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In this guide, family order follows Covell (1984); within families, similar-appearing caterpillars are grouped together. Each family is introduced with a synopsis that includes a diagnosis. To facilitate field identification, our descriptions emphasize macroscopic characteristics, although a hand lens is helpful when examining arrangements of the crochets on the abdominal prolegs and other minute features. Many families of Lepidoptera are characterized most readily by the arrangement of their setae ("chaetotaxy"), but we did not attempt a discussion of such because our effort would have fallen well short of what is already available in Stehr (1987). For heterogeneous groups like the owlet moths (Family Noctuidae), our family description is valid for many species, but recognize that exceptions are legion. In genera in which 2 or more species are depicted, general features or characteristics of the genus are discussed in the first species account.

For each species, both a common and scientific name are provided. When a different common name exists for the larval and adult stages of the same insect, we list the name for the larva, and include the name for the adult in brackets after the scientific name. For those few instances where a common name was not available, we created one that seemed appropriate given the species' larval morphology or habit. Species accounts include a telegraphic description of diagnostic characters that are present in the last larval instar. Descriptions are based on our slide images and literature cited in the back of this book—forms of highly variable species may not be identifiable. Many identifications in this guide are based on wild larvae that did not complete their development and yield an adult—we use question marks where some uncertainty remains about a species' identity.

Following the diagnosis, we often make mention of species that are likely to be mistaken for the one we describe. Occasionally we include comments on a species' natural history, especially if this information might be useful in identification. Food or host plant associations are extracted from the literature that we list in the back of this guide. We are less inclined to make mention of particular host genera and species for general feeders that are known to accept many food plant species. Detailed information on the number of yearly generations is lacking for many species, and hence our accounts are approximate.