An uncommon (or seldom encountered) ant of the genus Stenamma was caught in my pitfall trap on my second visit to the burned area. This is a genus of cryptic ants that live in leaf litter and apparently feed on soil microinvertebrates. They inhabit moderately humid to wet forest habitats throughout the Holarctic region (northern continents), and Middle America (Mexico, Central America and Caribbean), and part of northwestern South America (Colombia and Ecuador). I have only caught one other Stenamma at Daley Ranch in the leaf litter under a coast live oak.

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.

This is definitely a cryptic ant that is not commonly observed. According to Phil Ward at UC Davis (personal communication, 2021), it has been given the name of Stenamma mgb101. Its compound eyes are small, with less than a dozen ommatidia (facets). By contrast, Argentine ants have 82 to 110 ommatidia, flies have 4,000 to 5,000, and dragonflies have 30,000. Compared with other species, like Argentine ants with millions of workers, Stenamma nests are small with only a few hundred workers or less. In addition, they live in leaf litter and presumedly feed on soil microinvertebrates, so unless you are sifting through leaf litter, you probably won't see one in your lifetime! Since they are essentially subterranean ants, spending their lives in soil, leaf litter and rotting wood, they probably don't need larger multifaceted eyes. They are more common in colder forested regions of northern continents and Middle America. This was indeed a surprising ant discovery in coastal sage scrub, one of the most interesting in my ant career since retiring from Palomar College.

Based on the DNA phylogenetic tree (cladogram) for Stenamma species, the 2 most closely related ant genera are Novomessor (Aphaenogaster) and Veromessor (Messor). Veromessor populations were on Owens Peak, but unfortunately have declined in recent years. This was the main diet for coast horned lizards, which have sadly vanished from this area. I have photographed 2 species of Novomessor in Arizona, New Mexico and Texas, one of which is very friendly and curious, while the other bites aggressively.

During the March 2021 rainy season there were numerous Argentine ants (Linepithema humile) in my traps and under blackened boulders with their masses of eggs. These aggressive little ants can eliminate other species by their sheer numbers. I don't expect to find many different ant species in my traps unless the Argentine ants leave the area as the soil dries out by late spring.

According to Dylan O. Burge (2005), major workers of Pheidole clementensis may be distinguished from both P. californica and P. creightoni by the diagonal rather than longitudinal rugulae between the clypeus and the eye in lateral view. Unfortunately, I have not collected any major workers at the time of this writing to confirm this distinction (1 March 2022). So at this time I must conclude that my mystery ant could be P. clementensis or P. californica.

On Saturday 15 Oct. 2022, while sifting through soil near Pheidole nest on Bottlewort Trail, I found the partial head of a San Clemente Island big-headed ant (P. clementensis). This discovery confirmed my original hypothesis that we indeed have this common San Clemente Island ant species in the coastal sage scrub near the Palomar College Arboretum. Although the head is incomplete (missing eyes & mandibles), the parallel ridges (rugae) and frontal ridges (carinae) match illustrations by the late Edward O. Wilson in his remarkable book "Pheidole In The New World."

After confirming the closely related Pheidole californica in nearby Twin Oaks Valley is nocturnal, I suddenly realized that P. clementensis is nocturnal. This explains why I never encountered a live major worker on the Bottlewort Trail to confirm its identity!

On a dirt road north of the Salton Sea I attempted to reassemble (using Photoshop) the head of a major worker from exoskeleton parts recovered from a big-headed ant nest. Crawling in the sand prompted several drivers to stop and ask if I was OK! See following image of enormous head compared with minor worker. Major workers are essentially soldiers that defend colony against other ant species, such as army ants. Although their bodies are small, some big-headed ant major workers (soldiers) can even fight off larger ant species, such as harvester ants (Pogonomyrmex).

This is the 2nd Pheidole nest where I found the head of a major worker during a time when seed-bearing wildflowers were not available. I wondered if major workers are sacrificed when food reserves are in short supply; however, some references say this is not the case.

In 1859, Charles Darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. He defined natural selection as the "principle by which each slight variation [of a trait], if advantageous, is preserved."

