Ants (Family Formicidae)
Main Sources: Much of the source material for this introductory account comes directly from the landmark book, The Ants, by Bert Holldobler and Edward O. Wilson, 1990 (Belnap Press of the Harvard University Press), from which I draw not only most of the scientific information but also some of the drawings and some of the color photographs. Additional sources used for both text and illustration include The Audubon Society Field Guide to North American Insects and Spiders, by Lorus and Margery Milne, 1984 (Alfred A Knopf), Insects of the Southwest, by Floyd Werner & Carl Olson, 1994 (Fisher Books), A Field Guide to Desert Holes, by Pinau Merlin, 1999, Arizona-Sonora Desert Museum Press, and Ants at Work, by Deborah Gordon, 1999, The Free Press, which focuses on the Chiricahua Mountains. For Arizona specifically, we are indebted to the article by Richard Conniff, 2006, "Antsy in Madagascar", Discover March:44-51, not least because it led us to this remarkable website: Antweb -- which includes pages on Arizona ants.
The most diverse and abundant insect orders now existing are the beetles (Order Coleoptera), the flies (Order Diptera), and the bees, wasps, and their kin (Order Hymenoptera). Within the Hymenoptera ("membranous-winged" insects), the ants comprise a single Family, the Formicidae, one of the several lines of insects who evolved to become "eusocial" ---that is, to form trans-generational families that include division of reproductive labor (queens-and-kings versus workers). Eusocial insects have, since their origin sometime during the Cretaceous Era, become the predominating insect forms around the world, and the ants show the greatest local diversity of all social insects.
Ants evolved from non-social aculeate [stinging] wasps, becoming first of all strongly-social-bonded, soil-dwelling families of medium-sized wasps, and the most primitive known ants (genus Sphecomyrma, found in New Jersey Sequoia tree amber of Mid-Cretaceous times, ca 80 My old) show a combination of wasp-like and ant-like traits, as shown in the image at right (a fossil contained in amber):
Throughout the Cretaceous Era, ants were very rare (judging by fossil numbers), but in Oligocene and Miocene deposits of the Cenozoic Era they become very abundant (i.e., after 34 million years ago).
Eusocial behavior, a fairly rare development among insects, provides an advantage because such creatures can switch from individual to group response and back swiftly according to need, and this plus specialization of cooperative labor produces greater adaptive effectiveness. Ants are unique also because they are the only eusocial predators occupying the soil and ground litter (in contrast to termites, who are almost exclusively vegetarian). Anatomically a number of features link to this earthen adaptation: ants have become wingless (except for the mating stage), ants possess one or two waist-like constrictions between thorax and gaster ["abdomen"] that enable them to maneuver in tight spaces, ant mandibles have become working tools for a variety of purposes, and ants uniquely possess a metapleural gland (which produces an antibiotic/antifungal acid which ants spread through the moist, microbe-ridden nests most ant species occupy, thus protecting them and their larvae from earth-borne disease).
Most ants in a colony are wingless, sterile females. When the reproductive cycle occurs, winged males and winged females (virgin queens) typically fly out in swarms, where mating occurs, after which the males usually die and the females lose their wings and each goes off to form a new colony. Below we see some images of swarming from July 28, 2005, an event which occurred after very heavy rains in the immediate vicinity of Cascabel, Arizona (a recurrent pattern among ants, taking advantage of the availability of damp soils to dig into). We think most of these swarms were Pogonomyrmex species, see further below.
Below are two different swarms, on the left one forms over the Cascabel Road, where heavy rains had left the earth very damp; the roadway was filled for several miles with such swarms at sunrise. On the right, a swarm on the terrace above Hot Springs Canyon. This was a smaller group.
(Click on each image to enlarge it.)
Below, at left a swarm close-up; at right, two hours later no more ants are flying but the roadway has spots of these brown masses as the ants continue swarming there: (Click on each image to enlarge it.)
