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The caterpillars are categorized into two main groups based on the number of pairs of midabdominal prolegs -- less than four pairs, and four pairs. Each group is further divided by such obvious features as hairs, spines, color patterns, warts, and tubercles. A "worm" with more than four pairs of midabdominal prolegs is, with rare exception, something other than a caterpillar, namely a sawfly.

The groups of caterpillars may be approached through the use of the keys. The subgroups represent sets of species exhibiting similarities in appearance. The description of each subgroup can be used to find a best fit for identification of an unknown caterpillar. To identify a caterpillar, count the midabdominal prolegs and work through the Key to Families and Species Groups to a series of photos. Then locate those in the Photo Key to Species, and compare the caterpillar to the photos. While many of the caterpillars illustrated here are common, only a small percentage of the species in the Pacific Northwest are represented. Thus, you may collect a specimen that does not fit into any of the subgroups or match any of the photographs.