During last year’s extended visit to Argentina, I had the chance to spend the early part of April in the northern province of Chaco. Though much of this hot, arid plain has been converted to agriculture, remnants of thorn forest remain along fence rows and in small patches of Chaco Forest. Despite the decidedly tropical latitude of the region, however, the profuse bloom of Chilean goldenrod, Solidago chilensis, along these fence rows during the Argentine autumn is reminiscent of crisp fall days here in the eastern U.S., and like the goldenrod here the ubiquitous stands of yellow blossoms stretching across the Chaco Plain are equally attractive to a multitude of insects. Among those insects are the Cerambycidae, or longhorned beetles, and while the eastern U.S. cerambycid fauna of goldenrod boasts only a few (albeit spectacular) species in the genus Megacyllene, the Argentine cerambycid fauna that I found on these flowers included at least three species in various genera belonging to two different tribes.
Two of the species I saw are shown here, and their similarity of appearance is no coincidence, as both belong to the tribe Rhopalophorini (coming from the Greek words rhopalon = club and phero = to bear, in reference to the distinctly clavate, or club-shaped, legs exhibited by nearly all members of the tribe). In fact, a great many species in this tribe exhibit the same general facies—slender in form and black in coloration with the head and/or pronotum red to some degree. Since all of these species are diurnal (active during the day) and frequently found on flowers, one can assume that the members of this tribe represent an example of what Linsley (1959) called ‘group mimicry.’ In this simple form of Batesian mimicry (harmless mimic with protected model), a group of related species within a genus or even a tribe have a general but nonspecific resemblance to those of some other group of insects—in this case presumably small, flower-visiting wasps. Although the tribe is largely Neotropical, the nominate genus Rhopalophora does extend northward with one eastern U.S. representative, R. longipes. Among the numerous species occurring in South America, the individuals I saw in Argentina can be placed as R. collaris due to the relative lengths of their antennal segments and uniquely shaped pronotum (Napp 2009).
The second species could easily be mistaken for another species of Rhopalophora were it not for the distinct tufts of hair surrounding the middle of the antennae. These tufts immediately identify the beetle as a member of the large, strictly Neotropical genus Cosmisoma (derived from the Greek words kosmos = ornament and soma = body, a direct reference to the tufts adorning the antennae of all members of this genus). Three species of the largely Brazilian genus are known from Argentina, with the black and red coloration of this individual easily identifying it as C. brullei (Bezark 2o13). In the years since this genus was described, additional related genera have been described that bear remarkably similar tufts of hair not on the antennae, but on the elongated hind legs. The great, 19th century naturalist Henry Walter Bates “tried in vain to discover the use of these curious brush-like decorations” (Bates 1863), and nearly a century later Linsley (1959) conceded that their function still remained unknown. Antennal tufts are actually quite common in Cerambycidae, especially in Australia, and while experimental evidence continues (to my knowledge) to be completely lacking, Belt (2004) records observing “Coremia hirtipes” (a synonym of C. plumipes) flourishing its leg tufts in the air (presumably in a manner similar to waving of antennae) and, thus, giving the impression of two black flies hovering above the branch on which the beetle was sitting. This seems also to suggest a function in defense, with the tufts perhaps serving as a distraction to potential predators in much the same way that many butterflies have bright spots near the tail to draw the predator’s attention away from the head.
Bates, H. W. 1863. The Naturalist on the River Amazons. Murray, London, 2 vols.
Belt, T. 2004. The Naturalist in Nicaragua. Project Guttenberg eBook.
Linsley, E. G. 1959. Ecology of Cerambycidae. Annual Review of Entomology 4:99–138.
Napp, D. S. 2009. Revisão das espécies sul-americanas de Rhopalophora (Coleoptera: Cerambycidae). Zoologia (Curitiba) 26(2):343–356.
Copyright © Ted C. MacRae 2013