Below, mate-seeking ants walk along a tent surface.
According to Holldobler and Wilson, most of these flying ants die without mating, many killed by worker ants of other species and other, various predators. We saw one example of this on the tent shown above, where a small spider had captured its prey:
Some Distinctive Ants of our Zone
Ants strongly prefer warm temperatures, functioning poorly below 68 degrees F. (20 degrees centigrade) -- very few ant species have adapted to cold-temperate zones. In contrast, they are both diverse and abundant in the hottest and driest habitats on earth, so we should not be surprised that they thrive in our own sub-tropical and semi-arid zone in Cascabel. Ants seek heat for rearing their larvae, and they regulate temperature by constructing appropriate nests, migrating within and among nests, and by regulating their metabolic heat. Those ants who nest down in the soil exploit the universal circumstance that below a few inches depth, soil temperatures and humidity vary little throughout the year, a condition protecting them from both excessive heat and cold.
Since beginning this page, we have had the good fortune to discover the website AntWeb(see this link), which includes a series of pages on the ants of Arizona. A quotation from the main Arizona page on that website gives a more succinct account of the Arizona situation than we could ever muster: (thanks to Brian Fisher for permission to use this quote)
"More species of ants occur in Arizona than in any other state, with the fauna consisting of 8 subfamilies, 43 genera, and 318 native species. This extremely high number of species results from Arizona lying at the confluence of several biotic regions, combined with high topographic diversity (140-12,650 feet) that encompasses areas from hot desert to tundra habitat. On a north-south axis, Rocky Mountain and Canadian faunal components mix with the hyperdiverse neotropical assemblages of the Sierra Madre Occidentale in northern Mexico, while the east-west axis consists of a rainfall gradient across the Mohave, Sonoran, and Chihuahuan Deserts. The numerous isolated mountain ranges "sky islands" of southern Arizona also contribute to high species richness, as exemplified by 187 species occurring in the Chiricahua Mountains of southeastern Arizona."
In what follows below we will develop what we are learning from our own encounters with the ants on Saguaro Juniper lands.
(Subfamily Myrmicinae; genera Acromyrmex, Atta)
Adult ants of this type retain the basic ant predator/scavenger adaptation, but have recently been found to obtain substantial nutrition from the sap of the leaves they cut. More remarkably, these ants have developed the specialization of growing and eating fungus, a very rare but also very successful adaptation, and many varieties of leafcutter species have evolved. In tropical America, Leafcutters are the dominant herbivores, consuming far more vegetation than any other group of animals of comparable taxonomic diversity (including mammals), yet these insects remain relatively little studied. Of some 40 known species, only Acromyrmex versicolor and Atta mexicana have been found in Arizona.
Early observers who saw groups of leafcutter ants cutting leaves and then carrying the leaf-cuttings back into their nests first assumed that the leaves were used to thatch domes inside their subterranean dwellings, then that they carried the leaves below to eat them, but the far stranger truth was recognized by a naturalist in Nicaragua in the 1870s, who found that deep within Atta nests were garden chambers -- tended by the smallest worker ants in the presence of the ant larvae -- composed of masses of minutely subdivided leaf-pieces, withered and overgrown by a minute white fungus. A later naturalist saw that the tips of this unique fungus [Cyphomyrmex rimosus] produce bulbous swellings which are plucked and eaten by both the adult workers and larvae, and which have recently been shown to be rich in soluble nutrients. The ants in turn provide nourishing fertilizer for the fungus.
Acromyrmex versicolor builds nests with multiple entry holes, each of which is surrounded by a very distinct, well-rounded cone, looking like a tiny volcanic cinder cone. The cones vary in size from less than 6" to slightly more than a foot in diameter, and one of these will likely have some pieces of fresh leaves lying around it which mark it as the main entrance to the colony. (The other cones are the result of workers depositing their excavations of the chambers below, which spread out laterally and link parts of a whole which may reach depths of more than 20 feet.) Leafcutter nests may be closed up for weeks and seem abandoned, since a continuous food source is available to the ants down below. Here below is a closeup of a Leafcutter nest in Hot Springs Canyon, newly opened for business in January 2002 after a period of dormancy:
The workers who build the mounds and who may be seen cutting and carrying leaves look like this:
Note the many spines on the body, which is reddish brown in color and not shiny. These ants are from 3/16 to 3/8" long.
