Hiking a Juneau trail on a sunny August day we encounter a fat, inch-long fuzzy black and orange caterpillar. Within an hour we've seen more than a dozen. For a few weeks in late summer, these woolly bear caterpillars are everywhere. A little research reveals that they are not more abundant than normal, just more noticeable. There are two reasons.
Woolly bear caterpillars are the final larval stage of the spotted tussock moth, part of a larger group known as tiger moths. The caterpillar hatches from an egg, and the tussock moth caterpillar goes through five stages as it grows, these stages are called instars. The black and orange banded caterpillar is the fifth and final instar, and the only one with this distinct coloration. So these small black caterpillars don't look like woolly bears most of the time. In this final instar, the caterpillars are preparing to overwinter in a cocoon. They're on the move, looking for a good place, so they attract attention. Once the cocoon is built they will pupate inside and overwinter. Next spring they will emerge as adults.
In most temperate climates, caterpillars become moths within months of hatching, but in the north the summers are short, so the Woolly Bear must feed for several summers, freezing each winter before finally pupating. Some are known to live through as many as 14 winters.