It Makes Me Wonder

With the recent cold snap and significant dump of snow I am wondering just why they do it.

What makes some species of stoneflies, animals that are cold blooded and rely on favorable environmental conditions to maintain their metabolic activity, forego the warmth of summer and emerge under conditions that are marginal at best?

Emerging in the late winter or early spring exposes winter stoneflies to some very harsh conditions. Low temperatures mean that they are limited in their ability to move around; sudden changes in the weather can have life or death consequences; and there are not many places to hide out on the snowpack. Overall the risks are very high. Yet they have somehow managed to survive and thrive for hundreds of thousands of years. Clearly there must be some evolutionary advantage to their strategy.

Winter stoneflies do have several tricks that they use to survive the harsh conditions often encountered at this time of year. Most species are either black or some shade of dark brown. This allows them to absorb the maximum amount of the suns energy – both direct sunlight from above, and reflected light off the snow from below. Even on overcast days there is still enough UV light getting through to warm their bodies several degrees above the air temperature. At night winter stoneflies will take refuge in the thin layer of warm air under the snowpack. And even if their bodies do get chilled a few degrees below zero, the high concentration of glucose compounds in their body fluids prevent ice crystals from forming.

But all of these adaptations will only go so far. A sudden drop in temperature, especially to levels that are uncharacteristically low for the time of year (as we just saw), often results in at least some mortality. In addition, their dark coloration makes them an easy target when moving about on the snow. Add to this the fact that males lack functional wings and the odds seem to be stacked against them.

Utacapnia trava - adult (wingless) male with teneral female

So why emerge in the winter?

Bugs that emerge in the summer must run the gauntlet of a myriad of predators – everything from hungry trout and other predatory aquatic bugs, to birds, bats, and…well…everything else that eats bugs. A common emergence strategy is predator swamping (emerging en-mass) in hopes that at least a few will survive to carry on the bloodline. Winter is a more subdued time of year; there are few predators on the prowl and the trout are often found sulking in their winter lies. This allows winter stonefly hatches to trickle off over several weeks with minimal fear of becoming dinner for something else. The prolonged hatch period is added insurance against severe weather events that may wipe out a portion of the generation.

I am guessing that the recent cold snap took its toll on any winter stoneflies that have already emerged. But rest assured, the hatch will resume as soon as things warm up again.