In one form or another – eggs, larvae, pupae, nymphs, adults – insects are as much with us when it’s below zero as they are in July. Some, tucked away, become inactive before the first frost; others gather in metabolically warmed colonies or exist in a transitional stage.
One insect, however, the winter stonefly, finds an upland stream’s frigidity as acceptable in January as in summer. This intrepid insect crawls out of the water onto an exposed rock at midday in midwinter, its dark body absorbing the sun’s warmth to raise metabolism necessary for shucking its nymphal skin. It briefly takes wing, mates, and completes its life cycle, all in the course of a few short days.
Now is the time of year to be on the lookout for these unusual insects. Emergence begins in January, reaches a peak in February, and tapers off in March, the schedule determined and timed by increasing day length and an almost imperceptible rise in water temperature. An adult winter stonefly is about an inch in length, has long, overlapping wings folded flat on its back, and has a dark, flattened body. A pair of antennae faces forward, and a matched set of short tail filaments, called cerci, extends off the back.
For most of the year, winter stoneflies (also called boreal willowflies, Taeniopteryx nivalis) live in brooks in their nymphal stage, undergoing as many as 30 molts per year. In the partly frozen brook near my house, these sturdy nymphs clamber over stones and under rocks all winter, actively foraging for food. Stonefly nymphs have strong, biting mouth parts, and this winter species is mainly carnivorous, scurrying around after midge larvae and mayfly nymphs, though, if these aren’t plentiful, algae and plant fragments will do.
Single, fingerlike gills protrude from the winter stonefly nymph’s underside at the bases of its legs. The gills are quite small compared to the size of the insect’s body, meaning that winter stoneflies can only live in clean, highly aerated water. Their presence in large numbers indicates that a brook is healthy and relatively pollution-free.
Though there are some 400 different North American stonefly species, only 22 are found in Vermont and New Hampshire, and only two species hatch at this time of year. Any stonefly nymph can be recognized by its two long, straight tail filaments and flattened body and is easily distinguished from similar mayfly nymphs, which have three wide tail filaments.
In the water, a stonefly nymph’s outstretched legs reach out onto a rock surface like grappling hooks, each leg ending in a large, two-clawed foot. The legs themselves are flattened and held against the rock to offer as little resistance to the current as possible. The insect’s head and long body also present a very low profile, allowing the stonefly nymph to scuttle like a crab in the rock/water interface, where friction retards the current. It never rises on tiptoe, for to do so would mean being plucked off and whirled downstream.
When the full-grown nymph crawls out of the icy brook to shed its nymphal skin, a split appears down the middle of its back, and the body swells as air in its tracheal tubes inflates inner tissues. Holding on with its front feet, the emerging adult withdraws its long tail filaments from their thin sheaths, then the legs from theirs. The wings, previously existing only as pads on the nymph, are now exposed and begin to expand and harden in the wintry air. As the adult appears, it leaves behind the lining to major tracheal tubes and part of its gut, which remain attached to the rock along with the shed exoskeleton.
Once it’s time to fly, the winter stonefly takes wing slowly and awkwardly, alighting on bare bushes and tree trunks near the stream bank. Stoneflies tend not to fly much and, if disturbed, may scurry off instead. By midday, however, large numbers of dark adults will be fluttering short distances in preparation for mating.
The insects engage in courtship duets, males drumming their bodies against the surface they’re on, females responding with a drummed “I am here.” Mating soon follows.
The following day, females fly heavily back to the brook and drop large packets of eggs into the water. The eggs swell, separate, and are swirled by the current until they become caught under stones. Mortality is extremely high: a single female lays several thousand eggs, and though there are many females at work, barely enough young survive to ensure continuation of the species.
Winter being winter, the adult winter stoneflies don’t last much longer and never make it far from their nymphal brook home. They may nibble in desultory fashion on blue-green cyanophytes growing on tree bark, but they soon succumb to the cold, their work and lives complete.