Amazing Nature

Eusocial Insects - Ants

Originally published in Green Country, Issue 120 (Jun 2016)
Author: Green Power
Two ants on tree trunk
"ant" by Aleksey Gnilenkov is licensed under CC BY 2.0

Social insects live in groups, and forage and take care for their offspring together. Mention social insects, and bees, ants and aphids spring to mind. Some social insects, however, are more "genuinely social" than others. In biological terms, these insects are "eusocial", which has a more restricted definition. Ants are eusocial insects.

A worker ant carrying larvae
Worker moving larva about
"20100623h" by dw_ross is licensed under CC BY 2.0

As defined by American biologist E. O. Wilson, eusocial insects must meet all the following three criteria: overlapping generations within a colony of adults; cooperative brood care; and a division of labour into reproductive and non-reproductive groups – that is, only a certain group within the colony is responsible for reproduction, while other individuals are sterile (or have very low reproductive ability). The latter are responsible for taking care of the colony. According to the definition, not all species of bees and aphids are eusocial. But all species of ants and termites are eusocial.

One larger worker ant and one smaller worker ant
Workers may have distinctive variations in size
"Florida harvester ant polymorphic workers" by Bob Peterson is licensed under CC BY 2.0

The social structure of an ant colony is formed by a queen, workers and male ants. Most members of the colony are workers and they are all female, offspring of the queen. Their duties include taking care of the queen and all other duties in the colony, such as foraging, raising new generations, defending and mending nest. In some species, the workers are of different sizes; the smaller ones work in the nest while the bigger ones go out foraging. The largest are the soldiers, which take charge of defending the nest. Some soldiers have well-developed mandibles to attack invaders.

Soldiers ant with well-developed mandibles
Soldiers of some species have well-developed mandibles to attack invaders.
"Army ant warrior" by Axel Rouvin is licensed under CC BY 2.0

The queen is the head of the colony. Most colonies have only one queen, which is responsible for reproduction. The queen gives birth to certain winged male and female ants which are reproductive. In between spring and summer, on the days suitable for nuptial flight, they leave the nest and mate with the opposite sex. The male die after mating, while the female finds a suitable place to start a new colony and becomes a new queen. The queen-to-be lays the first batch of eggs after her wings are shed. These eggs are cared for by the queen herself. After the successful breeding of the first generation worker ants, the work of foraging, raising young and expanding the nest is taken on by the workers and the colony can thrive.

The female ants with wings
The female ants with wings in the colony are larger than ordinary female ants. Upon successful mating, they will become the new generation queens.
"ants" by Yisong Yue is licensed under CC BY-SA 2.0

Two ants touching each other by the antennae
Ants touching each other by the antennae are exchanging information through pheromones.
"Shattuck_C11745-2, Papyrius, Cairns, Queensland" by Steve Shattuck is licensed under CC BY 2.0

Communicating through pheromones

An ant colony may have thousands of ants. How can they communicate with each other? The secret lies in chemicals called pheromones. There are numerous glands on the body of the ant that secrete more than a dozen pheromones. The ants make use of these chemicals to communicate. You may have seen ants touching each other with the antennae - they are exchanging information through pheromones.

Ants foraging
Ants can make use of pheromones to share messages on food location, thus mobilising group foraging.

Apart from touching each other, ants may “leave messages” in the environment through pheromones too. Multiple information, such as on food location, gathering, alert and attack, can be shared. With pheromones, the queen can suppress the development of larvae into queens, or even establish and maintain complex social interactions among the colony, such as ratio of soldiers, social structure and hierarchy. In some species, ants can communicate by vision and sound as well as the chemical messages.