Stenomorpha marginata and S. rimata are two closely related species of Tenebrionid beetles from southwestern North America.
Description
Stenomorpha marginata and S. rimata are both about 25 mm long (Casey, 1912; DJS personal measurements) with masses just over 1 g (S. marginata = 1.03 +/-0.23 g (Slobodchikoff, 1978); S. rimata 1.19 +/- 0.14 g (DJS)). Similar to other Tenebrionids that inhabit arid environments, their elytra are fused and the beetles are flightless.
They are members of the S. marginata species group, which can be distinguished in the field by the following (Keller, 2014).
Lateral margins of the pronotum are broad and rugose (white box in photo to the left). The pronotum also has a well-developed posteromedial depression (white arrow).
The elytra have a distinct marginal carina, i.e., a ridge running along the edge of the elytra (indicated by black arrowheads in photo). It begins at the humerus and can extend from the first one fourth of the elytron to nearly the entire length.
Distinguishing the two species seems difficult. The features used by Casey appear to overlap, and neither Keller nor Smith provide details. There is considerable variation in elytral morphology among species identified as S.marginata, and my best guess is that those of S. rimata are considerably more punctate.
Range
The two species are concentrated in Arizona and New Mexico, with S. marginata appearing to be more widespread. They are often found at the same locations, although there is some apparent geographical separation. It is unknown whether the areas of segregation indicate ecological specialization.
Habitat and Ecology
Like other Tenebrionids, Stenomorpha species are detritivores dwelling in semi-arid locations. Experiments have shown that S. marginata is palatable to predators, including mice and skunks. Based on their size and appearance, they have been suggested as Batesian mimics of Eleodes obscura, gaining protection from their similarity to the chemically-defended species (Hetz and Slobodchikoff, 1988; Smith et al., 2015). There is evidence that predators are capable of distinguishing S. marginata from E. obscura, and feed on Stenomorpha preferentially (Hetz and Slobodchikoff, 1988).
Both species are present late summer and early fall (S. marginata: Hetz and Slobodchikoff, 1988; Slobodchikoff, 1983; S. rimata: DJS unpub). Of the large Tenebrionids found in the Santa Fe area, S. rimata can be found latest in the season, up to late October.
Taxonomy
Phylogenetic analyses utilizing morphology and molecular markers (Keller, 2014; Smith, 2015) indicate that S. marginata and S. rimata are closely related and part of a species complex. There are currently 16 species recognized in the marginata species group.
Family: Tenebrionidae
Subfamily: Pimeliinae
Tribe: Asidini.
References
Bousquet, Y., Thomas, D. B., Bouchard, P., Smith, A. D., Aalbu, R. L., Johnston, M. A. and Steiner Jr., W. E. (2018). Catalogue of Tenebrionidae (Coleoptera) of North America. ZooKeys 728, 1–455.
Casey, T. (1912). Memoirs on the Coleoptera. III. Lancaster (PA): The New Era Printing Company.
Hetz, M. and Slobodchikoff, C. N. (1988). Predation pressure on an imperfect Batesian mimicry 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.
Keller, M. F. (2014). A revision of the North American subgenus Stenomorpha Solier 1836 (Coleoptera: Tenebrionidae: Asidini).
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. Environ. Entomol. 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., Vaughan, T. A. and Warner, R. M. (1987). How Prey Defenses Affect a Predator’s Net Energetic Profit. J. Mammal. 68, 668–671.
Smith, A. D. (2013). Phylogenetic revision of the North American Asidini (Coleoptera: Tenebrionidae). Systematic Entomology 38, 585–614.
Smith, A. D., Wilson, J. S. and Cognato, A. I. (2015). The evolution of Batesian mimicry within the North American Asidini (Coleoptera: Tenebrionidae). Cladistics 31, 441–454.