Backyard Biology

Nature stories from my backyard and beyond

Nature Stories

Nature's Ornaments

Thursday, January 7, 2010

An excursion along the upper reaches of the North River in the dead of winter, visually, is a pretty bleak experience. The swamp maples, only recently blazing forth in the fiery reds and oranges of fall display, are now leafless and, apparently lifeless. Just a few dried twisted stems of last season’s wildflowers bear evidence of natures artistry. And the wild rice that so recently threatened to choke the river with its unchecked growth has completely vanished. There is not much to attract the eye along the river’s edge. Until one comes upon the Buttonbush.

The Buttonbush is a small tree - or large shrub – depending on your point of view. It grows to about 20 feet and is happiest right at the edge of the water, with its branches often providing the only shade for long stretches on a hot summers day paddle. In the dead of winter, however, there is no shade because, like other deciduous trees, the leaves have long since fallen.
I can still recall quite clearly, a number of years ago, paddling under a Buttonbush on a cold winters day and noticing what looked like Christmas ornaments dangling from its branches. They weren’t brightly colored of course; there is no real reason for producing bright colors in the dead of winter, but beautiful for their grace and delicacy none the less. They were a stark remindPromethia Moth chrysalis imageer on this bleak and barren winters day that nature was only biding its time before bursting forth in a frenzy of life with the need to fit a whole lifetime into one short summer’s season.

The ornaments dangling so delicately from the branches of the buttonwood are in fact the cocoons of one of our large silk moths – the Promethea Moth.
Swaddled in a protective layer of silk, which in turn is wrapped in a leaf anchored securely to a twig with more silk, the Promethea caterpillar is silently transforming its body into an adult moth. Disappearing are the prolegs that line the abdomen, useful for holding tight to the branches of its home. The chewing mouthparts also will be reabsorbed. They are no longer needed, for the moth will never feed again once it leaves the cocoon. The brilliantly colored tubercles that adorn the caterpillar will be lost as well, for though the adult Promethea is not brightly colored, she has a special beauty all her own.
Once these transformations are complete, the adult moth, still wrapped tightly, secretes a fluid that dissolves the silk threads at the top of the cocoon where she will force her way out, stretch her wings and take flight.

The female Promethea moth will lay a couple hundred eggs on the leaves of the trees that she has selected very carefully for her young caterpillars to eat. These include - as well as the buttonbush - wild cherry, sassafrass and spicebush. In their young caterpillar stages, they can be seen feeding lined up along the edge of the leaf, one next to the other. However, after a while, they will stray from their brothers and sisters and feed alone. Once fully grown, the Promethia Moth larva imagecaterpillars will fasten a leaf petiole securely to a stem, curl the leaf protectively around their body, spin a cocoon of silk, shed their last caterpillar skin and, now as a chrysalis, begin the transformation once again to the adult moth.
It all sounds so easy as I write about it, but this transformation is a very complicated process. So many things could go wrong in the myriad steps leading up to the formation of the adult moth that it is incredible that any moths ever emerge from their cocoons. And yet, perhaps the greatest challenge the promethea faces is the freezing temperatures of winter.

As I write this article, the temperature outside is about -4 degrees fahrenheit. Water freezes at +32 degrees fahrenheit. That means that it is 36 degrees below the temperature at which water freezes. In fact, the river behind my house has been frozen for some time.
The body of the Promethea chrysalis is about 3/4 water. Its cells are filled with water and much of the space between the cells is water. The only thing between the water in the moth and the 36 degrees below the temperate water freezes at is a little silk wrapping of the cocoon. So, by all rights, the water in the moths’ cells should be frozen solid. However, if this were to happen, the moth, and almost any other living creature, would die.

To see why ice in the cells would be fatal, try these two experiments:
On a cold winters day, go outside and breath on a window of your house. The moisture in your breath, on hitting the cold glass, will form ice crystals. These crystals, as you should be able to see, have very sharp edges. These sharp edges can wreak havoc on the delicate internal structures of the cell, possibly even rupturing the cell itself.
Place an unopened soda can in the freezer and leave it to freeze. When the water in the can turns to ice, it expands, rupturing the can. When water in a cell freezes, it too expands and ruptures the cell.
Both of these events would have a very negative effect on the moth chrysalis.

To stay liquid, and thus alive, during the winter, the Promethea chrysalis, as well as many other insects, employ a three part strategy.
As you may have noticed with your breath on the cold window, ice crystals like to form on other ice crystals. That is why your ice breath grew as one big crystal instead of many small crystals. Many insects produce special chemicals in their blood that attaches to the surface of any newly formed ice crystals and prevent them from growing and becoming dangerous to the cells.
During the winter, insects can build up high concentrations of sugars in their blood and cells. This lowers the temperature at which water will freeze - much the same way antifreeze works in the radiator of your car.
As stated earlier, ice likes to form on ice. If the insect gets wet from moisture in the air, this moisture can freeze and become a platform for the water within the insect to freeze. By staying dry, there is less likelihood of this happening. And while the silk wrapping of the cocoon is not very good insulation, it is very good at keeping the caterpillar dry, preventing ice from forming in the first place.

Employing these strategies, many insects can actually stay liquid at temperatures down to –40F. Four degrees below zero may seem mighty cold to us, but to the Promethea chrysalis, it is just another winter day.