While I may have already declared Plinthocoelium suaveolens (bumelia borer) as North America’s most beautiful longhorned beetle, any short list of top candidates for this title must also include the species Crossidius coralinus. Like most other members of this strictly North American genus, these gorgeous beetles emerge as adults during late August and September to feed on the profusion of yellow blooms put forth by their larval host plants, Ericameria nauseosa (gray rabbitbrush). Across the Great Basin and adjacent areas, the relatively large size, spectacularly long antennae, and stunning value contrast between red/black or orange/black beetles, yellow flowers, and blue skies combine to make the sight of C. coralinus a highlight on any fall insect collecting trip. If beauty alone isn’t enough, the species also exhibits an unusual level of polytopism across its range. Red in some areas (e.g., C. c. temprans), orange in others (e.g., C. c. monoensis), bigger or smaller, and varying degrees of development of the black areas that cover the basal edge and apical portion of the elytra, the species segregates into several described subspecies and many more unnamed but locally distinct populations. This post features photos of individuals from several populations that field-mate Jeff Huether and I encountered during last August’s Great Basin collecting trip.
One of the most impressive populations I’ve encountered is illustrated by the male and female individuals shown in the above photos, which were seen near the city of Cortez in southwestern Colorado. Linsley & Chemsak (1961) assigned specimens from this area to the nominate subspecies, characterizing them as “moderate-sized”; however, some of the individuals that we encountered at this site were truly gargantuan (exceeding 20 mm in length). Note how extensive the black areas are in these individuals, especially the female.
On the other side of the state near Fort Garland (southeastern Colorado) we encountered a population that Linsley & Chemsak (1961) considered representative of the subspecies C. c. jocosus. In contrast to the larger size and extensive black markings of the nominotypical population we found near Cortez, individuals in this population were considerably smaller in size and exhibited less extensively developed black areas of the elytra. Their small size also made them a little harder to notice—perhaps that is the reason we found so few individuals (~7 total at several sites along Hwy 160). We did note also, however, that the gray rabbitbrush flowers seemed to be well past their prime, so perhaps an earlier appearance of the rains upon which plant flowering and beetle emergence rely had us on the tail end of their activity period.
Linsley & Chemsak (1961) noted several populations across middle and southern Utah, but the only one to which they assigned a name was C. c. coccineus in Washington Co. (southwestern Utah). While we didn’t visit Washington Co. on this trip, we did look for these beetles at several sites north of Monticello in San Juan Co. (southeastern part of the state). Geography would place this population close to nominotypical populations, and while the beetles in this population resembled them in size they clearly differed in the greatly reduced black areas of the elytra. Note the male especially, with the black area reduced to little more than a sutural stripe in the apical half of the elytra. Linsley & Chemsak (1961) related specimens collected just a few miles further south from ours to an unnamed population in adjacent Wayne Co. (near Hanksville), both of which seem to be close to C. c. coccineus due to their robust size and the reduced black elytral markings of the male.
Crossidius coralinus is found commonly along the western edge of the Great Basin in the form of C. c. temprans. However, Linsley & Chemsak (1961) presented very few records of the species further east in Nevada. We stopped at several spots in central Nevada while traveling along Hwy 6, but despite an abundance of gray rabbitbrush stands in peak bloom we found but a single male and a single female, the latter shown in the above photograph. Geographically this female should be assignable to C. c. temprans, but the black area of the elytra is not nearly so expanded as is typical for that subspecies. The only record from central Nevada in Linsley & Chemsak (1961) is a single male from White Pine Co. (a little further to the east), but they related that specimen to an unnamed population near Marysvale in Piute Co., Utah of smaller size and with the black area of the elytra distinctly expanded in both sexes.
There are those who say “Subspecies, schmubspecies!” I concede they may be right for a large number of named subspecies, possibly including C. coralinus, and while the basin and range topology of the Great Basin and discontinuous distribution of host plants within that geography provide ideal conditions for the development of distinctive, geographically based populations, I suspect C. coralinus has sufficient mobility to allow gene flow across its range (with the possible exception of populations in California’s Central and Owens Valleys). Moreover, the inability of Linsley & Chemsak (1961) to segregate the central Great Basin populations into discrete taxonomic units suggests that the subspecies concept may not be applicable. Nevertheless, it cannot be denied that distinctive, localized populations of this species do exist. Moreover, I hesitate to dismiss subspecies in problematic taxa such as C. coralinus, because doing so makes it easier to ignore variability and presume (possibly incorrectly) no geographic component. Variability is interesting and should be thoroughly evaluated to determine its basis regardless of its basis. Geographically based variability is especially interesting because it suggests the existence of distinct genetic traits that contribute to the genetic diversity of species. Such traits are valuable to protect, and the use of subspecies provides a convenient shorthand for referring to the populations that contain them in both taxonomic and conservation contexts.
Linsley, E. G. & J. A. Chemsak. 1961. A distributional and taxonomic study of the genus Crossidius (Coleoptera, Cerambycidae). Miscellaneous Publications of the Entomological Society of America 3(2):25–64 + 3 color plates.
© Ted C. MacRae 2014