Wild Side: How insects survive Martha’s Vineyard winters

Wild Side: How insects survive Martha’s Vineyard winters

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If you think you’re glad winter is over, imagine how insects feel. With hardly any ability to regulate their body temperature, insects cool and warm in concert with their surroundings. So how to survive cold weather is a central problem for any insect living outside the tropics.

It isn’t just a question of dealing with cold. Hibernating animals of many kinds slow their entire systems down enough so that they can conserve resources and survive even a prolonged chill. But when your body cools below the freezing temperature of water, you run into real problems: as ice crystals form, their sharp points damage membranes and delicate organs. Sure, you’ll thaw out when temperatures warm, but your innards will have been reduced to mush.

A few insects, like many vertebrates, avoid this fate by leaving. The most famous example is the monarch butterfly, migrating south and west to traditional wintering grounds in Florida, Mexico, or California. A few of our other butterflies migrate in a somewhat less organized manner; each year, their populations expand northward from a frost-free core range, so the area occupied by these species undergoes seasonal fluctuation. The northward push, in some species, is echoed by a southern migration in the fall. A few dragonflies exhibit this pattern, as well.

But migration requires enormous energy and mobility and demands some sort of navigational system, meaning that most non-tropical insects simply need to deal with freezing.

Two basic approaches have evolved. Some species avoid freezing in the first place, producing chemicals — literally, biological antifreeze — that lower the freezing point of the water in their cells and prevent ice from forming. The other approach is to make your body resistant to damage from freezing: for example, transferring water out of the body’s cells so that the cells don’t freeze, even if the fluid surrounding them does.

More help for surviving winter comes from the life cycles of many insects — the progression, or metamorphosis, from the very simple structure of an egg, to a somewhat more complicated larval structure, to the elaborate structure of the full adult. The simpler structures are less susceptible to damage, so it is no surprise to find that the vast majority of our insects overwinter in an immature stage of their development.

Which brings us to the interesting case of the grasshoppers, which employ what is termed partial metamorphosis. Once a grasshopper egg hatches, the immature insect closely resembles a small version of an adult, developing through a series of larger but basically similar stages until it reaches maturity. This is a marked contrast to the case of, say, a butterfly, where the larval form (a caterpillar) must metamorphose into a dramatically different adult form.

In keeping with the rule that simpler forms are less susceptible to damage, one might predict, correctly, that most grasshoppers overwinter in the simplest possible stage, as eggs (indeed, as eggs somewhat insulated from the cold by virtue of being underground). But a few grasshoppers do things the hard way, overwintering as half-grown nymphs, hiding under leaf litter, emerging when temperatures start to warm in spring, even becoming active on warm, sunny winter days.

The advantage of this system? Well, you get an early start in spring, with no competitors around and an ample supply of highly nutritious young plant growth to eat. But despite the advantages, the physiological challenge of this lifestyle is daunting, and hence this is a relatively rare strategy.

About 40 grasshopper species appear to be possible on the Vineyard, based on their geographical ranges (the number actually present here is probably lower than that). Of those 40, only four species are known to overwinter as nymphs.

One of these species, though, is common and widespread on the Vineyard: the northern green-striped grasshopper, known more precisely by its scientific name, Chlortophaga viridifasciata. Experiments have shown that adult Chlortophaga, though they look similar to nymphs, have lost the ability to survive freezing. But while still nymphs, these grasshoppers readily tolerate being repeatedly frozen solid and thawed.

Odds are good that if you check sunny, sheltered spots in dry, grassy habitat, you can find this species in late winter or early spring, while most of our other grasshoppers are still underground as eggs. (You won’t be the only one looking: the nymphs, less than half an inch long, seem to be a popular snack item among the grackles now migrating through, at least in my yard.)

I found my first Chlortophaga nymphs on March 7, an unseasonably mild day, as they were crawling and hopping around an untidy flower bed in my yard. As March progressed, they became increasingly easy to find, with the strengthening sun making them active even on chilly days. They’re a highly unusual case in the bug world, showing that there is more than one way to deal with a difficult season.

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