Social insects are among the most dominant and prolific of all organisms on earth.  In South America, leafcutter ants (Atta spp.) consume more foliage than mammalian herbivores; in the southwestern United States, more seeds are collected and eaten by harvester ants than by all other forms of life; in some tropical habitats, the biomass of ants and termites exceeds that of all other animal species combined; in the African savanah, a single colony of driver ants may contain as many as 20 million workers; in Japan, a supercolony of Formica yessensis with 45,000 interconnecting nests contained more than a million queens and 306 million workers within an area of 2.7 square kilometers.

Many insects exhibit "social" behaviors (e.g. feeding aggregations, parental care of the young, and communal nest sites).  In a broad sense, any insect that interacts with another member of its own species could be called a social insect.  But as a rule, entomologists do not regard these behaviors as sufficient justification for classifying a species as truly social (i.e. eusocial).  In order to qualify as eusocial, a species must exhibit all four of the following characteristics:

1. Share a common nest site
2. Individuals of the same species cooperate in caring for the young
3. Reproductive division of labor -- sterile (or less fecund) individuals work for the benefit of a few reproductive individuals
4. Overlap of generations -- offspring contribute to colony labor while their parents are still alive

Species that lack one or more of these characteristics are classified as presocial.  Within this category are subsocial species (in which the parents care for their offspring) and parasocial species (which have a common nest site but lack one or more of the other eusocial characteristics).  The table below helps clarify the various terms used for social and presocial insects.  Click on each term for a formal definition.

A Classification System for Presocial and Eusocial Insects

	Common nest site	Cooperative brood care	Reproductive castes	Generational overlap
Solitary	No	No	No	No
Communal or Subsocial	Yes	No	No	No
Quasisocial	Yes	Yes	No	No
Semisocial	Yes	Yes	Yes	No
Eusocial	Yes	Yes	Yes	Yes

Living in social groups has both advantages and disadvantages.  Large colonies are especially vulnerable to the spread of contagious pathogens, nest sites may be exploited by "social parasites" who steal food or attack the brood, and member individuals must compete with each other for space and resources (e.g. food).  But on the other hand, cooperative behavior can accomplish feats that would be impossible for solitary insects:  construction of huge nest sites, widespread foraging for food, and constant vigilance against predation or parasitism.  For social insects, the benefits outweigh the liabilities.  In the final analysis, social behavior is an adaptation that promotes survival and reproductive success of the species.

Relatively few insects are classified as eusocial -- the distinction is limited to the following groups:
Order Isoptera	Order Hymenoptera
Termites -- all species	Ants -- all species (family Formicidae) Bees -- about 600 species in the family Apidae Wasps -- about 700 species in the family Vespidae
Click on the links below for more information about each group.

Termites

Ants

Bees

Wasps