HONEY I SWELLED THE KIDS
Ambitious ants store liquid food for dry times
Randy C. Morgan
Myrmecologists, naturalists and bugwatchers have long been fascinated by honey ants. Colonies of these amazing insects develop specialized workers, called repletes or honeypots, with tremendously swollen abdomens for nectar storage. Some repletes also hoard water, fats and body fluid from insect prey. The repletes gorge on food that is collected outside the nest and brought to them by normally-proportioned worker ants. Deep in underground chambers, the repletes hang quietly in clusters, literally imprisoned by their globose abdomens ballooned to the size of small grapes!

Worldwide there are several very different kinds
of ants that exhibit extreme repletism; all are properly called honey ants. One outstanding group; the genus Myrmecocystus, occurs exclusively in western North America. Myrmecocystus contains a diverse ensemblage of at least 28 closely related species, many of which are broadly distributed.

It follows that these honey ants are very common insects.Most human residents or visitors to the American Southwest have probably strolled atop a honey ant nest completely unaware of the treasure trove hidden below. But those lucky enough to view repletes first hand can't help but want to find out more about these astonishing creatures.

HIGH SOCIETY

Honey ants are highly social insects. They live in efficiently organized colonies or groups of cooperating individuals. Their societies are matriarchal or female-dominated families. A typical honey ant colony contains a single fertile queen and thousands of sterile female workers. The queen is the mother of the colony and is specialized for egg production, while the workers, the queen's daughters and hence all sisters, are responsible for colony labor.
Worker honey ants are polymorphic, meaning that they come in different sizes, ranging ten-fold or more in body weight. This variation, in turn, permits more effective task specialization. For example, the smallest workers, called minims, tend to remain in the nest as nursemaids caring for the queen, the developing brood (eggs, larvae and pupae) and the repletes. Medium and larger size workers are more apt to be involved with nest excavation, food acquisition and colony defense. The largest workers, called majors,
are the individuals most likely to develop into repletes.Any animal society requires an effective system of communication to perform smoothly. Honey ant colonies are coordinated largely by pheromones, glandular secretions produced by the ants that function as chemical signals. Pheromones regulate complex social behavior and broadly serve to identify colony members, recruit nestmates to food sources, and elicit alarm and defensive behaviors. For example, honey ant workers instantly distinguish nestmates from intruders based on their smell (colony odor); the former might be groomed and offered food while the latter would be attacked and killed or driven from the nest.

Depending on the season and colony cycle (Fig. 1), honey ant societies may also contain numerous non-working reproductive forms, or winged males and virgin queens. Winged forms, called alates, are produced when colony populations reach an optimal size and become reproductively mature. Then, a portion of the brood raised each year is converted into individuals designed to mate, disperse and start new colonies. Mating flights typically are nocturnal or crepuscular, and generally occur only once a year following the first heavy precipitation of the summer rainy season.

The reproductive forms pour out of the nest and swarm into the air, mating on the fly. Young queens take only one mating flight; they store viable sperm long-term in a special organ, called a spermatheca, which is then used for fertilizing eggs throughout the remainder of their lives.
The males, their primary function completed, die shortly after mating. The inseminated queens drop to the ground, twist off their now useless wings, and scamper across the surface of the warm wet ground, looking for a place to burrow. The vast majority of the queens fall victim to a barrage of predators including birds, lizards, spiders and especially, other ants. Surviving queens must be well below ground before the rising sun relentlessly dries and bakes the surface.
Honey ant queens, like most highly evolved ants, utilize the claustral mode of colony founding. The queens seal themselves inside humid, subterranean cavities and do not forage; rather, they rear their first brood of tiny workers, called nanitics, relying solely on their stored fat reserves and metabolized flight muscles. Colony growth is slow at first, but soon rapidly accelerates as additional ants are added to the worker force.

HONEY, I'M HOME!

Myrmecocystus nests are found in a diversity of arid or semi-arid environments. Some species live in brutally hot lowland deserts, others reside in transitional habitats, and still other species can be found in montane woodlands where it is relatively cool although still dry for much of the year.

