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Interesting Ants In Palomar College Arboretum & Adjacent Coastal Sage Scrub
© W.P. Armstrong (Updated 20 March 2022)
 1.  Unusual Ant In Comet Fire Post Burn Area  
 2.  A Big-Headed Ant Near The Bottleworts  
 3.  One Of Smallest Ants I Have Photographed  

1. Unusual Ant During Comet Fire Post-Burn Survey
Ant Caught In Pitfall Trap: A New Genus For
Owens Peak & Twin Oaks Valley Species List

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 Stenamma Found At Daley Ranch  

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.

Michael G. Branstetter (2012). "Origin and Diversification of the Cryptic Ant Genus Stenamma Westwood (Hymenoptera: Formicidae), Inferred From Multilocus Molecular Data, Biogeography and Natural History." Systematic Entomology Vol. 37 (3): 478-496.

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.

My 2nd Stenamma Discovery In Burned Area

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.


2. Big-Headed Ant Near Bottleworts!
Since retiring from Palomar College I have been identifying and photographing ant species on Owens Peak, in nearby Twin Oaks Valley, and on road trips to desert area of California & Arizona. To my surprise, I found a tiny black ant in its nest near the bottlewort location. Initially I thought it was a new species of Cardiocondyla (i.e. new to me); however, after carefully keying it out to genus in the Phil Ward, UC Davis Ant Key, I am fairly certain it is a minor worker in the enormous genus Pheidole. These ants are called "big-headed ants" because the major workers have huge heads compared with the minor workers. The famous biologist Edward O. Wilson wrote an entire 800 page book about this genus in the New World. The total number of described species exceeds 1,000, so identifying it to species is difficult. I also included an image of the nest of an Arizona species showing the remarkable difference between major & minor workers.

Phil Ward California Key To Ant Genus Pheidole

Postpetiole present.
Eye Not reduced.
Eye less than 1/2 head length.
Antenna with 12 segnents.
Hind tibia spur simple or absent. (Not Pogonomyrmex).
Lateral portion of clypeus not a raised ridge (Not Tetramorium).
Petiole with anterior peduncle & dorsal node.
Dorsum with standing pilosity.
Anteromedian portion of clypeus not abruptly elevated.
Antennal club 3-segmented.
In profile promesonotum domed & elevated above propodeal dorsum.
.....Pheidole (Major Workers Not Seen--as of 12 Feb. 2022.

I found only 3 minor workers near the bottlewort location. They appear to be in the Pilifera Group and possibly the Pheidole californica complex. They superficially resemble the latter species I photographed in Twin Oaks Valley, except they are concolorous dark brown (black) with yellow appendages. They match Antwiki images of the closely-related P. clementensis on San Clemente Island and coastal southern California, including San Diego County. Hopefully I can collect a major worker in my pitfall traps. According to Roy Snelling (AntWiki), minor workers on San Clemente Island are nocturnal, "starting to forage shortly before sundown." This species is reported in coastal San Diego County south of Palomar College and adjacent Orange County to the north. P. vistana occurs on nearby Owens Peak; however, its major & minor workers are unmistakably different from the Pilifera Group.

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.

Dylan O. Burge. 2005. "Taxonomy, Biology, and Distribution of Seed Harvesting Ants in the Pheidole californica Complex (Hymenoptera: Formicidae)." Journal of Hymenoptera Research Vol. 14 (2): 137-150.

The following image is a major & minor worker tentatively identified as Pheidole californica from a pitfall trap in Twin Oaks Valley. The minor worker has a yellowish body compared with the above Pheidole images from bottlewort trail. The original description of P. clementensis on San Clemente Island describes the body color of minor worker as dark brown to black; however, body color may be variable in ant species.

Most references I have consulted do not separate these 2 closely related species by the size and color of minor workers in their dichotomous keys, probably because these are variable characteristics. In my comparisons of both species caught in pitfall traps, Pheidole cf. clementensis are a little larger and much darker. These color & size differences between minor workers are also described by E.O. Wilson on pages 564 & 572 of Pheidole in the New World: A Dominant, Hyperdiverse Ant Genus. Hopefully, I can confirm these species identifications if & when I collect a major worker of P. clementensis in the coastal sage scrub near Palomar College.

This female California glowworm larva (Microphotus angustus) was caught in a pitfall trap near the bottlewort location. It is a remarkable bioluminescent beetle that retains its larval body form as an adult. The small ant attached to its leg is a Pheidole cf. clementensis worker.


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!

Big-headed major & minor workers near Willcox, Arizona.
    


If I calculated frequency of occurrence for Argentine ants (Linepithema humile) on the bottlewort trail, it would be at least 90%. In other words, practically every pitfall trap would contain this ant. Argentine ants are a serious deterrent to the establishment of other native ant species.

The decline of black harvester ants (Veromessor andrei) on Owens Peak, and red harvester ants (Pogonomyrmex) in Twin Oaks Valley, was caused (at least in part) by introduced Argentine ants . This resulted in the demise of endangered coast horned lizards (Phrynosoma coronatum) because harvester ants were their primary diet. Horned lizards are not vegetarians. They do not eat lettuce, as many people who caught them for pets discovered.

Remember When: I took this picture of a coast horned lizard 40 years ago (circa 1982) in a field near Palomar College. Because of urbanization and the elimination of its diet of native harvester ants by the Argentine ant, it is now extinct in 45% of its original range in southern California.

  Argentine Ant & Demise Of Native Harvester Ants  

San Clemente Island

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.

NOTE: A botanical endemic is a plant confined to a particular region, such as an island or mountain. Endemic species are often distinct due to genetic variation during long periods of geographic isolation. See following image of San Clemente Island brodiaea (Brodiaea kinkiensis) off the coast of southern California. Owned and operated by the U.S. Navy, this fascinating island includes several federally endangered plant species found no where else on our planet. I was able to visit this island courtesy of the U.S. Navy and accompanied by two former ex-military students.


3. One Of The Smallest Ants I Ever Photographed!

Note: I Encountered This Minute Ant While Crawling Along Bottlewort Trail 6 March 2022.
Dedicated to Dennis Astl For His Comment About My Unusual Bottlewort Hunting Method!
While observing a trail of Argentine ants near the bottleworts I noticed a minute orange ant walking with the much larger Argentine ants. I attempted to collect the tiny ant, but my attempts to pick it up were unsuccessful. I returned 3 days later to check my pitfall traps. Just as I was about to discard the dirt in my traps I noticed a tiny orange dot about the size of 2 average grains of ordinary table salt (NaCl). It was my tiny orange ant!

The tiny ant is a "thief ant" (Solenopsis molesta), so named because of their habit of robbing from other ants--in this case, undoudtedy the nearby Argentine ant nest. I have found these ants before, most recently in my living room; however, their typical length is 1.5 mm. Although my thief ant in trap was in a rolled up condition, I am quite certain its total body length was only 1 mm or less. I was afraid to straighten its fragile body because it might break into pieces.