Eleodes obscura is a large, common, and widespread Tenebrionid beetle. It may be the most well-studied species of Eleodes, with more than two dozen papers describing its taxonomy, behavior, ecology, and metabolism.
Description
At 28-35 mm in length and around 1.4 grams (Lease et al., 2012; Scilman et al., 2008; Slobodchikoff, 1983; Triplehorn et al., 2015; my own observations), E. obscura is one of the largest Tenebrionids in its habitats. The following details come largely from Triplehorn et al. (2015).
The pronotum is relatively smooth and convex. The anterior angles of the pronotum are obtuse, meaning that they are rounded at the edge. This contrasts with the sharp angles of E. sponsa, which overlaps in range and is otherwise quite similar. There is a bead around the edge of the pronotum, but the edges are not at all reflexed. The prothoracic femora of males are dentate, possessing distinct spines, whereas those of females are sinuate.
Elytra sides are rounded and without marginal bead, which distinguishes E. obscura from E. acuta. Patterning of the elytra varies by subspecies:
Elytra have little or no indication of striae in E. o. dispersa (four corners region) or E. o. glabriuscula (west Texas). Elytra of both subspecies are irregularly punctate, but those of E. o. dispersa are muricate whereas E. o. glabriuscula is coarsely punctate.
Elytra are sulcate in E. o. sulcipennis (intermountain region) and E. o. obscura (Great Plains). Intervals are convex in E. o. sulcipennis, flat in E. o. obscura.
Range
Eleodes obscura comprises four subspecies, and is widespread throughout western North America.
Habitat and Ecology
[cribbed from Triplehorn]
Ecological studies include Wise (1981) on interspecific competition and population size, Kenagy and Stevenson (1982) who related body temperature to seasonality and daily activity rhythms, Slobodchikoff (1983) on temperature preferences, Marino (1986) on activity period and microhabitat, and Whicker and Tracy (1987) on ambient conditions and activity period. Rogers et al. (1988) studied the diet. With regard to habitat, LaRivers (1943) found them to be common on sand dunes in Nevada and Tanner and Packham (1965) trapped them in abundance both in Grayia-Lycium and Pinyon-Juniper habitats. Evidently they hide in animal burrows in the daytime because Wickham (1890) encountered them frequently at entrances of prairie dog holes and Hyslop (1912) similarly states that they are usually found around ground squirrel and badger burrows. Cress and Lawson (1971) described the defensive repertoire against predators, and yet Slobodchikoff (1978) described how captive skunks can learn to eat them.
Taxonomy
Subfamily: Blaptinae
Tribe: Amphidorini
Subgenus: Eleodes.
References
Bernett, A. (2008). The Genus Eleodes Eschscholtz (Coleoptera: Tenebrionidae) of Eastern Colorado. Journal of the Kansas Entomological Society 81, 377–391.
Brown, K. W. (1971). A population approach to computer taxonomy with applications in the genus Gonasida. Ph.D Thesis. U.C. Riverside.
Hetz, M. and Slobodchikoff, C. N. (1988). Predation pressure on an imperfect Batesian micicry complex in the presence of alternative prey. Oecologia 76, 570–573.
Hetz, M. and Slobodchikoff, C. N. (1990). Reproduction and the energy cost of defense in a Batesian mimicry complex. Oecologia 84, 69–73.
Johnston, M. A., Fleming, D., Franz, N. M. and Smith, A. D. (2015). Amphidorini Leconte (Coleoptera: Tenebrionidae) of Arizona: Keys and Species Accounts. The Coleopterists Bulletin 69, 27–54.
Kaiser, A., Klok, C. J., Socha, J. J., Lee, W.-K., Quinlan, M. C. and Harrison, J. F. (2007). Increase in tracheal investment with beetle size supports hypothesis of oxygen limitation on insect gigantism. Proc. Natl. Acad. Sci. U.S.A. 104, 13198–13203.
Kenagy, G. J. and Stevenson, R. D. (1982). Role of Body Temperature in the Seasonality of Daily Activity in Tenebrionid Beetles of Eastern Washington. Ecology 63, 1491–1503.
