When the little streams start to open up in late winter and early spring, the snow on either side may be dotted with small, dark objects crawling slowly along. Look more closely, and see that they are slim, winged insects that range in length from a few millimeters to about a centimeter. Sometimes there are hundreds of them, generally moving away from the stream into the surrounding vegetation. They may crawl fifty or a hundred feet or even more away from the water. After mating, females go back to the stream to lay their eggs.
These are the so-called “winter stoneflies,” of which there are reported to be about six species (in four different taxonomic families) in Southeast. The larvae are active in cold water. In fact, they apparently cannot handle warm conditions. After spending their larval lives in the water, they transform into adults and their emergence may be triggered by increasing day length. They emerge below the surface ice where lowered water levels have created a little space and then find small openings in the ice so they can move onto land. Adults are more cold-tolerant than the larvae and they are exposed to more temperature extremes. Most adults don’t live very long, perhaps a few days or a few weeks, and their main goal is reproduction. The adults of at least some of these species eat algae that grows on tree bark.
No one seems to know why these species emerge so early in the year, when temperatures regularly drop below freezing. They are rather conspicuous on the snow and would be ready prey for hungry birds — but there aren’t as many of those around in February and March as there will be in May, June and July, when ordinary stoneflies emerge and mate. So some researchers suggest that predator avoidance might make early emergence advantageous — leaving open the question of why more species don’t emerge so early. Other researchers suggest that some as-yet-undetermined negative conditions in the water might drive these species to metamorphose and emerge. Here is a ready-and-waiting research opportunity!
While the “winter stoneflies” are emerging and mating, the numerous other species of stonefly are still in the creek, as larvae. There has been a fair amount of research on the ways in which stonefly larvae (and many other invertebrates) cope with the cold of winter. It turns out that most Alaskan stoneflies are not especially tolerant of cold.
The salient exception is a stonefly called Nemoura arctica, the only Alaskan stonefly known to tolerate being frozen. This species develops slowly in Alaska, requiring at least two years of larval life before metamorphosing into an adult. Specimens were collected from the headwaters of the Chandalar River, where temperatures in the streambed in midwinter reached –12.7 degrees C (about 9 degrees Fahrenheit). Larvae collected in winter were encased in ice, but most of them survived and became active again when thawed. “Anchor ice” on the streambed is exposed to less extreme temperatures than surface ice, because it is buffered by flowing water.
For comparison, fewer stoneflies tolerate being frozen than midges (Chironomidae), danceflies (Empididae), and some dragonflies (in several families).
Freezing commonly kills organisms because ice inside cells expands and tears cell membranes, ruining the integrity of essential physiological processes. How, then, do dance flies, midges, dragonflies, and one stonefly manage to survive the ice?
Freeze-tolerant insects generally produce ice-binding proteins outside of the cells. Ice then forms around the cells in the extra-cellular spaces, taking up much of the water, so the remaining extracellular fluid becomes very concentrated. Water molecules then move from inside the cells to the outside by osmosis, and the cells effectively become partially dehydrated. In some cases, as much as ninety percent of body water may be lost. These insects run a risk of death from desiccation, if temperatures drop too low and too much water leaves their cells. Furthermore, being encased in ice necessitates being tolerant of low oxygen levels.
Freeze-tolerant insects also produce various types of antifreeze, often small sugar molecules or certain proteins that lower the freezing point of body fluids, protecting enzymes and lipid (fat)-containing parts of the cells (such as the cell membrane).
Still other insects, including some stoneflies, are not freeze-tolerant but would be very susceptible to freezing, except that they make use of the strange phenomenon called supercooling. Extremely small volumes of water (such as those in a small insect) can often be cooled many degrees below the normal freezing point of body fluids (i.e. about – 3 or – 4 degrees C) before freezing occurs. So, for example, a supercooled insect might survive a body temperature of minus fifteen or twenty degrees Celsius. These insects also produce antifreezes that limit the growth of ice crystals in the cells. The risk to supercooling is that virtually any contact can disrupt or circumvent the antifreeze function and allow formation of lethal ice crystals. For instance, soil particles, micro-organisms, or even food in the gut can serve as nuclei for deadly ice crystals. So, these insects can’t be very active at all.
Research has shown, however, that the majority of freshwater insects in Alaska try to avoid extreme cold by burrowing down into the substrate below the frost line, or remaining in habitats where water does not freeze. These larvae, including the larvae of “winter stoneflies,” stay active through the winter, feeding, molting and growing.
• Mary F. Willson is a retired professor of ecology.