Aphaenogaster occidentalis ( Emery, 1895 )

Shattuck, Steve & Cover, Stefan, 2016, Taxonomy of some little-understood North American ants (Hymenoptera: Formicidae), Zootaxa 4175 (1), pp. 10-22 : 13-14

Aphaenogaster occidentalis ( Emery, 1895)

Stenamma (Aphaenogaster) subterraneum occidentale Emery, 1895: 301 . Syntype or holotype worker(s), Pullman , Washington [46°44′N 117°11′W] , United States (T. Pergande) (probably Museo Civico di Storia Naturale, Genoa, not seen) [types of junior synonym A. subterranea borealis Wheeler, W.M. 1915 examined: 13 syntype workers, Lardeau (as Lardo), Kootenay Lake , British Columbia [50°09′N 116°57′W] , Canada (J.C. Bradley) (Museum of Comparative Zoology) [http://www.antwiki.org/wiki/ Aphaenogaster _ occidentalis ]].

Aphaenogaster subterranea valida Wheeler, W.M., 1915: 411 . Worker, queen and male syntypes, North Cheyenne Cañon, near Colorado Springs [38°48′N 104°53′W], Colorado (W.M. Wheeler) (Museum of Comparative Zoology) [http://mczbase.mcz.harvard.edu/guid/MCZ:Ent:20609].

Creighton (1950) considered A. occidentalis and A. valida to be geographic “races” of A. subterranea , the former occurring from California and Nevada northward through Oregon and Washington and into British Columbia while the latter occurred from Colorado and Wyoming northward into Montana and then west into southern British Columbia . He reported a broad area of integration between the two forms, from British Columbia, eastern Washington, northern Idaho and western Montana, where they showed greatly increased variability. He separated the two based on size, A. valida ranging up to 6mm in length while A. occidentalis reaching only 4.5mm, and color, A. valida being “usually castaneous brown” and A. occidentalis “usually piceous brown.” However, he noted that large numbers of workers were required to separate the two forms with certainty and that it was often impossible to identify isolated workers.

Little has changed since Creighton’s study. Examination of recent collections suggests that while western populations do average slightly smaller than eastern populations, there is considerable overlap, making reliable separation impossible. Color shows a similar pattern with geographic differentiation but considerable overlap. It seems apparent that these two forms are variants of a single variable species. We therefore consider A. subterranea valida Wheeler to be a junior synonym of A. occidentalis (Emery) .

This species is found from British Columbia south to California and east to Montana and Colorado.