Atta mexicana looks similar in color, tone, shape, and size:
Below, an active Leafcutter nest is operating in November of 2007near the Red Tank at left, while an older opening to the same nest was abandoned before rainfall largely degraded it.
Below left, while the mound opening is littered with old mesquite leaves, the dominant current activity concerns ants carrying pieces of a small Mexican Oregano plant which is being defoliated a few yards off-camera. Below right, a closeup view of two worker ants near the opening. One carries a piece of Mexican Oregano leaf, the other some kind of seed. (Click on each image to enlarge it.)
(worldwide, occur in four different subfamilies)
Harvester ants eat other insects, but are distinct in the fact that they regularly feed on seeds as part of their diet, and consequently they (by accident) play an important role in seed dispersal. Old Testament writers were impressed by the diligence of these insects, who are prominent inhabitants of warm-region deserts and drier grasslands throughout the world. "Go to the ant, thou sluggard, consider her ways...."
Pogonomyrmex species (Subfamily Myrmicinae) are the most prominent Harvesters in our region, and they display an extreme aggressive belligerence, using both their powerful, seed-cropping mandibles to bite encroaching predators, and an autotomizing sting (one which breaks off in the body of the victim) that injects the most toxic venom of any known insect poison (at least to mammals, including humans, who find their sting even more painful than that of southern Fire Ants). In sufficient quantity, these stings can be deadly to many kinds of small animals. Pogonomyrmex colonies usually fight foreign conspecifics when they meet them during forage, and colonies adjacent to one another practice territorial avoidance, producing a high regularity of species spacing. These ants are popularly called "Pogos", a term people generally utter with grudging respect.
Despite their fame for aggressiveness and sting toxicity, remarkably another resident of our lands, the Horned Lizard, successfully preys upon them, consuming large numbers by swallowing them whole after snagging them with a long sticky tongue. How this is possible without the lizards being severely stung by these dangerous ants can be understood by consulting that link.
Foraging for seeds usually peaks in the cool of morning, then again in early evening (with variations due to temperature and humidity). Pogonomyrmex scouts explore the terrain, guided by visual landmarks, position of the sun, and odor marks. When a worker finds a patch of seeds she carries one back to the nest, depositing an odor trail from the tip of her abdomen (her sting gland). Nestmates then follow, often in large numbers, and if the patch persists a trunk trail develops which can remain active for weeks. At left, a group of Pogos move a fairly large seed toward their nest during a September evening in 2004. (Click on the image to enlarge it) Few foragers range much beyond 40 feet from the nest exit. Nest sites are spread out in several directions, in such a way that the trunk trails of different colonies do not overlap one another.
