The genus Crossidius is limited to North America. Fifteen species are now known and all are presumed to feed in the larval stage on roots of perennial, shrubby, fall-blooming Compositae, especially Chrysothamnus, Haplopappus and Gutierresia. The distribution of the genus centers in the western half of the continent, principally in the Great Basin and adjacent areas. All of the species are sexually dimorphic and nearly all sexually dichromatic. Individuals of both sexes are variable in size, coloration, vestiture and sculpturing. The host range of all species is limited and within this narrow range varying degrees of specificity are expressed among species, subspecies and local populations. The broken topography of the Great Basin area and discontinuities in host plant distribution provide considerable isolation for many populations and a high degree of polytypy is evident in many species. The interpretation of color variation is complicated in some cases by what appears to be introgressive hybridization between species.
A key is provided to the known species, species and well-defined subspecies are described, and local populations of various kinds are characterized to the extent available material will permit.