Extreme sexual dimorphism in Buprestidae: Xenorhipis hidalgoensis

Yesterday I received in the mail a package of specimens for identification from Dan Heffern in Houston, Texas. I first began corresponding with Dan about 25 years ago—not long after I myself became interested in Coleoptera, and as an avid collector of longhorned beetles (family Cerambycidae) Dan knows a thing or two about jewel beetles (family Buprestidae) as well. Over the years, Dan has come up with a number of great buprestid species from Texas, especially through rearing, many of which he has graciously sent to me examples for my cabinet.

Xenorhipis hidalgoensis (male, length 5.3 mm) | Hidalgo Co., Texas

The present package was no exception, as it contained a male/female pair of one of North America’s least frequently encountered buprestids, Xenorhipis hidalgoensis. Dan had reared them from dead branches of Condalia obovata collected in south Texas as part of a multi-year beetle survey in the Lower Rio Grande Valley. He had mentioned to me that he would be including them in the sending, but that did nothing to diminish my excitement upon seeing the specimens when I opened the box. The species was first described from Hidalgo Co., Texas (Knull 1952), and other than a single record from the neighboring Mexican state of Tamaulipas (Nelson et al. 1981) subsequent mentions of the species have been limited to catalogue listings and inclusion in keys to species. Thus, these specimens represent not only a nice addition to my collection, but also the first reported larval host for the species.

Xenorhipis hidalgoensis belongs to the tribe Xenorhipini, the males of which exhibit a distinctive specialization of their antennae in that the distal segments are highly modified into a very extended flabellate or lamellate condition. Females, in contrast, retain the serrate antennal condition that is more typical throughout the family.  Differences in antennal morphology aren’t the only sexual differences exhibited by members of the tribe, as males and females often exhibit differences in coloration as well. This is especially true in the genus Xenorhipis, and I’m not aware of a more striking example of this than X. hidalgoensis (compare these photos to those of male/female pairs of X. bajacalifornica, X. brendeli and X. osborni in A new species of Xenorhipus from Baja California).

Xenorhipis hidalgoensis (female, length 6.4 mm) | Hidalgo Co., Texas

The highly modified male antennae are interesting from both an evolutionary and functional standpoint. Similarly modified male antennae are found in a few other groups of Buprestidae, including the genus Knowltonia in western North America (four species), the monotypic genera Mendizabalia and Australorhipis in South America and Australia, respectively, and two species in the enormous Australian genus Castiarina (Bellamy and Nylander 2007). The Xenorhipini, however, with 20 species distributed throughout the Western Hemisphere, is by far the most diverse group exhibiting the condition. Despite the similarity of their antennae, the taxonomic distance between these groups and differences in the detailed structure of the numerous olfactory sensillae that cover the modified segments (Volkovitsh 2001) suggest that the modifications have arisen and evolved independently in these several disparate taxa.

All Buprestidae exhibit sensory structures on their antennae, presumably with olfactory and/or thermosensory functions; however, Wellso (1966) provided strong evidence that females of X. brendeli release pheromones that are highly attractive to males and detected by their elaborate antennae. Caging studies with virgin and mated females revealed that males were highly attracted to virgin but not mated females, and that mated males were not attracted to either virgin or mated females and died shortly thereafter.  Further, more than 80% of males were attracted within a 1-hour period just past midday. It is assumed that chemoreceptors for detecting female pheromones are located on the male antennae, as males with their antennae removed were not attracted to caged virgin females but were able to mate when caged with them. This suggests that the male antennae evolved in response to selection for greater surface area, which allows placement of more sensillae to detect female pheromone. Wellso observed also that adults are very short-lived, with no individuals living longer than 48 hours (perhaps due to reliance on pheromone attraction rather than longevity for mate location). This, along with their very short daily activity period, may explain  why adults of species in this tribe are so seldom encountered in the field.

Male with highly flabellate antennae

Female with unmodified serrate antennae



REFERENCES:

Bellamy, C. L. and U. Nylander. 2007. New genus-group synonymy in Stigmoderini (Coleoptera: Buprestidae). The Coleopterists Bulletin 61(3):423–427.

Knull, J. N. 1952. A new species of Xenorhipis from Texas (Coleoptera: Buprestidae). Entomological News 63(7):177–178.

Nelson, G. H., D. S. Verity, and R. L. Westcott. 1981. Additional notes on the biology and distribution of Buprestidae (Coleoptera) of North America. The Coleopterists Bulletin 35(2):129–152.

Volkovitsh, M. G. 2001. The comparative morphology of antennal structures in Buprestidae (Coleoptera): evolutionary trends, taxonomic and phylogenetic implications. Part 1. Acta Musei Moraviae, Scientiae biologicae (Brno) 86:43-169.

Wellso, S. G. 1966. Sexual attraction and biology of Xenorhipis brendeli (Coleoptera: LeConte). Journal of the Kansas Entomological Society 39(2):242–245.

Copyright © Ted C. MacRae 2012

About Ted C. MacRae

Ted C. MacRae is a research entomologist by vocation and beetle taxonomist by avocation. Areas of expertise in the latter include worldwide jewel beetles (Buprestidae) and North American longhorned beetles (Cerambycidae). More recent work has focused on North American tiger beetles (Cicindelidae) and their distribution, ecology, and conservation.
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11 Responses to Extreme sexual dimorphism in Buprestidae: Xenorhipis hidalgoensis

  1. The antenna are very interesting, but any reason for the color difference?

  2. The dorsal view of the male reminds m a bit of Pikmin, but this might be a bit of a stretch.

  3. Dave says:

    Very neat. Convergence and parallelism (if they are actually different) are a bane to understanding the history of life, but they are also fascinating. I wonder if there is a tendency for buprestids without hypertrophied male antennomeres to mate on the host and for the ones with large combs to need to track down females elsewhere? One would think that there is some correlate that has to do with mating and with some increased distance, more complex chemical habitat, or something similar.

    Well, maybe that is too much the need to tell a story. When I was a student, my advisor used to advise me that not everything you see on an animal is the direct result of selection. Tough advice to follow.

    • I’m sure your right – unfortunately we know nothing about where these guys are spending their time during their very brief life.

      Personally I have a tough time with the concept that energetically expensive structures can evolve without some type of positive selection to balance out the negative pressures.

      If this story gets picked up by the major science news reporting outlet (fat chance!), they’ll probably retitle it something like, “Male beetles evolve bizarre sniffers for smelling mates!”

  4. Jon Q says:

    Yet again i am happy to have paid a visit to your blog. I did a Lucanus mazama experiment and the males almost always chose the unmated female over the potentially fertile one. This might just suggest that there are ways a Beetle knows, probably through instinct, the wiser decision when dealing with mating.

  5. Henry Hespenheide says:

    I’d say that the REALLY extreme sexual dimorphism is in those parthenogenetic buprestids in which males do not exist. What’s more extreme than something vs. nothing?

  6. Dennis Haines says:

    I remember my excitement when first spotting our local member of the group, Hesperorhipis albofasciatus. I had a difficult time convincing my old entomology professor that they were indeed in Buprestidae. Since then I’ve encountered a number of specimens (usually females) coming to our fruit fly traps baited with methyl eugenol. One of these days I’m going to place some baited traps in better habitat and see what happens.

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