Curiously, it is not unusual to find both diurnal and nocturnal honey ant species coexisting within the same habitat, sometimes even nesting closely to one another. Further, the diurnal species often are darkly pigmented, while the nocturnal species tend to be lightly colored. A good example of this phenomenon is readily observable on the desert flatlands bordering the Chiricahua Mountains in southeast Arizona. Here, M. mimicus and M. mexicanus nests are found in abundance. The former is relatively small, fast-running, jet black in color and diurnally active, though it avoids surface exposure during intense midday heat. In contrast, M. mexicanus is larger, slower-moving, golden amber colored and is principally nocturnal, although it may be above ground on rare cloudy days. Both honey ant species consume similar types of food, but by foraging at different times, apparently avoid competing with one another.
Honey ant nests almost always have only a single surface opening surrounded by a small accumulation of excavated substrate. Each species has a characteristic nest entrance. For example, M. mimicus builds a low sandy mound while M. mexicanus constructs a turret of coarse gravel. Nest openings of the latter species are very distinctive and not easily confused with those built by other kinds of ants in the area. The amount of soil surrounding a honey ant nest entrance is deceptively small, for the nest tunnels and chambers frequently extend deep into the ground, often reaching several yards or more below the surface.
Nests of M. mexicanus have been studied in detail and have a surprisingly distinct and seemingly adaptive architecture. The nests typically are built in hard rocky soil, and consist of a subsurface labyrinth of tunnels and a vertical array of small domed chambers interconnected by one narrow vertical passageway. The maze of tunnels just below the surface probably serves as a staging area for foraging parties and mating flights and as a site for brood incubation. The lower chambers, deep enough to be within the level of permanent soil moisture, house the queen, much of the worker population and the majority of repletes. The lone interconnecting passage probably makes individual nest chambers easier to defend or seal off from enemy invaders.

FINDING FOOD & FENDING FOES

Honey ants are generalized scavengers and predators. Sugary carbohydrates are needed for energy. The ants visit flowers and extrafloral nectaries for the sweet rewards, and "milk" homopterans like aphids and scale insects for their sugary exudations called honeydew. Once engorged with liquid sugar, workers return to their nest to share with other colony members.

Protein sources also are necessary, particularly for the growing larvae. Foraging honey ants search for live or recently dead invertebrates, predominantly arthropods; small soft-bodied insects like termites and caterpillars are especially fair game. Individual workers head homeward with pieces of food clasped within their mandibles, while groups of workers may cooperate when retrieving large or struggling prey items.

Like all formicine ants, honey ants lack stingers: rather, they spray fine droplets of formic acid from their abdominal tip. The caustic acid is used, along with pinching mandibles, to help subdue uncooperative prey or to deter colony enemies. Grappling with prey or facing off against their foes, attacking honey ants spread and brace their legs, rapidly curl their abdomens underneath pointing forward, aim and fire. Their chemical weaponry is most effective against small targets; it is useful against mammals or other large animals only if sprayed into highly sensitive tissues like eyes or nostrils.

Given an abundance of sweet repletes, it is not surprising that badgers and other desert creatures sometimes burrow into and plunder honey ant nests. Even some honey ant colonies will raid one another, stealing not only repletes but also brood, that when mature, is "enslaved" as part of the pirating colony's worker force.
As recently as early this century, various Native American tribes and Mexicans regularly excavated honey ant colonies to obtain the savory morsels stockpiled therein. Sweet-toothed human predators typically hold a replete's head and thorax with the fingers, bite off or rupture the fragile abdomen, then suck its contents into the mouth. The so-called "honey" tastes somewhat like molasses, while the ants' bodies, being bitter, are discarded. Perhaps fortunately for honey ants, this practice seems to have fallen by the wayside due to the advent of commercially available candies. These not only taste better but are much less work to procure.

PLEASE REPLETE YOURSELF

Many social insects, notably honey bees and some wasps, collect and store liquid sweets for later use. However, these insects stow their food within the confines of their nest combs. Honey ants are unique in using their own bodies as living storage vessels (Fig. 2).

But why develop repletes? In arid regions, food and water usually are obtainable only for very limited periods. But when available, as during the brief summer rainy or growing season, these resources are present in overwhelming abundance. Under favorable conditions, surplus food gathered by the colony is fed to the repletes for long-term storage. Then, in times of need, the food is regurgitated back to colony members.

While the formation of food-storing repletes seems remarkable, it is merely an exaggerated form of typical ant behavior. Most ants possess expandable abdominal crops (stomachs) for temporarily carrying liquid food back to the nest. When solicited by a hungry nestmate, they regurgitate this liquid and offer a small droplet held between their mandibles. Close observation will reveal that the donor ant's abdomen shrinks as the recipient's abdomen expands. Thus, the crop serves as a "communal stomach" and repletism simply as extreme development of this expandable organ.

