A few months ago I discussed Trichinorhipis knulli of the tribe Xenorhipidini (family Buprestidae). Members of this tribe exhibit highly sexually dimorphic antennae, with the distal segments of the male antennae highly modified into a very extended flabellate or lamellate condition. The surfaces of the flabellae/lamellae are covered with numerous, presumably olfactory sensillae that are lacking on female antennae (which retain the unmodified serrate condition), strongly suggesting a function involving detection of female sex pheromones. Although chemosensory structures are present on the antennae of nearly all buprestids, the extreme modification exhibited by the males of species in this tribe is not a common occurrence. Nevertheless, similar modifications have evolved independently in a few other genera within the family, including Knowltonia (four species in western North America), Mendizabalia and Australorhipis (monotypic genera in South America and Australia, respectively), and two species of the enormous Australian genus Castiarina. Indeed, males of Knowltonia and the two Castiarina species possess what might be termed ‘bipectinate’ or ‘biflabellate’ antennae due to dual projections from the terminal antennomeres (see Bellamy & Nylander 2007 for a more complete discussion of male antennal modifications in Buprestidae). The tribe Xenorhipidini is the most diverse group in which these modifications have arisen, comprised of the monotypic Trichinorhipis from California and the closely related Hesperorhipis (four species in Arizona and California) and Xenorhipis (until now, 14 species from North and South America and the West Indies).
Photos by Steve Valley (Oregon Department of Agriculture).
In a recent issue of the online journal Zootaxa, Rick Westcott (Oregon Department of Agriculture) describes a new species of Xenorhipis from the Cape Region of Baja California Sur, Mexico. Although assigned to the genus Xenorhipis, the new species – X. bajacalifornica – seems to bridge the gap between the genera Xenorhipis and Hesperorhipis. As currently recognized, Xenorhipis is distinguished from Hesperorhipis by the shape of the posterior coxal plates, which are scarcely narrowed laterally in the former genus, while in the latter genus they are triangular and with the hind margin strongly oblique. In X. bajacalifornica the posterior coxal plates are somewhat triangular but not as acute laterally as in some species of Hesperorhipis. Xenorhipis bajacalifornica also differs from other described Xenorhipis in its strongly abbreviated elytra, which in males barely reach the second ventrite – similar to species of Hesperorhipis. Other described Xenorhipis exhibit less abbreviated elytra, which cover at least the first three ventrites and in some species almost the entire abdomen. Despite these similarities to Hesperorhipis, a consistent distinguishing character between the two genera was found in the male antenna – in Xenorhipis the flabellar processes begin with the second antennomere, while in Hesperorhipis they begin with the third. It was on this basis that the new species was assigned to the genus Xenorhipis. (The genus Trichinorhipis differs from both Xenorhipis and Hesperorhipis by its rounded rather than quadrate pronotum and its unabbreviated elytra that cover the entire abdomen and has, as a result, been placed in its own subtribe.)
The photos left show the male (L) and female (R) of Xenorhipis brendeli, the only species in the tribe occurring in eastern North America (west to Minnesota and eastern Texas). Adults of this species are not commonly encountered and have been collected on a variety of deciduous hardwoods but reared almost exclusively from species of hickory (genus Carya). These individuals were reared from dead branches collected in southeastern Missouri – the male from pecan (Carya illinoensis) and the female from shellbark hickory (Carya laciniosa). The male exhibits the scarcely abbreviated elytra that cover almost the entire abdomen (as discussed above). Stan Wellso reported large numbers of males attracted to caged live females in Texas, apparently responding to sex pheromones released by the females.
This is another species in the genus – Xenorhipis osborni – known from west Texas. Joseph Knull described the species in 1936 from specimens collected in the Davis Mountains on whitethorn acacia (Acacia constricta), but larval hosts remained unknown until I reared a series of these specimens from dead branches of black acacia (Acacia rigidula) collected above the Pecos River in Val Verde County. I’ve also reared a few specimens from dead branches of catclaw acacia (Acacia greggii) collected in Big Bend National Park, and I wouldn’t be surprised if it breeds in other species of acacia. Again, in this speices the elytra are only slightly abbreviated, though more so than in Xenorhipis brendeli above and also more so in the male (L) than in the female (R). The male of this species is one of the prettiest I’ve encountered in the tribe.
The genus Hesperorhipis is illustrated here by these photos of H. albofasciatus. These specimens were reared by Rick Westcott from dead branches of walnut (Juglans sp.) – its only known host – collected in Tulare County, California. The elytra in this species are much more abbreviated than in Xenorhipis brendeli and X. osborni but similar to those of X. bajacalifornica – again with the male (L) exhiting greater abbreviation than the female (R). The three remaining species of Hesperorhipis exhibit even more highly abbreviated elytra than H. albofasciatus.
Dr. Charles Bellamy (California Department of Food and Agriculture) is currently revising the tribe. It will be interesting to see how, ulimately, he treats Xenorhipis and Hesperorhipis, given the blended characters exhibited by some species.
Westcott, R. L. (2008). A new species of Xenorhipis LeConte and of Mastogenius Solier from Mexico, with a discussion of Chrysobothris ichthyomorpha Thomson and its allies and notes on other Mexican and Central American Buprestidae (Coleoptera) Zootaxa, 1929, 47-68