Ants

Diana E. Wheeler and Steven W. Rissing

Ants are familiar creatures. Although they are small as individuals, they are social, living in cooperative colonies, and these colonies are huge. Ant colonies are made up entirely of females, and include one or more queens and many workers. All of the ants streaming in and out of a nest entrance are workers, who protect the colony, collect food, and care for the larvae. Even though the workers are females, they lack the reproductive abilities of the queen, who lives deep within the nest and does little besides produce eggs.

Harvester ant

All ants’ bodies are divided into three parts. The head includes the antennae that detect smells, compound eyes, and capable jaws. All legs are attached to the mid-section, called the thorax. The gaster is the last segment and contains most of the internal organs, including defensive organs, including the glands that produce formic acid. When working with ants, beware the gaster!

Ants are most closely related to wasps. Imagine a wasp without its wings — it looks like an ant. Flying ants (the new queens and males) are often mistaken for wasps.

Order:: Hymenoptera
Family:: Formicidae
Sonoran Desert genera:: Pogonomyrmex, Messor, Pheidole, Solenopsis, Myrmecocystus, desert Neivamyrmex
Spanish name:: hormigas

Life History

Ants hatch from eggs laid by the queen and go through a series of larval stages before becoming adults, just as do butterflies. Ant larvae cannot move about on their own, however, and are completely dependent on workers for their care.

Just as individual ants go through stages of development, so do ant colonies. An adult ant colony raises reproductive (sexual) male and female forms, which can be recognized (in the desert) by the presence of wings. Reproductive females (who will become queens) look like workers but are 2 to 3 times as big; males are smaller than workers with even smaller heads and gasters. Mating occurs at species-specific times of the year. Most species fly immediately after summer rains; a few (Messor and some Myrmecocystus) fly in the winter. Reproductive individuals released within one area congregate at a mating swarm where one female may mate several times; sperm are stored for the lifetime of her colony, which may be more than 20 years!

Many desert ants (especially some Messor, Acromyrmex, Myrmecocystus, Solenopsis, Pogonomyrmex, and Pheidole), cooperate in founding colonies. Multiple new queens work together to start a small colony and raise their first workers. By working together, these queens can get underground faster (avoiding heat and predators) and produce more workers to more rapidly establish the territorial limits for their colony.

The period of colony founding — when the new queens must fly, mate, locate a good nest site, and avoid predators — is when most colonies fail. Only a few will survive. Once established, a desert ant colony may live for a decade or more. This means that the queens that establish the colonies are among the longest lived insects we know.

Long-lived, underground nests protected by thousands of ants devoted to bringing in food, offer attractive environments to other insects besides the ant-architects themselves. A variety of beetles, roaches, crickets and silverfish have evolved the ability to live in ant homes. Should we be surprised that they live in our homes as well? Perhaps the most interesting of these guests are other ant species. Some ants are “kitchen thieves” that are small and secretive. They live in the cracks and other hide-aways of regular ant nests and eat the crumbs of food that are left out. Other ants are more insidious. They sneak into a host nest of a closely related species and lay eggs destined to become reproductive males and females. Workers of the host colony raise these eggs as if they were their own. When it comes time to fly and reproduce, these social parasites propagate their own genes, not those of the colony that reared them.

Ecology

The Sonoran Desert is a great place to watch ants. They are the most abundant animal in this habitat and lack of ground cover makes them easy to see. In the desert most species of ants build underground nests that protect them from the harsh conditions. Here they can store or even grow food, find ample water, and avoid the environmental extremes of the soil’s surface.

One way to look at ant diversity is to classify them by the foods they eat:

Seed-harvester ants

Many desert ants (especially Messor, Pheidole, Pogonomyrmex, and Solenopsis) harvest seeds that they use as food for their larvae. Seeds of several grasses and annual plant species are preferred; seeds of perennial plants — especially cacti — seem not to be preferred. Seeds are stored in chambers toward the top of nests where dry conditions discourage germination. Ants have interesting behaviors when learning the different types of seeds that are available to them.

Pogonomyrmex workers have large squarish heads that contain powerful muscles for crushing seeds. The workers are ½ inch (13 mm) in length and are brick red to black. They have unforgettable stings. The typical nest of many species has a prominent cleared area, with a central opening and several permanent trails radiating from it. Another ant, Messor pergandei, is the only species of this worldwide genus that extends into the Sonoran Desert. Workers are ¼ to ½ inches (6 to 13 mm) long and are a shiny, jet black; they do not sting.

Over a dozen species of Pheidole live in the Sonoran Desert. Their workers come in two distinct forms: a small “minor worker” class and a much larger “soldier” class that crushes seeds, and sometimes enemies too.

Our desert Solenopsis are related to the infamous “imported fire ant” that is the scourge of the southeastern United States. The desert fire ant is a natural part of the Sonoran Desert community; unfortunately, it does resemble its eastern relative in its aggressive behavior and annoying sting. Most fire ants are ⅛ inch (3mm) long with some as large as ⅓ inch (8 mm); they are shiny brick red to black.

Leaf-cutter ants

Another common group of ants in the Sonoran Desert are the leaf-cutting or fungus-growing ants. Acromyrmex ants are related to the larger leaf-cutting ants of the tropical Americas. Acromyrmex versicolor is common in the Sonoran Desert. Its workers collect leaves and other plant parts to insert into fungus masses, which they grow in chambers deep within their underground nest. The fungus is completely dependent upon the ants for its care and propagation; the ants, in turn, eat a portion of the fungus as their sole source of solid food. Long columns of leaf-cutter ants search across the desert for plant matter for their fungus gardens when conditions permit in the fall and spring and on cool summer mornings; at other times, they remain underground. The fungus garden is started from a small “plug”of fungus brought by the queen from her home colony.

Honey pot ants

Another common food source in the desert is the liquid nectar of plants and the “juice” of other insects; both of these, however, are available only seasonally. Honey pot ants (Myrmecocystus) have solved this seasonal problem with specialized members of the colony that store liquid food in their engorged gasters. When other members of the colony need food, these living storage vessels share their stored reserves.

Army ants

The ants described above eat seeds, fungus or nectar. Some ants prefer meat; these are the desert army ants (genus Neivamyrmex). These ants raid the nests of other desert ants and occasionally take other prey as well. Because they are predatory and deplete the prey in any one area, they are nomadic and move from place to place. They have no permanent nest structures, and instead tend to live in temporary quarters such as hollows under trees or kangaroo rats’ nests.

Ants’ ability to live in colonies and excavate deep nests where the seasonally abundant food of the desert can be stored has made them remarkably successful in the Sonoran Desert. Further, in this environment, ants are easily collected and observed; this has made them model organisms for studies of development, behavior, and ecology.