Although I want people looking up caterpillars they see in September to find information about the Bear Caterpillar family who grow up to be Tiger Moths, the good news is that, in North America, none of these species can be considered a real pest. Most are general feeders who eat mostly weeds and don't stay on one plant long enough to do any damage. None is economically important. Most have short bristly hairs that can be irritating; none of ours can really be said to sting. Bear Caterpillars are shaggy (and spend the winter sleeping in tiny dens), and most Tiger Moths have colorful black, yellow, white, brown, or pink stripes.
Although I'd like to be more specific about this family, the fact is that it's hard to find the kind of specific information about them that's available about pest species like gypsy moths. Many people think the black-and-brown-banded "Woolly Bears" are cute, but, once people learn that there are no stinging spines hidden in their coats, few people are really interested in the Bears. Months have passed since I e-mailed my county extension office to request any information they could share about Bear Caterpillars, beyond what everybody in Scott County already knows. They've not replied.
What Internet searches for these species shows is that there are dozens of different species that all look and behave somewhat alike, and not much information is out there about the differences between them. I've known for a long time that some of the big harmless "Black Bears" have black skins underneath their bristly, brush-like, all-black hair, and some have reddish skins. Are those two distinct species? If anybody knows the answer to that question, please e-mail or comment; I didn't find any answers on the Internet.
Which of the Tiger Moths do Black Bears mature into? Again, if the information's out there, I didn't find it.
Black Bears are the ones that can be two or three inches long, and are dark and hairy enough to be confused with our not really confusing pests, the tent caterpillars, gypsy moth caterpillars, and stingingworms. Once you know that Black Bears have long stiff straight black hair and are active in autumn and winter, you're not likely to confuse them with the pest species. Black Bears look like little bottle brushes on legs. They feel like brushes, too, should you ever inadvertently touch one. The hair is stiff and prickly and protects the caterpillar from contact with the rest of the world.
But there are lots of other caterpillars in the family. Although they continue eating and growing during the winter, and individuals reach different sizes, Black Bears are probably the biggest--some may be three inches long. A few varieties of Brown Bears, and the Woolly Bears, can also be more than two inches long. Most of the Bears are smaller. Some are also more colorful. I'm guessing that the gaudy little specimen Kayre photographed is one of the smaller Bears...
John Fowler's gallery contains something similar that he classifies as an Arctiid, although he also calls it a "Milkweed Tussock Moth." Some experts use "Tussock Moths" to refer to a different scientific family:
Butterfliesandmoths.org is generally a good place to find images and descriptions of moths. Their page for a common species found in North America and Europe will work as a portal:
If you have a lot of time to spend on this project, you can look up the hundreds of other species in the family. A few of the species names are synonyms; for example, Woolly Bears, or Isabella's Tiger Moths, are listed as either Isia isabella or Pyrrharctia isabella.
In general, Tiger Moths fly and mate in early summer. Eggs may hatch in as little as two weeks, but young caterpillars usually stay well hidden. Once in a while one sees a very small Bear caterpillar in July or August. Usually we see the Bears when they're close to their full size, in September up until frost. When the weather gets cold they burrow into tiny shallow underground dens. Throughout the winter, Bears come out to nibble on weeds during warm afternoons. They're always gambling on being able to get back into their holes before they freeze, but somehow enough of them do that the species survive. Mature Bears pupate in spring and emerge as moths in May, June, or July.