Tiger beetles in Argentina’s Chaco forest

The day after I photographed Brasiella argentata on the mud/san banks of the Rio Paraná in Corrientes, Argentina, I decided to drive westward into the heart of Chaco Province. The destination: Chaco National Park, where some of the best remaining examples of the original “Gran Chaco” remain. Once covering nearly a million square kilometers in northern Argentina, Paraguay and Bolivia, this distinctive ecoregion has been largely converted to a vast, hot sea of cotton fields and mesquite fence-rows. A unique plant community in the Gran Chaco is the quebracho forest that takes its name from quebracho colorado chaqueño (Schinopsis balansae)—a tall, massively-trunked tree (related to, of all things, poison ivy!) with beautiful red wood that has been logged relentlessly wherever it occurs. Chaco National Forest is unique for the largely intact example of this forest it preserves and the mature quebracho trees that it contains.

Odontocheila chrysis (Fabricius, 1801) | Chaco National Park, Argentina

Insect life was not abundant as I walked the dark forest path. The lateness of the season (early April) and long-enduring drought occurring in the region had taken their toll, and I was content to see just about anything. At one point, a flash of movement caught my eye, and as I scanned the forest floor in the area where I had seen it, the familiar silhouette of a Golden Forest Tiger Beetle—Odontocheila chrysis (Fabricius, 1801)—became visible. I was already familiar with this species, having seen fairly good numbers of them at another location further east during my first visit to the area 12 years earlier. Long-legged and fast-flying, this tiger beetle occurs throughout much of South America, where it lives in more shaded areas of forest clear-cuts, secondary forests, savannas, and open scrublands (Erwin & Pearson 2008). The ground-dwelling adults are known to congregate along paths and at large openings on the forest floor, and indeed I had seen them in their greatest numbers on a shaded dirt road around the margins of a temporary mud puddle. When disturbed, the wary adults fly up from the forest floor to land in adjacent bare area of substrate or on the leaves of understory plants.

Superposition eyes are an adaptation to the dark forest environment where this species lives.

I faced a bit of a quandary when I saw this individual—do I collect it as a voucher and studio photograph backup, or do I go ahead and try to get the much more desirable in situ photograph of an unconfined adult in its native habitat. Considering that I had already collected a sufficient number during my earlier trip, I opted for the latter. I am fortunate that I got these two quite acceptable photographs before the adult flashed away in the blink of an eye right after I took the second shot, because I never saw another one the rest of the day or even the trip.  

An interesting feature of O. chrysis is its superposition eyes. In such eyes, each rhabdom (light sensitive unit) in the compound eye receives light through many ommatidial facets. This is in contrast to apposition compound eyes, where each rhabdom receives light from only a single facet. Superposition eyes are designed to increase photon capture, which is an advantage in the dark forest habitats where this beetle prefers to live (Brännström 1999).

Brasiella argentata (Fabricius, 1801) | Chaco National Park, Argentina

Shortly after photographing O. chrysis, I came upon a small opening where the path was a little wider and sunnier and the soil a little sandier and drier. Immediately I saw the small, zippy flits of the same tiger beetle species I had photographed the previous day on the banks of the Rio Paraná—Brasiella argentata. I could not find in the literature whether this species has superposition or apposition compound eyes, but considering that the species occur in great numbers on sunny river banks and that the few individuals I saw in the forest were in a sunny opening, I’m betting it’s the latter.

I couldn’t help but make another attempt to photograph this species, considering the difficulty I’d had the previous day (and that I wasn’t completely satisfied with any of the photos that I had obtained). More good fortune, despite there being only a few individuals to work with, as I managed to get the above photograph, which I consider far better than any that I already had. These beetles, too, quickly disappeared, and I never saw them again, but knowing I had the photos that I wanted made that okay.

Quebracho forest habitat for Odontocheila chrysis and Brasiella argentata.

REFERENCES:

Brännström, P. A. 1999. Visual ecology of insect superposition eyes. Unpublished Ph.D. Dissertation, Lund University, 142 pp. [abstract].

Erwin, T. L. & 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. [Amazon description, book review].

© Ted C. MacRae 2014