A Google search reveals numerous research articles about these remarkable ants. According to Phil Ward at the University of Calif., Davis (personal communication, March 2021): "This is a Stenamma worker. The species-level taxonomy of the North American Stenamma species remains somewhat unsatisfactory, especially for those occurring in California. I would say that this is close to the species that Branstetter (2012) calls Stenamma mgb101." Dr. Ward also referred me to detailed taxonomic reference about the genus Stenamma in Systematic Entomology by Michael G. Branstetter (2012). Two species of Stenamma (S. californicum and S. diecki) were reported by Krista H. Pease and Robert N. Fisher (2001): "Report on Pitfall Trapping of Ants at the Biospecies Sites in the Nature Reserve of Orange County California." U.S. Geological Survey, San Diego Field Station. Argentine ants (Linepithema humile) were also caught in my pitfall traps.

The origin of the taxa Stenamma mgb101 is explained in Michael Branstetter's scholarly 2012 article containing cladistical phylogenetic trees based on DNA sequencing: " ... Similarly, S. mgb101, which is nested inside of the ‘debile’ clade, was thought originally to be a dark form of the widespread North American species S. diecki, but the molecular data clearly show that S. diecki Emery collected from near the type locality (Yale, British Columbia), belongs to the ‘brevicorne’ clade." Stenamma californicum also belongs to the ‘brevicorne’ clade and is closely related to S. diecki.

According to Michael Branstetter (2012): Stenamma is monophyletic and tentatively is sister to a group of New World species placed currently in the genus Aphaenogaster ….Divergence dating and biogeographic reconstruction show that Stenamma is most likely to have originated in the Nearctic (North America--including Greenland) at the Eocene–Oligocene boundary (35 million years ago) and diversified into 2 isolated monophyletic groups, the Holarctic Clade (HOC) and Middle American Clade (MAC). Potential environmental factors affecting the evolution of Stenamma include the intense global cooling of the late Eocene combined with aridification and mountain building.

An exceptional characteristic of Stenamma is that many species seem to be well adapted to cool, wet environments. Also, it has been found that Stenamma can be the most common ant genus in leaf-litter samples collected from very wet and cool, cloud forest localities. These ecological traits are in contrast to the pattern seen in ants generally, in which diversity and abundance decrease with elevation. Biogeographic results indicate that Stenamma originated in the Nearctic, potentially preadapting it to thrive in cool montane forest environments (Branstetter 2012).

Most Stenamma species have very cryptic habits. Nests are usually small, and workers are slow moving, often becoming immobile upon disturbance. Consequently, Stenamma is rarely found by the casual observer and most collections are made by sifting leaf-litter from the forest floor. This fact has given Stenamma its stereotype as a “leaf-litter ant genus.” Despite being common in moist forest habitats, the small colonies of this genus are not often encountered except through specialized collecting techniques. Considering the above observations, finding Stenamma in the dry coastal sage scrub near Palomar College is especially noteworthy.