(c) Anthony K. Grafton 2003

Wooly Bear Winters

Have you recently noticed fat, hairy, brown and black caterpillars crossing the roads? These are wooly bear caterpillars, and they’re out looking for a nice place to spend the winter.

Wooly bears are the larval form of the brownish-yellow Isabella Tiger Moth (whose scientific name is Isia Isabella). In the caterpillar form, they spend their time eating grass and other small plants. Two rounds of offspring are produced each year by the adult moths, and the later group (the ones seen crawling around now) overwinters in the caterpillar form. In the spring, the caterpillar will emerge from its winter rest and pupate (make a cocoon for itself) and then metamorphose into an adult moth.

It has long been a superstition that these caterpillars can be used to predict the weather. People who believe this say that the thickness of the middle brown band on the caterpillars is related to the severity of the upcoming winter. Unfortunately, there’s no scientific evidence to support this idea.

In reality, experiments indicate that the width of the brown band on wooly bears is related to the conditions in which the caterpillar has lived as well as the caterpillar’s maturity. So the wetter the conditions have been and the more mature the caterpillars are, the thinner the brown stripes will be.

Superstitions like this can arise by drawing conclusions from too little data. For instance, if you walk under a ladder and experience bad luck later that day, you might begin to believe the superstition that walking under ladders is bad luck. But consider how many other days you might experience bad luck without ever having walked under a ladder and you’ll quickly realize there’s no connection between the two. Similarly, if you’ve been thinking about someone and they suddenly call you on the phone, you might think you’re psychic, but how many times have you thought about people who didn’t call?

So somewhere in the past, someone noticed that where he lived the wooly bear caterpillars had thin brown bands (or thick ones, depending on the version of the superstition), and later that year the area had a bad winter. From this limited set of experiences, a superstition was born. Do you know of any other common superstitions?