These Harvester Ants make circular clearances of all plants to a diameter of several feet around their single nest entrance. Beyond this point, they surround their nests with low crescents of plant and other debris which they have extruded from their nest. The hole, typically more than an inch across, lies flat on the ground surface, but the nest may extend some ten feet down, its many chambers (some storing seeds for food) lying directly below the surface clearance area. Below is a view of a typical Pogo nest pattern, with the small entry hole roughly in the center of the reddened earth (these ants are still cleaning out the nest after early April rains of 2004): (click on the image for a close-up view of the hole)
In the Arizona deserts, Pogonomyrmex colonies definitely aid the dispersal of such seeds as Plantago insularis and Schismus arabicus, since these plants are found in much denser concentration near the nests than elsewhere. Below, a nest photographed a month later (May 11, 2004) shows a periphery strewn with the seedpods ejected after their work (the red "X" indicates a rock above the entry hole):
As a genus, "Pogos", as they are called, take this shape (note the contrast between head and abdomen shapes in the image at right):
The Rough Harvester Ant, Pogonomyrmex rugosus, is common in our area. Its workers are about 1/4 to 3/8 " long, and its color is reddish brown. Note also the white bar across the top of the head:
In a study of colonies of Red Harvester Ants (Barbatus species) located in a valley east of the Chiricahua Mountains in southeastern Arizona, Deborah Gordon (cited above) found that each colony has a life-span of 15 to 20 years, which is the life-span of its queen. (The sterile female workers live about a year, males for only a few weeks.) During her lifetime, a queen pumps out ants in numbers sufficient to build the colony to a maximum population of about 10,000 within 5 or six years, the colony then maintaining its numbers until she dies (after which the colony also dies). Each year, when the summer rains begin in strength, all of the colonies in a local area send their winged virgin queens and males out on a single mating flight, and each mated queen who survives will build a new nest and produce all of the new colony's future ants from the sperm of her single mating.
(Subfamily Formicinae, genus Camponotus)
These ants, poorly studied for most species, are so labeled for their tunneling into deadwood (tree trunks, felled logs, etc.) for building their nests (though some Camponotus species --not in our region -- construct their nests almost entirely of larval silk, making complex three-dimensional mazes of chambers and passagesways made mostly of silken sheets covering leaves, and are known as "Weaver Ants"). Among the Texas Carpenter Ants (Camponotus festinatus) of our area, the mated queen begins her nest in dead wood and tends its first brood. As the female workers mature, they extend the nest galleries, tending eggs and young and hunting for insect food (as well as eating sweets from rotting fruit etc.), which they regurgitate to feed the queen. Large colonies include soldiers and workers of various sizes.[I --RNH --found a small, scouting, group of them inside a sealed canvas bag I had hung from a tree in Muleshoe Spring, but which had fallen to the ground and been torn partly open by a carnivore.]
Texas Carpenter Ants are rather large -- to 1" depending on caste -- and a brownish yellow in color (sometimes banded as shown above right).
Many other kinds of ants -- including some tiny kinds of "Army Ants" -- populate our area. We hope to collect images and specimens to elaborate this page as opportunity permits.
Some Ways of Collecting Ants for Study
Holldobler & Wilson (cited above, p. 630-1) recommend picking up ants with stiff, narrow foceps with pointed but not oversharp tips, and recommend placing ant specimens in labeled vials containing rubbing alcohol. Numerous specimens may be placed in a single vial, each indicating location, habitat type, and date.
For surveying a particular locality, both day and night collections should be made. Tree and bush branches may be raked with a sweep net -- a fine-meshed circular-mouthed net with a deep pocket -- and branches broken open. Vegetation may be shaken with large squares of white cloth placed underneath, and the resulting haul collected with a "pooter" -- a pair of fine rubber hoses set into a collecting bottle that enable the student to suck insects into the bottle. More elaborate methods entail collecting leaf litter and placing it into funnels that channel the arthropods contained in the mass downward by heat into a bottle containing alcohol -- a small version is called a Berlese-Tullgren funnel. The Conniff article cited above (p. 47) describes the much larger-scale work of Brian Fisher, who uses a device called a Winkler Bag -- a very substantial cloth container with a rectangular opening more than a foot across, into which flat-rectangle open-mesh bags filled with litter are inserted in a way that allows each one to dry and the top of the larger bag is then closed. As drying proceeds, the arthropods try to escape by heading downward, where they fall through the open mesh and into the bottom of the Winkler Bag, where a hole at the bottom leads only to the alcohol trap. Fisher also uses pan traps -- plastic bowls filled with soapy water -- and malaise traps -- tentlike mesh devices placed along natural flight paths that catch insects and trap them in alcohol. We will provide more details on these methods when we have found time to use them.