3. Notes on Dinacoma Casey, 1889:Will the real marginata please stand up?

My taxonomic review gained substance with the addition of the heretofore "unknown" female morphology. As I mentioned in Part 2., since the time T.L. Casey proposed the genus, well over 100 years ago, females have remained undescribed. As a result, prior taxonomic diagnoses of Dinacoma, including comparisons with other closely related genera, were based exclusively upon a composite of male morphological characters. This hadn't been a significant problem because only two species were considered to comprise the genus. So, to prior authors and collectors, it was either/or. However, some recent studies of other conterminous genera of Melolonthini, for example, Phyllophaga Harris (Woodruff 2004), and Polyphylla Harris (La Rue 1998), had shown that females have morphological characters that are supplemental to interspecific diagnostic criteria. I suspected the same in Dinacoma. My hopes were that they would assist in determining the taxonomic status of several extant demes that didn't quite "fit" the prior morphological definition of D. marginata as mentioned in Part 1.

Dinacoma caseyi Blaisdell
Male ~ Female
Desert Wash
Colorado Desert, Riverside County
Habitat of Dinacoma caseyi Blaisdell
Although provided with fully developed metathoracic wings, female Dinacoma do not exhibit flight behavior. Similar behavior has been observed in some species of closely related Melolonthini including Polyphylla Harris, Thyce LeConte, and Amblonoxia Harris, among several others. It has been posited that loss of flight is favored among females because it permits greater allocation of metabolic resources toward production of ova while flight is retained in males because it increases the probability of mate location. This is supported by the fact that males have larger antennal structures and longer lamellate antennomeres, therefore greater surface area. For this reason, males are presumed to have a superior ability in chemoreception of pheromones emitted by prospective mates.

Other theories suggest an evolutional preference to flightlessness where dispersal ability is not essential toward long term survival of species in ecologically stable environments ("environmental homogeneity" Roff 1990). Thus, the success of progeny is enhanced by preserving genomes that are favorable in the existing environment. Moreover, among desert insects, flightlessness may have evolved because dispersing individuals experience high rates of mortality as well as higher desiccation from flight activity (Wagner and Liebherr 1992).

End of Part 3.

Literature Cited
La Rue, D.L. 1998. Notes on Polyphylla Harris with a description of a new species. (Coleoptera:Scarabaeidae:Melolonthinae). Insecta Mundi 12(1/2):23-37

Roff, D. A. 1990. The evolution of flightlessness in Insects. Ecological Monographs 60(4):389-421.

Wagner, D. L. & Liebherr, J. K. 1992. Flightlessness in Insects. Trends in Ecology and Evolution 7:216-220.

Woodruff, R.E. 2004. Revision of the Phyllophaga of Hispaniola (Scarabaeidae: Melolonthinae). Insecta Mundi 18(1-4):1-154.

© Delbert La Rue 2011. All Rights Reserved.