Winter may seem harsh — especially for us New Englanders — but take a moment to consider how much worse it would be if you were an insect. The laws of physics would be conspiring against you. Insects are very small, and the smaller an object the more surface area it has relative to is volume (this is because volume increases as a cube and surface area increases as a square). It is through this exposed surface area that animals lose heat to the outside world. In short, insects have very little mass to hold heat and a proportionately huge amount of surface area across which to lose heat.
Not to mention insects are ‘cold-blooded’ and generally lacking in the ability to maintain their own body temperature. Furthermore, food supplies — in the butterflies’ case, flowers — are extremely limited in winter. I could keep painting this picture, but I think we’ve all got an intuitive understanding that winter is a difficult time for life, and butterflies and their kin are in an especially difficult position.
As it turns out, Lepidoptera (the order of insects comprising butterflies and moths) have devised a few tricks to deal with winter.
One of the most striking features of Lepidopteran biology is their distinct life stages: egg, caterpillar (larva), pupa, and butterfly/moth (adult). They pass from their larval form to their adult form by undergoing a complete metamorphosis. Imagine if, instead of just growing some awkward body hair, your puberty had involved completely remaking your body and sprouting wings. Aside from sheer wow-factor, this complete metamorphosis has had a huge impact on the success and adaptability of Lepidoptera (and insects in general).
Because they have three distinct life stages, butterflies have been able to adapt their life cycles so that one of these three stages is specially equipped to deal with winter (‘winterized’, if you will), whereas the other two stages don’t need to maintain the costly investments into winter survival.
In most cases, the butterflies themselves don’t even survive winter. They live only a few months to lay their eggs, then they die off, and leave caterpillars or pupa to survive the winter. The familiar woolly-bear (i.e. tiger moth caterpillar) is one such winterized life stage. It waits out winter as a frozen, fuzzy little hockey puck hidden away under a thin layer of leaf litter.
Woolly bears (caterpillars of the tiger moth) use chemicals to endure freezing solid in wintertime.
Another species, the eastern tent caterpillar moth, has winterized eggs. It leaves bundles of its eggs — full of glycerol as an anti-freeze agent on exposed twigs all winter long. They brave the elements and hatch is spring.
Still others have winterized pupa. One such example is the cecropia moth. Their caterpillars build big bundles of silk and leaves to serve as cocoons and winter shelter, passing the lean seasons in this transitional state to emerge as adults when the sunshine and warmth return.
The real rarities are the Lepidoptera who actually overwinter as adults. Three quick examples come to mind. Some nocturnal noctuid moths are actually active in winter. On comparatively mild nights (that’s right, nights), they shiver with their powerful flight muscles to raise their body temperature up to active levels. Then there are mourning cloak butterflies, which take refuge in hollowed-out trees and other shelter to wait out the winter.
Mourning cloak butterflies overwinter as adults, which is uncommon among Lepidoptera.
Of course, I’ve saved the best for last. Unlike with birds, long-distance seasonal migrations are actually a rarity in butterflies. Which is what makes the monarch butterfly so remarkable. Every fall, all the adults from the eastern half of N. America — up into Canada — make a journey south to some cool hilltops near Mexico City to overwinter in gigantic clusters on the trees there. Perhaps most remarkable, it actually takes them several generations in the spring to travel all the way back up to the northernmost part of their range, but all of the monarchs will make the trip south in a single shot come fall.
Monarch butterflies are remarkable for their long-distance seasonal migration.
All of this variation in adaption is present just in the order Lepidoptera, and it’s but a sampling of the mind-boggling diversity displayed by insects. There’s a reason they are a dominant, if-easy-to-overlook, life form here on Earth.
Shameless self-plug: if this answer has whetted your appetite, check out the more formal exploration of insect survival in New England winter as published here.
Winter World by Bernd Heinrich
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