The purpose of this stulty was to investigate the biology of as many U.S. skippers as possible, compare morphological features of their immature stages, and produce standardized descriptions. This revision summarizes of our current knowledge of immature stages of North American Hesperiidae, and provides a framework for future taxonomic and systematic studies.;About 300 species of skipper butterflies (Hesperiidae) have been reported from the United States. Most of the fauna is composed of the subfamilies Pyrginae and Hesperiinae, but Pyrrhopyginae, Heteropterinae, and Megathyminae are also represented.;Species of Hesperiidae found in the U.S. feed on 31 families of plants as larvae. These include 8 families of monocots and 23 families of dicots. Most (92%) of these butterflies are limited to only 1 of the 31 families of plants, but a few skipper species feed on plants in 2, or very rarely, as many as 6 different families. Species having relatively broad host ranges include Lerema accius (35 spp.), Epargyreus clarus (31 spp.), Thorybes pylades (27 spp.), and Megathymus yuccae (27 spp.). One hundred and thirty-eight new host records are reported for 55 species of hesperiids. Some skippers are important pests of beans, rice, sugarcane, and pasture grasses. Host plants remain to be discovered for 22% of the U.S. Hesperiidae.;The eggs of 49 species, the larvae of 156 species, and the pupae of 120 species of Hesperiidae of the U.S. are described in this thesis. These include the eggs of 16 species, the larvae of 41 species, and the pupae of 35 species that were previously unknown. Twenty-six percent of the U.S. skippers are totally unknown with respect to their egg, larval, and pupal stages. About half of these species with undescribed immatures are breeding residents. The other half are neotropical species that rarely enter the U.S. and probably do not reproduce within its limits.;Larval characters that may prove useful to systematists conducting phylogenetic analyses include the type of mandible articulation, setal types, sculpturing of the head, lenticle distribution on the larvae, wax gland patterns, and pigments. Differences in the sculpturing patterns of hesperiid eggs have been known for some time, but have not been extensively surveyed. Promising characteristics of the pupae include the shape of the pupal cap, lenticle distribution, relative length of the antennae and proboscis, cremaster shape, type of thoracic spiracle guard, and presence of crenulations on the posterior margin of the prothorax.
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