How Swallowtail Butterflies Survive The Winter
Fall is a hectic time of year in Montana. There are a thousand things to do before the first snow, and not nearly enough time. The days shorten, tourists leave, seasonal shops close, kids romp at the playground one last time, flower beds are mulched, snow tires put on the car, storm windows polished and mounted. You can feel the change in season just walking down the street: people hurry from building to building, head down and quiet, bundled up and hunched against the cold, looking for warmth, a fire, hot cocoa.Last fall, on a crowded sidewalk, I nearly stepped on a huge, ugly, disregarded caterpillar. The number of pedestrians on the sidewalk guaranteed it a quick death by squishing, so I picked it up. It was even uglier close up. An unfortunate shade of brown, two inches long, and half an inch thick, the caterpillar writhed in my palm. A huge vein ran the entire length of its back, barely under the skin, pumping what I could only assume to be some disgusting bug juice. However, in my hands it was a little safer and a little less overlooked because this caterpillar wasn’t just any caterpillar, it was a swallowtail butterfly larvae. It, too, could sense the coming of winter and was in a hurry to find a place to hole up, just like I was.
Winter is hard on animals for two simple reasons: it’s cold, and there’s not a lot of food. The easiest way to deal with these issues is to migrate, leave, go south where it’s warm and there’s plenty to eat. Monarch butterflies are well-known for their migration to Mexico where they gather to hang from trees in the mountains. But not every species can migrate. For most insects, the journey is too long and too dangerous, and they must find a way to survive here, in the bitter, dry cold.
Insects have some truly amazing ways of pulling this off. Some overwinter as eggs; their small size makes it harder for them to freeze. Some burrow into the ground, others into plant stems, effectively insulated from the cold and protected from predators. To further lower the danger of freezing, many insects make sugars like ethylene glycol in their blood, the same chemical used in many brands of car antifreeze. Yet others make proteins that bind to small ice crystals as they form, preventing the ice from growing larger, a method that is also used by fish in Arctic and Antarctic waters. At the other end of the spectrum, some insects don’t avoid freezing at all, and actually hasten the formation of ice in their bodies in a controlled manner that protects their cells from damage.
The particular species of swallowtail I had found spends the winter inside a chrysalis, also known as a cocoon, hanging from a tree where it is camouflaged from predators but otherwise exposed to wind, snow and sleet. Relatively safe in its little home, the developing butterfly survives without eating or drinking by lowering its metabolic rate to the bare minimum. To keep from freezing, it makes glycerol, which acts as an antifreeze in its blood.
Back on the street, I took out my knife and cut off a few small branches from a nearby tree. After walking home, I put the branches and caterpillar into a box. A few days later the larva had made its chrysalis and hung silently from a stem, looking more like a dead twig than a living organism, ready for winter. That was five months ago. It’s early spring now, and soon it will be warm and flowers will dot the hills. If everything has gone right, Brown and Ugly will soon be Yellow and Gorgeous, a welcome sight after a long, cold winter.
"Field Notes" is produced by the Montana Natural History Center.