The lepidopteran fauna on goldenrod, Solidago altissima, in central New York is extraordinarily diverse (63 species in 43 genera and 13 families, representing 61% of the total insect fauna on S. altissima), as determined through over four years of sampling at 43 sites. To best illustrate the functional structure of the community, caterpillars are divided into both feeding and sheltering guilds. We studied patterns in caterpillar size, feeding guild, sheltering guild, host range, phenology, abundance, and taxonomic relationships to gain insights about community assembly on this group of native caterpillars on a prominent native host species. Size distribution among caterpillar species is highly skewed toward the small end and does not differ between caterpillars with broad and narrow host ranges. There are relatively few species that feed on stems and rhizomes, suggesting challenges in adapting to those plant parts. Host range is a highly conserved trait in several lineages of goldenrod feeders (relatives of polyphagous species are polyphagous, and relatives of specialists are specialists). Polyphagous species and exposed-feeding species are usually rare; some specialist species are also chronically rare. There is a dearth of species in mid-summer, between the time that many leaf-folding and leaf-tying species complete development and the time that flower-feeding species appear. Species with high intimacy of association (gallmakers, stem borers, and leaf miners) tend to be specialists, as compared with exposed feeders, and they are necessarily small in size. Among exophages, size relations are constrained by phylogeny. Lineages have characteristic behaviors and means of association with the host; as a result, the structure of the fauna is determined by an interaction of ecological factors and evolved constraints in the lineages that have adapted to feed on Solidago.