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Myrmecocystus mexicanus is a species of ant belonging to the genus Myrmecocystus, which is one of the six genera that bear the common name “honey ant” or “honeypot ant,” due to curious behavior where some of the workers will swell with liquid food until they become immobile and hang from the ceilings of nest chambers, acting as living food storage for the colony. M. mexicanus is found in the southwestern U.S. and parts of Mexico.

Eusociality
M. mexicanus like many ant, wasps and bees, is a eusocial insect species. Eusocial insects are characterized by distinct caste systems, where some individuals breed and most individuals are sterile helpers, and overlapping generations so mother, adult offspring and immature offspring are all living at the same time. In a eusocial colony, an individual is assigned a specialized caste before they become reproductively mature, which makes them behaviorally (and sometimes physiologically) distinct from other castes. The honey pot ants exhibit all of these characteristics within a colony: a queen and males make up the reproductive caste, and the rest of the individuals are sterile female workers.

Colony life cycle
After conducting a 35 year census of M. mexicanus, among other ant species, in degraded grassland in southern Arizona, Chew found that the M. mexicanus colonies average a life span of 8.9 years and have a maximum life span of 23 years.

Reproduction
Only two castes in the colony reproduce, the queens and the males. Workers are sterile and tend to the queen and brood. Mating occurs during nuptial flights, where winged queens and males swarm outside the nest.

Prenuptial activity
The reproductives (winged queens and males) emerge from preexisting nests several days prior to the nuptial flight, and are encouraged back into the nest by workers. Some reproductives die after not returning to the nest. Hours before the nuptial flight, queens, males and workers emerge from the nest and swarm around the entrance hole. Workers will even nip at queens on the ground to encourage them to take flight.

Nuptial flights
To mate, M. mexicanus winged queens and males emerge from an existing colony and swarm in nuptial flights. These flights occur in late July in the evening, typically at about the same time of day when the colony workers begin foraging. The flights usually occur the day after a rain. A normal nuptial flight lasts about an hour and half. After the flight, unmated queens and males return to the nest. The fact that some queens and males return from these flights unmated suggests that aerial union is difficult. The mated queens lose their wings, and the mated males die.

Colony Founding/ Development
After mating, a queen digs a nest chamber. About two days after her flight, she seals the nest chamber. Conway found that many mated queens after digging and sealing nest chambers did not reappear later on, which could indicate a high mortality rate among newly mated queens. Around five days after the flight, the queen lays eggs in clusters of 5-10. The larvae hatch around 20 days, and begin pupating in cocoons around 10 days after that. Adult workers emerge around 63 days after eggs are laid. Larvae are feed and cared for by the queen and workers. It is believed that the larvae may even be fed eggs and dead workers.

The workers in the first brood tend to be smaller than workers in subsequent broods, which is believed to be an adaptation to promote rapid population growth. The average worker lives from 11 to 170 days.

Population structure
Conway surveyed sixty six nest colonies located near Garden of the Gods, Colorado Springs, CO. He found that a typical nest consisted of about 5,000 ants. Typically, about 75% to 78% of the total population consisted of workers. About 22 -25% of the colony population consisted of replete workers. In preparation for nuptial flights, nests produce a large number of reproductives. 100-110 males were found in one nest, and 209 queens were found in another.

Role within the colony
The name honeypot ant comes from the peculiar development of replete workers, whose abdomens become so swollen with food that they are used by the rest of the colony as living food storage. They are “drained” during famine, usually the wintertime, to sustain the colony. This behavior is an example of the caste system within ants and other eusocial insects. Repletes are a subset of the sterile "helper" caste.

Development
In M. mexicanus, replete workers develop from the largest workers in the colony. In fact, if the repletes are removed from a colony, the next largest workers quickly become repletes. Typical workers and callows can develop into repletes in about two weeks. Other workers feed the developing repletes with plant nectar collected in the evenings. The repletes become so swollen that they become immobile, and hang from the ceilings of dome chambers in the underground nest.

