Identification And Behavior

All fire ants have two segments in their narrow waist and antennae with 10 segments. The workers range in size from small to large (for S. invicta, about 2—5 mm in length; queens are about 7 mm long). To be able to sting, fire ants must first gain leverage with their mandibles by biting; they then curve around the abdomen to insert the stinger. The fire ant injects venom consisting mainly of piperidine alkaloids that produce a burning sensation. Shortly thereafter, a red spot is usually visible. The burning sensation is short-lived, followed by itching. In most people, a white pustule will develop at the site within a few hours. These pustules are sometimes called "sterile pustules" because they are not produced by infectious bacteria. The pustules can last from days to weeks and can become infected if they are scratched. The venom also contains a small amount of protein (about 1%) that can cause anaphylactic shock in susceptible individuals. Fire ants can sting repeatedly; therefore, stinging ants should be brushed off rapidly.

Mounds and Foraging Behavior

When undisturbed, the typically dome-shaped mounds of S. invicta can reach heights of 30 cm or more above the ground. These mounds allow the workers to respond to local conditions by moving up and down with their brood and queens according to temperature and humidity. Exit holes are usually not apparent on the mounds themselves, but foraging trails extend outward from the mound just below the surface. During floods, fire ants move to the upper parts of the mound. If the water gets any higher, the ants grasp each other to form floating rafts that carry the brood and queens downstream. During droughts, fire ants can extend their tunnels down 6 m or more in search of moisture.

Fire ant workers can feed only on liquids: they have filters in their digestive tract that prevent the ingestion of solids. Only the fourth instar can digest solids directly, and it is the only path for processing of solid food particles in the colony. Workers deposit insect parts and other solids on the larva's "food basket." After feeding on these solid foods, the larva secretes liquids that are licked up by workers and distributed around the colony.

Mating Behavior

Like most other ants, fire ants have mating flights. In the southeastern United States, flights are most frequent in the spring following rain and subsequent sunshine. After the rain, workers fill the queens with food to prepare them for the flight. Dissection of queens at this time shows a large drop of yellow oil in their crops. In midafternoon the workers open large exit holes in the mounds to allow quick exit of the males and females. Workers become very agitated and start chasing the reproductives, which then climb vertical objects nearby from which they fly. Mating occurs in the sky. The males drop to the ground and die shortly thereafter. The queens also land, quickly shed their wings, and search for a place to dig a tunnel. The queen will close the tunnel and start to lay eggs, producing her first workers in about one month. Queens typically live 6 or more years. Because they mate only once, they must store live sperm for the rest of their lives. For this purpose, they have a transparent sac in their abdomens called the spermatheca that is filled with over a million sperm after mating. When the queen lays an egg, she can open a valve on the spermatheca, allowing the escape of sperm to fertilize her eggs. These diploid eggs give rise to females, either workers or new queens. If she does not release sperm, the egg she lays is haploid and becomes a male, as is typical for all haplodiploid social Hymenoptera.

Number of Queens

There are two forms of S. invicta in the United States. Originally this species was thought to be monogyne, having one queen per colony. Polygyne, or multiple-queen fire ant colonies, were first described from Mississippi in the early 1970s. In these colonies there can be dozens of fertilized queens. The queens are not aggressive toward each other and are frequently together in one part of the nest. One or more of these queens may be dominant, laying more eggs and receiving more food than the others. Polygyne queens in a colony are not closely related, suggesting that they are adopted from outside sources. They are also smaller on the average than monogyne queens and lay fewer eggs. However, the total number of eggs laid by all the polygyne queens in a colony exceeds that produced by a single monogyne queen. On the average, polygyne colonies also have smaller workers: there is a negative correlation between the number of queens and the average worker size. Furthermore, the polygyne form is not aggressive toward conspecifics, whereas the monogyne form will fight with nearby conspecifics. The polygyne form can bud off new colonies of queens and workers and is thereby able to quickly populate an area with fire ants. The polygyne form is now predominant in Texas and has also been found in Florida, Georgia, and even South America.

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