Lease, H. M., Klok, C. J., Kaiser, A. and Harrison, J. F. (2012). Body size is not critical for critical PO2 in scarabaeid and tenebrionid beetles. Journal of Experimental Biology 215, 2524–2533.
Marino, P. C. (1986). Activity Patterns and Microhabitat Selection in a Desert Tenebrionid Beetle (Coleoptera: Tenebrionidae). Annals of the Entomological Society of America 79, 468–471.
McIntyre, N. E. (1999). Use of Pogonomyrmex nest-sites by Tenebrionid beetles (Coleoptera: Tenebrionidae) for oviposition and thermoregulation in a temperate grassland. The Southwestern Naturalist 44, 379–382.
Schilman, P. E., Kaiser, A. and Lighton, J. R. B. (2008). Breathe softly, beetle: Continuous gas exchange, water loss and the role of the subelytral space in the Tenebrionid beetle, Eleodes obscura. Journal of Insect Physiology 54, 192–203.
Sheldon, J. K. and Rogers, L. E. (1984). Seasonal and Habitat Distribution of Tenebrionid Beetles in Shrub-Steppe Communities of the Hanford Site in Eastern Washington. Environmental Entomology 13, 214–220.
Slobodchikoff, C. N. (1978). Experimental Studies of Tenebrionid Beetle Predation By Skunks. Behav 66, 313–322.
Slobodchikoff, C. N. (1979). Utilization of Harvester Ant Debris by Tenebrionid Beetles. Environmental Entomology 8, 770–772.
Slobodchikoff, C. N. (1983). Water Balance and Temperature Preferences, and Their Role in Regulating Activity Times of Tenebrionid Beetles. Oikos 40, 113.
Slobodchikoff, C. N. and Wismann, K. (1981). A Function of the Subelytral Chamber of Tenebrionid Beetles. Journal of Experimental Biology 90, 109–114.
Slobodchikoff, C. N., Vaughan, T. A. and Warner, R. M. (1987). How Prey Defenses Affect a Predator’s Net Energetic Profit. Journal of Mammalogy 68, 668–671.
Stapp, P. (1997). Microhabitat Use and Community Structure of Darkling Beetles (Coleoptera: Tenebrionidae) in Shortgrass Prairie: Effects of Season Shrub and Soil Type. American Midland Naturalist 137, 298.
Tanner, V. M. and Packham, W. A. (1965). Tenebrionidae beetles of the Nevada Test Site. Brigham Young University Science Bulletin 6, 1–41.
Thomas, D. B., Smith, A. D., Triplehorn, C. A. and Aalbu, R. A. (2014). Walker’s Eleodes (Coleoptera: Tenebrionidae). Zootaxa 3835, 583–592.
Triplehorn, C. A., Thomas, D. B. and Riley, E. G. (2009). The Genus Eleodes Eschscholtz (Coleoptera: Tenebrionidae) in Texas. The Coleopterists Bulletin 63, 413–437.
Triplehorn, C. A., Thomas, D. B. and Smith, A. D. (2015). A Revision of Eleodes Subgenus Eleodes Eschscholtz (Coleoptera: Tenebrionidae). Transactions of the American Entomological Society 141, 156–196.
Uhey, D. A., Riskas, H. L., Smith, A. D. and Hofstetter, R. W. (2020). Ground-dwelling arthropods of pinyon-juniper woodlands: Arthropod community patterns are driven by climate and overall plant productivity, not host tree species. PLoS ONE 15, e0238219.
Uhey, D. A., Bowker, M. A., Haubensak, K. A., Auty, D., Vissa, S. and Hofstetter, R. W. (2022). Habitat Type Affects Elevational Patterns in Ground-dwelling Arthropod Communities. Journal of Insect Science 22, 9.
Whicker, A. D. and Tracy, C. R. (1987). Tenebrionid beetles in the shortgrass prairie: daily and seasonal patterns of activity and temperature. Ecological Entomology 12, 97–108.
Wise, D. H. (1981). Seasonal and Yearly Patterns in the Densities of Darkling Beetles (Coleoptera: Tenebrionidae) in a Montane Community. Environmental Entomology 10, 350–358.