SAGUARO JUNIPER ANTS

Main Sources: Most of the material for this brief 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 text information but also most 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), and A Field Guide to Desert Holes, by Pinau Merlin, 1999, Arizona-Sonora Desert Museum Press.

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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, 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:

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.

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: 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 disease).

SOME DISTINCTIVE ANTS OF OUR ZONE

Ants strongly prefer warm temperatures, functioning poorly below 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 zone. 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, protecting them from both excessive heat and cold.

LEAFCUTTER ANTS : (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 is a closeup of a Leafcutter nest in Hot Springs Canyon, newly opened for business last January 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:

It will be interesting to see if both of these species are living on our lands, and if so, where.

HARVESTER ANTS (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, possessing 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). 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.

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. 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.

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.

As a genus, "Pogos", as they are called, take this shape:

The Rough Harvester Ant, Pogonomyrmex rugosis, is common in our area. Its workers are about _ to 3/8 " long, and its color is reddish brown:

CARPENTER ANTS (SubFamily Formicinae, CAMPONOTUS genus)

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).

SOME WAYS OF COLLECTING ANTS FOR STUDY

The experts 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. Obtain the ants using stiff, narrow forceps with tips that are not too sharp. See the main reference source referred to at the outset, for further details.