In my first post about Monroe Canyon in the Pine Ridge of northwestern Nebraska, I featured the sumptuous Cicindela lengi (blowout tiger beetle) – a target species for the trip and one of six tiger beetle species that Chris Brown and I would find at this quarter-mile long sandy roadside embankment. Another species we found there was C. denverensis (green claybank tiger beetle), unexpected given its preference for clay soils, but like C. lengi also a target species and thus a welcome find. We also saw some more common species – the nominotypical forms of C. formosa (big sand tiger beetle) and C. scutellaris (festive tiger beetle), both reliable residents of sand habitats throughout the Great Plains, as well as a few individuals of the often ubiquitous C. punctulata (punctured tiger beetle). The most numerous of all, however, was C. purpurea audubonii (Audubon’s tiger beetle). Despite being uncommon in other parts of its range (in fact, nominotypical populations are feared extirpated in some parts of the eastern U.S.), C. purpurea audubonii is one of the most commonly encountered tiger beetles in grassland habitats throughout the central Great Plains and Rocky Mountains. This species belongs to a complex of several that are generally green in color and fond of clay soil habitats, such as C. denverensis, C. decemnotata (badlands tiger beetle), C. limbalis (common claybank tiger beetle), and C. splendida (splendid tiger beetle). All of these species can co-occur with C. purpurea audubonii in the Great Plains, but the latter is distinguished by its faint purple tinge and distinct metallic purple border around the edge of the elytra and its reduced elytral markings consisting of a short, oblique middle line and a white rear tip at the edge of the elytra (Pearson et al. 2006). All of these characters can be seen easily in classic pose in the above photograph.
Despite its commonness, however, I actually did look forward to seeing this species – the reason being the occurrence of occasional all-black individuals in the population. Tiger beetles as a whole are a variable lot – polytopism (geographically based variability) is the rule! Despite this, for the most part individuals within a given population usually exhibit a fairly uniform appearance. Occasionally, populations of some species – generally those at intergrade zones where different subspecies meet – will show variation on a continuum between two extremes. The occurrence of two distinct morphs within a population, however, is rather unusual. I lacked good field photographs of the all-black morph of C. purpurea audubonii (and also the green morph, for that matter), so I was pleased to encounter several individuals of the black morph while we were at Monroe Canyon. As can be seen in the above photograph, black morph individuals are truly all-black, perhaps with a purple reflection but without a trace of green anywhere on the body. They do retain the same pattern of reduced white markings exhibited by the green morphs. As a result, these individuals can be confused with some other black species that also occur in the Great Plains, such as C. nebraskana (prairie long-lipped tiger beetle) and, at higher elevations, C. longilabris (boreal long-lipped tiger beetle). These latter species were also targets for our trip, so we had to pay close attention to any black morphs that we saw to confirm their identity (C. purpurea audubonii black morphs are distinguished by their distinctly hairy frons).
I’ve not been able to find any additional information about these black morph individuals and the possible causes for their occurrence. Cicindela purpurea is a so-called “spring-fall” species, emerging in the fall as sexually immature adults and then spending the winter in burrows before re-emerging in the spring to mate and lay eggs. As spring-fall species go, it is one of the earliest to appear in the spring and last to disappear in the fall.¹ This immediately brings to my mind a possible thermoregulatory function. Low temperatures may be a challenge for the adults during early spring and late fall, and the black coloration could be an adaptation to maximize absorption of solar radiation for heat gain. This idea seems to be supported by the fact that the incidence of black morphs is greater at more northern latitudes and in the higher elevations of the western part of the subspecies’ range (as much as 20-40%), where overall lower temperatures would be expected to occur.
¹ Karl Werner even amusingly stated that this species “rather delights in chilly weather” (Acorn 2001).
Acorn, J. 2001. Tiger Beetles of Alberta: Killers on the Clay, Stalkers on the Sand. The University of Alberta Press, Edmonton, xix + 120 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 2011