed this moment of metamorphosis. I look intently at the swimming larvae and the subimagoes ascending from the dark surface. In a fraction of a second the split skin of a former life is left behind for that brief time in the air. I try to make out the actual dividing line, the instant of the aerial insect's emergence from the husk of the aquatic, gill-breathing larva. This is not the long-drawn-out, laborious work of a dragonfly extricating itself from the exoskeleton of a nymph. I follow some larvae as they rise to the surface and see them as they touch it, but I cannot see the actual departure: one moment there is a larva with its back just breaking the water, the next a mayfly in the air, with the shed skin floating below. Transformations, dividing lines—is there any greater metamorphosis, any narrower dividing line, than that of the invisible instant separating life and death?

Frogs are abundant and active throughout the pool, catching mayflies as they rise into the air or struggle upward through mazes of overhanging branches and sedges. A hermit thrush works the shoreline, snatching up larvae before they transform. This woodland bird becomes a wading bird, albeit in mere films of water. I have seen robins, red-winged blackbirds, and grackles, who know precisely the places and timings of these migrations, pursue larvae in this way each spring at the overflow of the vernal pool. Perhaps one thing that guides these long migrations is an adaptation to move to fishless waters for this elemental, vulnerable metamorphosis. But I see nothing that I could interpret as

Mayfly metamorphosis.

an effort to elude predators; nothing in this critical timing allows for delay or avoidance. It is not about the individual but about a species en masse, as one identity: numbers plotted against a common fate. In this communal going forth lies the persistence of an ancient order of life.