There are thousands of species of insects that begin life as caterpillars. It is nearly impossible to identify them without seeing them, and even then it can be quite difficult. Features to note are whether they are smooth, hairy, bristled, or spiny; presence or absence of knobs, bumps, a rear horn, fleshy filaments, or tufts; sluglike or more typical caterpillar form; and host plant.
Here are some resources that may help:
Caterpillars of Pacific Northwest Forests and Woodlands
Caterpillars of Eastern Forests
Web Images of North American Moth Species
Caterpillar Hostplants Database
We hope this helps,
eNature Naturalists