Morphology
When a replete worker fills with food, her crop, or portion of her digestive track, swells and displaces other abdominal organs. The crop of replete workers expands about four to five times its normal linear dimension when they are fully engorged with food. In M. mexicanus, the size of a replete worker’s abdomen ranges from 6-12mm in length. As repletes are drained of their food stores, they become “flaccid depletes”. M. mexicanus repletes also vary in the content of their “honey” or food they carry. They can be classified according to the colors of their distended abdomens: dark amber, amber, milky and clear. Each color is thought to correspond with a particular food source. Dark amber and amber repletes contain sugars such as glucose and fructose, mostly likely from the nectar of flowers. Milky repletes contain large amounts of protein and oils, believed to be from insect prey. Clear repletes contain mostly water and sucrose (sugar), so they are thought to act as water storage for the arid climate. Burgett and Young found that a small percentage of repletes had two visible layers of liquid in their crops– one consisting of sugars, and the other of lipids, glycerol and cholesterol esters.

Food sources
M. mexicanus collects food mainly in the form of nectar from yucca plants (Yucca glauca) and sugary galls formed on scrub oaks (Quercus gambellii). The gall wasp (Holcaspis perniciosus) forms the galls (rotund growths) along scrub oak branches, and these galls leak a clear sugary liquid which the ants feed on. Additionally, these ants feed on a variety of flower nectars, cacti fruit, and the excretions of aphids feeding on yucca plants. Workers will also collect dead insects and attack small, soft bodied insects. In particular, M. mexicanus tends to feed on dead and moribund harvester ants (Pogonomyrmex occidentalis).

Foraging behavior
As worker forage, their abdomens become clear and distended, similar to replete workers. When they return to the nest, they regurgitate their collected nectar into replete workers for storage.

March through November is the typical foraging time for M. mexicanus. The summer months are when most nectar is collected. The ants forage in cooler temperatures ranging from 0.5o to 30o Celsius (33o to 86o Fahrenheit) and mainly during the night. The lower temperature preference is most likely an adaptation to allow for nocturnal foraging in desert climates.

Workers forage up to 20 meters (65.6 feet) away from the entrance of their nest. They exhibit both radial and path foraging patterns. Radial foraging (when the ants spread out radially from the nest entrance) happens mainly early in the season, when the ants are searching for nectar sources. Path foraging (when the ants follow frequently traveled paths) occurs later in the season once paths to nectar sources have been established.

Interference
Other species of ants such as Conomyrma biocolor has been known to drop stones around the entrances of M. mexicanus nests. This behavior usually happens if the two species have colony entrances within 3 meters of each other. The two species have similar foraging times and food sources which creates competition between the colonies. The foraging of a M. mexicanus colony can decrease drastically due to this stone dropping technique.

Nest location
Founding queens appear to prefer place their nests on top of ridges and mesas.

The nest entrances are typically located out in the open, and the colonies will move their entrances if their original entrance hole (tumulus) becomes shaded. M. mexicanus are nocturnal foragers and prefer lower temperatures between 15o to 20o Celsius (59o to 68o Fahrenheit). Therefore, it may be critical for them to have uncovered entrances so that the soil can cool off quickly after sunset and allow them to forage sooner.

In a study conducted by Dr. Cole, Haight and Wiernasz, M. mexicanus nests have been observed to be closely associated with nests of Pogonomyrmex occidentalis (harvester ants). M. mexicanus feeds on dead and nearly dead P. occidentalis workers, accounting for this close association. Of the 145 food items collected by M. mexicanus workers, 110 items were dead P. occidentalis workers and two were living workers. Nests of M. mexicanus on average were spaced 5 – 10 meters away from P. occidentalis nests, but a decent fraction of nests were found closer than 3 meters away.

Predation
M. mexicanus appears to not have many natural predators. Coyotes and badgers have been observed digging up colonies of ants.