The first hard frost of the season — and the Island, by this point, has experienced several — marks a major change in the natural world. Wildlife unequipped to deal with sub-freezing temperatures has departed, gone dormant, or died. This means that the plants and animals still active here are members of a select club — organisms that have evolved specialized physiology or behavior that make cold temperatures tolerable.
Indeed, in the case of certain insects that are active in winter, temperatures around freezing aren’t just tolerable, they’re actively preferred. For these species, it’s warm weather, with its associated risk of overheating and desiccation, that is the danger.
If you keep a close eye on your porch light during the coming weeks, you may be able to spot a sterling example of this unusual lifestyle. December is peak season on the Vineyard for winter crane flies, a small, highly specialized group adapted to life in cold climates. Long-legged and seemingly fragile, adult winter crane are often attracted to lights, which makes them easy to find and observe.
Crane flies in general resemble oversized mosquitoes, which in fact are fairly close relatives of this group. But winter crane flies are in a different family — Trichoceridae — than our more familiar warm-weather crane flies. In addition to their penchant for cold, winter crane flies are distinguished from their summertime relatives by small size (about a half-inch long), the arrangement of the veins in their wing, and the presence of tiny light-sensing organs, called “ocelli,” on top of their heads (other crane flies lack these).
Trichoceridae is a small family, little studied even by entomologists. World-wide, about 160 species exist (next to nothing for a fly family); fewer than 30 species inhabit North America, most of them in a single genus, Trichocera. I don’t know how many species occur on the Vineyard; quite possibly there’s only one. Identification of this group, when it’s possible, depends on dissecting their genitalia under a microscope, and it’s not even clear that an adequate identification key exists for our region.
Whatever their precise identity, winter crane fly adults begin turning up at lights in mid-November and can be found in moderate weather through the winter. They never seem to be numerous, if the numbers turning up my lights are any indication, but they don’t seem fussy about habitat and are probably possible at any porch light on the Island. Despite the low temperatures, males and females manage to find each other to mate (some species reportedly concentrate into mating swarms, though I’ve never observed this).
Little else is known about their reproductive lives: eggs hatch in late winter or spring, larvae are scavengers that feed on decaying vegetation, and after a brief period of pupation, adults emerge again in fall. The small amount of research available on these insects suggests that winter crane flies are very flexible in the timing of their life cycle: larvae and pupae can speed up or slow down their development in response to changing conditions, and presumably do so in order to emerge as adults when conditions are most favorable for mating.
One area of study that has been pursued with these insects is their ability to function in the cold, which is truly remarkable. Some species have survived temperatures well below zero, and many are fully active and capable of flying in temperatures several degrees below the freezing point of water. Ours seem happiest in temperatures from the 30s to the upper 40s; when temperatures approach 60, adults risk overheating and are unlikely to be active.
The muscle tissues in these insects have probably evolved to contract well at low temperatures. And winter crane flies, like most other low-temperature insects, produce natural antifreeze: proteins that lower the freezing point of water and help prevent ice crystals from forming. The real risk to an insect in freezing conditions comes not so much from the cold itself, which a bug can often just sleep through, but from the formation of ice crystals within body tissues. These pointed crystals can pierce cell walls or damage cell organelles, and if you can keep the crystals from forming, cold weather survival becomes much easier.
As with other crane flies, adult winter crane flies have rudimentary mouth parts and eat little or nothing. Indeed, while they may have lively times when they’re out of sight, the ones I’ve seen appear to do little more than perch on a wall or window screen, waiting for a potential mate to arrive.
Given their quiet habits, obscurity, and seasonality, I don’t foresee a Trichoceridae fan club forming. These are not charismatic insects, and their roles in the ecosystem, as scavengers and not doubt sometimes as prey, are ordinary and probably easily filled by other species. But to my mind, there’s something elegant about an insect designed to flourish when all the world is frozen.