Natural selection is the process through which populations of living organisms adapt and change. Individuals in a population are naturally variable, meaning that they are all different in some ways. This variation means that some individuals have traits better suited to the environment than others. Individuals with adaptive traits—traits that give them some advantage—are more likely to survive and reproduce. These individuals then pass the adaptive traits on to their offspring. Over time, these advantageous traits become more common in the population. Through this process of natural selection, favorable traits are transmitted through generations.

In chapter 7 of his Origin of Species by Means of Natural Selection, Darwin devotes several paragraphs regarding his concern over sterile ant castes with very unique shapes & sizes. How do these castes develop if their traits are not passed on. In fact, he even questions natural selection, the primary theme of his book on evolution.

The development of ant castes is a complex subject. Darwin was a brilliant scientist to question this in 1859. Some authorities now think this is an example of epigenetics, the study of heritable traits that do not depend upon the primary sequence of DNA. I have included brief examples on Wayne's Word, including the following link to an Arizona road trip in 2017.

Scientists at McGill University, Canada have found the answer to a question that perplexed Charles Darwin. So much so, that it actually led him to doubt his own theory of evolution. He wondered, if natural selection works at the level of the individual, fighting for survival and reproduction, how can a single colony produce worker ants that are so dramatically different in size – from the “minor” workers with their small heads and bodies, to the large-headed soldiers with their huge mandibles – especially if, as in the genus Pheidole, they are sterile? He discusses this in Chapter 7 of his On the Origin of Species by Means of Natural Selection (1859).

The answer, according to a paper published in the journal Nature (2018), is that the colony itself generates soldiers and regulates the balance between soldiers and “minor” workers thanks to a seemingly unimportant rudimentary “organ” which appears only briefly during the final stages of larval development. And only in some of the ants – the ones that will become soldiers.

Rajakumar, R., Koch, S., Couture, M. et al. "Social regulation of a rudimentary organ generates complex worker-caste systems in ants." Nature 562, 574–577 (2018)

In chapter 7 of On the Origin of Species by means of Natural Selection, Darwin comments on some of the sterile worker ant castes (that he calls "neuters") with remarkably different body sizes (big-headed soldiers) that look completely different from other members of the colony. The following quotations are from Darwin in 2 paragraphs of pages 238 and 239 of 1st edition):

"But we have not as yet touched on the climax of the difficulty; namely, the fact that the neuters of several ants differ, not only from the fertile females and males, but from each other, sometimes to an almost incredible degree, and are thus divided into two or even three castes. The castes, moreover, do not generally graduate into each other, but are perfectly well defined; being as distinct from each other, as are any two species of the same genus, or rather as any two genera of the same family."

"It will indeed be thought that I have an overweening confidence in the principle of natural selection, when I do not admit that such wonderful and well-established facts at once annihilate my theory." He is also referring here to the food storage workers (repletes) of honeypot ants (Myrmecocystus) with enormous swollen abdomens (gasters) containing sweet liquid food that is fed to other workers during times of famine & drought.

From the Bakersfied Skeptic Society: A honeypot ant "replete" engorged with nectar. Like a living larder, It remains deep in the nest and supplies other workers with liquid food during times of drought. Running fast is no longer an option for this ant! This tasty, sweet morsel is highly prized by aborigines in several countries where honeypot ants live.

A gentleman that I met at the Comfort Inn in Tucson said he remembered eating these tasty ants as a child in Mexico. The Spanish translation for honey ant is "hormiga de miel". He was surprised to know there was a nest in the riverbed near the hotel!

I have included the following 2 images of major & minor workers of a big-headed ant near Willcox, Arizona. This species belongs to the Pilifera Group, and the smaller minor worker superficially resembles the ant I found near bottlewort location. I am especially fond of big-headed ants. This is probably because the ant that inspired my interest in biology at age 10 in Arcadia, CA was a species of Pheidole!

The specific epithet for Pheidole clementensis refers to San Clemente Island, the type locality at south end of island (top of photo) where the original type specimen was collected. It is not truly endemic to San Clemente Island because it also occurs in coastal southern California (Orange and San Diego Counties). P. californica occurs in CA, NV, OR, WA & ID.