Monday, October 26, 2015

Woolly Bears of Winter

Woolly bear- Bob Moul

Woolly bear playing defense
What could be more warm and fuzzy than finding a woolly bear caterpillar?  When I pick one up it will usually roll up into a tight defensive ball and lay still until it decides I am not going to eat it.  Then it straightens out and starts back on its journey to an unknown destination.  It is hard to know which end has the head until it starts moving again.  It doesn't seem to have a reverse gear.

While having a warm and fuzzy image, this caterpillar phase of the Isabella Tiger Moth, (Pyrrharctia isabella), is built for winter.  On a WOLF School march through Wilson's Creek Battlefield, the were a lot of bears on the trail, out searching for the perfect patch of duff for a winter home.

Adult Isabella Tiger Moth - Bob Moul
P. isabella pupa -
The caterpillar will undergo six molts (instars) before forming a cocoon and emerging as an adult moth.  It has only a few days to meet and mate before dying.  One or two generations each year go through their life cycle in a few months but the last generation overwinters as a caterpillar.  Its body produces the cryoprotectant chemical glycerol (antifreeze), letting it survive even after being frozen solid.  In the spring it will crawl out of hiding, eat a variety of plants for a few days and then pupate, forming a cocoon from which the moth will emerge.

Head - Wikimedia
A woolly bear has another challenge when it lives in the Arctic.  Due to the short growing season, food is available for only brief periods.  Each instar may have to face the winter before molting into the next instar.  Reaching adulthood may take several years. The record is an amazing 14 years!

The describes how the woolly bear got its reputation for weather forecasting.  Folklore says that the wider the orange band, the warmer the winter.  It turns out that the caterpillar does indicate the length of the winter, but the previous winter.  The longer the previous growing season, the wider the black bands.  A mild winter lets it get out and start eating early, giving the caterpillar a head start that year and a longer time to feed.

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