Where siblings mingle: Ellipsoptera marginata vs. E. hamata
November 25, 2012 14 Comments
When Erwin & Pearson (2008) formally broke up the great genus Cicindela by elevating most of its former subgenera to full genus rank, it caused a bit of consternation amongst some North American cicindelophiles. The argument went something like, “Now we have all these new genus names to learn, and we’ll have to relabel and reorganize everything in our collections, and how do we know the names won’t change again, and we can’t even tell them apart in the field anyway, and blah blah…” Pardon me, but since when did taxonomy become more about slotting species into fixed, easy-to-learn categories and less about best reflecting dynamic knowledge of complex evolutionary relationships? In the case of Cicindela and its former subgenera, however, even these arguments don’t hold up to close scrutiny—tiger beetle enthusiasts in North America should have already been quite familiar with the former subgenera due to their inclusion in the widely accepted Pearson et al. (2006) field guide, many of which actually do present a unique suite of morphological/ecological characters that facilitate their recognition in the field, and I personally find that nomenclatural recognition of individual lineages helps my attempts to learn and understand them much more than dumping them into a large, all-encompassing genus based on superficial resemblance. As for insisting that names don’t change, well that has never been a tenet of taxonomy. Stable, yes, but fixed and immutable, no.
Enough waxing philosophic. One of the more distinctive of the former subgenera is Ellipsoptera. Morphologically the genus is defined by details of male genitalia, but the 11 North American species are generally recognizable in the field by their relatively “bug-eyed” look and long legs (Pearson et al. 2006) and, as a group, seem ecologically tied to extreme habitats with sandy and/or saline substrates that are nearly or completely devoid of vegetation. Coastal marshes and mudflats, saline flats, sandy river banks, and deep sand ridges representing ancient coastlines are some of the habitats where species in this genus are most commonly encountered. Most of the species exhibit a fairly uniform facies but differ in the details of maculation and dorsal coloration, but two species that stand apart from the rest are E. marginata (Margined Tiger Beetle) and E. hamata (Coastal Tiger Beetle) due to the highly diffuse middle band of their elytra. These are both eastern coastal species and presumably represent sibling species that have diverged based on geographical range partitioning—E. marginata along the Atlantic Coast and E. hamata along the Gulf Coast. In the field, the two species are almost identical in appearance but nonetheless easily identifiable based on geographical occurrence. There is, however, a small stretch of coastline—the lower Gulf Coast of Florida—where the ranges of the two species overlap and geography alone isn’t sufficient for species determination.
Fortunately, despite their strong resemblance to each other, field identifications in areas where these species co-occur are still possible due to the presence of small but distinct sexual characters present in one species but absent in the other. Close examination is necessary to see the characters (or their absence), so it is best to net a few individuals and examine them in the hand or, as I have done here, look at them through the viewfinder of a camera. The photos in this post include the male and the female of both species, each showing the presence or absence of the distinguishing character.
In most tiger beetles, male individuals are distinguished by a number of secondary sex characters, but easiest to see in the field are the brush-like pads on the underside of the front tarsi (“feet”). Males of E. marginata and E. hamata are further distinguished from each other by the presence (E. marginata) or absence (E. hamata) of a distinct tooth on the underside of the right mandible. Photo 1 above shows a male E. marginata from Pinellas Co., Florida, and the tooth is easily seen in that relatively distant view. Photo 2 above shows a male E. hamata lacerata (Gulf Coast Tiger Beetle)—the Floridian subspecies, and while a small bump can be seen on the underside of the right mandible, it is not nearly as well developed into a distinct tooth as in E. marginata.
Female tiger beetles, on the other hand, lack the brush-like tarsal pads present in the males and are further distinguished by the “mesopleural coupling sulcus”—an area just behind the side of the pronotum that receives the male mandible during mating and is thus devoid of setae (compare the females in Photos 3 and 4 with the males in Photos 1 and 2). Neither E. marginata nor E. hamata females possess the mandibular tooth found in E. marginata males, but they can be distinguished from each other by their elytral apices. In E. marginata females (Photo 3), the elytra are curiously “bent” at the tips, forming a distinct indentation at the apex of the elytra where they meet, while female E. hamata (Photo 4) lack this indentation.
Are there other tiger beetle sibling species groups for which you would like to see comparative posts such as this one?
p.s. I completely neglected to mark yesterday’s 5th anniversary of Beetles in the Bush! I don’t know how I missed a milestone as big as five years—hopefully my ability to provide interesting content is faring better than my middle-aged memory!
Erwin, T. L. and D. L. Pearson. 2008. A Treatise on the Western Hemisphere Caraboidea (Coleoptera). Their classification, distributions, and ways of life. Volume II (Carabidae-Nebriiformes 2-Cicindelitae). Pensoft Series Faunistica 84. Pensoft Publishers, Sofia, 400 pp.
Pearson, D. L., C. B. Knisley and C. J. Kazilek. 2006. A Field Guide to the Tiger Beetles of the United States and Canada. Oxford University Press, New York, 227 pp.
Copyright © Ted C. MacRae 2012