Stigmatomma oregonense

Stigmatomma oregonensis is found in shaded, mixed coniferous forest, where it nests in large rotten logs. Colonies are polygynous and often rather diffusely distributed throughout the log. The principal prey appears to be geophilomorph centipedes. Alates emerge in August and September.

Identification
Worker: inner border of mandible straight; anterior clypeal margin straight; large Stigmatomma species (HW 1.21-1.40), with broad head (CI 0.94-1.01).

S. oregonense has sometimes been misidentified as Stigmatomma pallipes (Ward 1988). The two species differ in the shape of the clypeus and mandibles (see the identification key). California S. pallipes are smaller and have narrower heads than S. oregonense, but some eastern S. pallipes overlap with S. oregonense in these dimensions (Ward 1988).

Distribution
Southern British Columbia through northern California; coastal in the northern part of its range, moving away from the coast into the Sierra Madre in California (Ward 1988).

Distribution based on Regional Taxon Lists
Nearctic Region: Canada, United States.

Biology
Ward (1988) - California records of S. oregonense come from regions of mixed coniferous forest, mostly in the rich transition zone of the Sierra Nevada, dominated by conifers of the genera Pseudotsuga, Pinus, Libocedrus and Abies, together with maple (Acer) and oak (Quercus). Elevation records (where given) range from 580 m to 1740 m. At two localities (in Plumas and Tuolumne Counties) I discovered populous, multiple-queen colonies of S. oregonense, with abundant brood, under the bark of large, moist, rotten coniferous logs, in shaded forest. Remains of a geophilomorph centipede were seen in one cluster of larvae.

Laboratory colonies of S. oregonense, maintained in small plaster-of-Paris nests and established from polygynous colony fragments collected in the field, also readily accepted geophilomorph centipedes. The centipedes were captured, stung, dragged into adjacent brood chambers, and, in some instances, cut into several pieces which were fed upon by larvae. Lithobiomorph centipedes did not elicit such a strong reaction and were not dragged over to the brood pile. Workers and queens were observed to assiduously lick the surface and exposed cuts of geophilomorphs. On one occasion three freshly laid eggs appeared on a geophilo morph, 24 hours after capture. Nevertheless, I was unable to sustain healthy brood production in these laboratory colonies of S. oregonense. All such colonies experienced considerable larval mortality and declined in size, although several adults (one queen, four workers) were kept alive on a diet of small arthropods (mostly Drosophila) for 16 months.

One of the laboratory colonies of S. oregonense was started from a field collection of workers and cocoons in late July 1984. Alate queens and males eclosed in this colony between August 6 and 14, 1984. Dates of field-collected S. oregonense alates from California and Oregon span the period August 22 to September 18. An alate queen from Forest Grove, Washington Co., Oregon bears the annotation "4 p.m., swarm before rain", suggesting that S. oregonense alates may fly earlier in the day than those of Stigmatomma pallipes. Alternatively, queens of both species might simply be released before males.

Wild (Notes From Underground, on-line, 23 May 2005) provides an engaging and lavishly illustrated account of a captive colony. Workers killed centipedes by repeated stinging, then carried the prey back to the nest for a prolonged period of mastication (one to several hours). During this time the workers consumed any droplets of hemolymph that oozed from the centipede. Larvae were then piled onto the centipede. Queens, and occasional workers, fed on larval hemolymph, puncturing the larvae to do so. Larvae developed scars at the puncture sites but otherwise seemed unharmed. This non-lethal cannibalism increased when the colony ran out of centipedes.

Nomenclature

 *  oregonense. Stigmatomma pallipes subsp. oregonense Wheeler, W.M. 1915b: 389 (w.q.) U.S.A. Combination in Amblyopone: Brown, 1960a: 169; in Stigmatomma: Yoshimura & Fisher, 2012: 19. Raised to species: Brown, 1960a: 169. See also: Ward, 1988: 105.

Taxonomic Notes
This species was described as a subspecies of Stigmatomma pallipes; it was raised to species by Brown (1960), who felt that the morphological differences between S. pallipes and S. oregonense were consistent enough to indicate that the two forms were probably separate species. Brown's intuition was validated by Ward (1988), who showed that the two forms occurred together in part of northern California.

Worker
Slightly larger than the typical pallipes; head somewhat shorter; antennal scapes longer, so that their reflected tips reach the lateral margin of the head two-thirds the distance from the anterior to the posterior corners; eyes distinctly larger; teeth on the anterior border of the clypeus smaller and more numerous (9 to 10). Petiole from above as broad as long, with convex sides. In the typical form it is distinctly longer than broad, with rather straight, subparallel sides. Sculpture, pilosity and color as in the type.

Queen
(dealated). Closely resembling the worker; eyes much larger than in the typical pallipes.

Type Material
Olympia Washington, Kincaid 1949.