In the presence of excess food, repletes can develop from any newly emerged worker, apparently before its exoskeleton and intersegmental membranes harden and become less flexible. In young colonies, it is not uncommon to find small workers serving as somewhat ineffective repletes, with their tiny abdomens distended to the bursting point. However, on an individual basis, major workers make the most efficient honeypots. Hundreds to a thousand or more large repletes can be found in mature honey ant colonies, each patiently hanging and waiting to serve up dinner when called upon by its nestmates.

DESERT DIGGERS

In July 1985, a dozen Cincinnati Zoo members and Steve Prchal first went looking for honey ants (SASI was not yet born but a similar trip in 1986 included SASI members). Our hope was to excavate and transplant entire colonies into observation nests at the Cincinnati Zoo Insectarium, and then develop captive management and display techniques.

We were after M. mexicanus, chosen in part because it was the same species formerly studied in Colorado by John Conway, our expedition co-leader. It also is a common inhabitant of the desert flats near the Chiricahua Mountains, conveniently allowing our group to be based at the nearby American Museum of Natural History's Southwest Research Station.

Because of limited time, our collecting team worked in shifts around the clock. Lanterns were used at night to illuminate the deep craters laboriously dug into the hard rocky soil. After sunup, canvas tarpaulins strung crudely overhead served to protect us from the dangerously intense sunlight.
Attracted by our questionable behavior, visitors passing the dig site thought we had gone crazy from the heat, but their skepticism turned to amazement upon spying the results of our labors. Deep within their subterranean nest, now finally uncovered after hours of grueling work, honey ant repletes hung from chamber ceilings in golden clusters. In the interplay of our flashlights the repletes sparkled and glistened like living jewels. The spectacle was enough to make anyone pause spellbound, temporarily forgetting the oppressive heat and irritating dust.
Numerous worker honey ants and brood, and ultimately the queen, were collected and placed in large plastic field containers for holding and transport. The delicate repletes were painstakingly removed by hand, or gently lifted with small spoons and held, along with a few attendant workers, in plastic petri dishes lined with damp toweling for perching. But even with great care, many repletes were damaged during collection.

Their fragility further thwarted successful colony transplantation into observation nests, typically a simple procedure for many kinds of ants. Repletes were carefully hung in their new homes, but they often tried to move to different locations, slipped and fell, sometimes becoming trapped in unnatural positions. Soon many of the repletes had ruptured, eventually fouling nest chambers. Already stressed, transplanted colonies began a slow decline and ultimately perished.
Fortunately this sad turn of events is not the end of the story. Our desert visit coincided with the start of the rainy season, when moisture laden monsoons blow out of the east, drenching the landscape with life-giving water. Many desert creatures are programmed to reproduce at this time, including honey ant colonies, and during this and subsequent years we easily collected newly mated queens following their nuptial flights.
But more importantly, young queens readily adapted to life in captivity. They organized nest cavities in small sand-filled vials, laid eggs and reared worker ants, and eventually gave rise to honey ant colonies housable in observation nests. Interested bugwatchers could now enjoy an intimate view of honey ant social behavior and their incredibly swollen repletes!
Author's note: A description of honey ant observation nest construction, captive care and a detailed bibliography can be found in my article referenced below, available from me upon request.



SELECTED REFERENCES AND ADDITIONAL READING

Conway, J.R (1983): Nest architecture and population of the honey ant, Myrmecocystus mexicanus Wesmael (Formicidae), in Colorado. Southwest Naturalist. 28(1):21-31.

Hölldobler, B. and Wilson, E. O.,1990. The Ants. Cambridge, MA: Belknap Press of Harvard University Press

Morgan, R.C. (1991): Natural history, field collection and captive management of the honey ant, Myrmecocystus mexicanus. International Zoo Yearbook 30:108-117.

Snelling, R.R. (1976): A revision of the honey ants genus Myrmecocystus (Hymenoptera, Formicidae). Science Bulletin Natural. History Museum of Los Angeles County. 24:1-163.

Wilson, E.O. (1971): The insect societies. Cambridge, MA: Belknap Press of Harvard University Press.

Randy Morgan is the Headkeeper of the Cincinnati Zoo Insectarium and a regular contributor to Backyard BUGwatching. A Charter member of SASI, he has been actively involved in many aspects of the organization's development.