What Are Social Insects?

The true social insects—all ants and termites, and some bees and wasps—comprise 75 percent of the world's insect biomass, according to E.O. Wilson. A colony of social bees can number in the tens of thousands, and hundreds of millions of ants can live together in a supercolony of interconnected nests. 

So what makes social insects behave the way they do? There are several theories, as well as varying degrees of social behavior.

Advantages of Social Behavior in Insects

Why have some insects evolved to live in large, cooperative colonies? There's strength in numbers. Social insects gain several advantages over their solitary cousins. Social insects work together to find food and other resources and to communicate their findings to others in the community. They can mount a vigorous defense of their home and resources when under attack.

Social insects also can outcompete other insects, and even larger animals, for territory and food. They can quickly construct a shelter, and expand it as needed, and they can divide chores in a manner that ensures everything gets done expeditiously.

Characteristics of Social Insects

So how do we define social, when speaking of insects? Many insects exhibit social behaviors, such as aggregating in large numbers at times. Gregarious behavior does not, by itself, mean an insect is social.

Entomologists refer to true social insects as eusocial. By definition, eusocial insects must exhibit all 3 of these characteristics:

1. overlapping generations
2. cooperative brood care
3. a sterile worker caste

To give an example, think of termites. All termites are eusocial insects. Within a single termite colony, you will find individuals at various stages of the termite life cycle. Generations of termites overlap, and there is a constant supply of new adults prepared to assume responsibility for the colony's care. The community cares for its young cooperatively.

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Termite communities are divided into three castes. The reproductive caste is comprised of a king and queen. The soldier caste of both males and females is specially adapted for defending the colony. Soldiers are larger than other termites and are sterile. Finally, the worker caste consists of immature males and females that do all chores: feeding, cleaning, construction, and brood care.

Solitary insects, by contrast, don't exhibit any of these social behaviors. 

Degrees of Sociality in Insects

As you may realize by now, many insects don't fit in either category. Some insects are neither eusocial nor solitary. Insects fall somewhere on a spectrum of sociality, with several degrees between solitary and eusocial.

Subsocial Insects

Just a step above solitary insects are the subsocial insects. Subsocial insects provide limited parental care to their offspring. They may shelter or guard their eggs, or even stay with their young nymphs or larvae for a time.

Most subsocial insects don't use nests to shelter their young, though there are exceptions to this rule. Giant water bugs fall into the subsocial group. The female deposits her eggs on the male's back, and he is charged with protecting and caring for the offspring until they hatch.

Communal Insects

Communal insects share a nest site with others of the same generation. This social behavior may be exhibited in one particular stage of the life cycle, such as in the larval stage of some moths. Communal insects use sophisticated forms of communication and gain certain advantages from nesting together. Communal living may help them avoid predation, assist them with thermoregulation, or enable them to find and use resources more efficiently.

Communal insects never share in caring for offspring, however. Tent-making caterpillars, such as the eastern tent caterpillars, build a communal silk tent, in which they all shelter. They share information about food sources by creating chemical trails, allowing their siblings to follow the scent to its location.

Quasi-social Insects

A slightly more advanced form of social behavior is exhibited by quasi-social insects. These insects do exhibit cooperative care of their young. A single generation shares a common nest. Certain orchard bees function as quasi-social groups, with multiple females sharing a nest and caring for their young together. Though all the bees share in brood care, not all bees lay eggs in the nest cells.

Semi-social Insects

Semi-social insects also share child-rearing duties with other individuals of the same generation, in a common nest.

As in true social insects, some members of the group are nonreproductive workers. However, this generation will leave their nest before the next generation emerges. The new adults will disperse and construct new nests for their offspring. For example, paper wasps are semi-social in the spring, with nonreproductive workers helping expand the nest and tend to the brood in a new colony.

Primitively Eusocial Insects

The sole difference between eusocial insects and primitively eusocial insects lies in the sterile worker caste. In primitively eusocial insects, the workers look the same as queens, with little or no morphological differences between the castes. Some sweat bees are primitively eusocial.

Bumblebees, for example, are also considered primitively eusocial, although they're an unusual example in that the queen is slightly larger than her workers, and therefore can be differentiated.

Table of Sociality in Insects

The following table illustrates the hierarchy of sociality in insects. The chart ranges from the lowest degree of sociality (solitary insects) at the bottom, to the highest degree of sociality (eusocial insects) at the top.