Insect societies

Many of the most robust, thriving species today owe their success in great part to the benefits that they reap from living in organized groups or societies. Nowhere are the benefits of group living more clearly illustrated than among the social insects. Edward O. Wilson (1929–2021), one of the foremost authorities on insect societies, is often called the father of biodiversity. He estimated that more than twelve thousand species of social insects exist in the modern world. This number is equivalent to all known bird species and mammals combined. Although insect societies have reached their pinnacle in the bees, wasps, ants, and termites, many insects show intermediate degrees of social organization—providing insights regarding the probable paths of the evolution of sociality.

Ant, Wasp, and Bee Societies

Scientists estimate that eusociality has evolved at least twelve times: once in the Isoptera, or termites, and eleven separate times in the order Hymenoptera, comprising ants, wasps, and bees. In addition, one group of aphids has been found which has a sterile soldier caste. Although the eusocial species represent diverse groups, they all show a high degree of social organization and possess numerous similarities, particularly with regard to division of labor, cooperative brood care, and communication among individuals. The organization of a typical ant colony is representative, with minor modifications, of all insect societies.

A newly mated queen, or reproductive female, will start a new ant colony. Alone, she digs the first nest chambers and lays the first batch of eggs. These give rise to grublike larvae, which are unable to care for themselves and must be nourished from the queen’s own body reserves. When the larvae have reached full size, they undergo metamorphosis and emerge as the first generation of worker ants. These workers—all sterile females—take over all the colony maintenance duties, including foraging outside the nest for food, defending the nest, and cleaning and feeding both the new brood and the queen, which subsequently becomes essentially an egg-laying machine. For a number of generations, all eggs develop into workers and the colony grows. Often, several types of workers can be recognized. Besides the initial small workers, or minor workers, many ant species produce larger forms known as major workers, or soldiers. These are often highly modified, with large heads and jaws, and are well suited for defending the nest and foraging for large prey. Food may include small insects, sugary secretions of plants or sap-feeding insects, or other scavenged foods. After several years, when the colony is large enough, some of the eggs develop into larger larvae that will mature into new reproductive forms: queens and males. Males arise from unfertilized eggs, while new queens are produced in response to changes in larval nutrition and environmental factors. These sexual forms swarm out of the nest in a synchronized fashion to mate and found new colonies of their own.

With minor modifications, the same pattern occurs in bees and wasps. Workers of both bees and wasps are also always sterile females, but they differ from ants in that they normally possess functional wings and lack a fully differentiated soldier caste. Wasps, like ants, are primarily predators and scavengers; bees, however, have specialized on pollen and plant nectar as foods, transforming the latter into honey that is fed to both nestmates and brood. The bias toward females reflects a feature of the biology of the Hymenoptera that is believed to underlie their tendency to form complex societies. All ants, wasps, and bees have an unusual form of sex determination in which fertilized eggs give rise to females, and unfertilized eggs develop into males. This type of sex determination, known as haplodiploidy, generates an asymmetry in the degree of relatedness among nestmates. As a consequence, sisters are more closely related to their sisters than they are to their own offspring or their brothers. Scientists believe that this provided an evolutionary predisposition for workers to give up their own personal reproduction in order to raise sisters—a form of natural selection known as kin selection.

Despite the social nature of most insects in these groups, there are also many solitary speciesemerald wasps (family Hymenopteran), sand wasps (group Bembicini), wool carder bee (Anthidiummanicatum), Loosestrifes (Genus Lysimachia), velvet ants (family Mutillidae), and ants in the genus Myrmecia.

Termite Society

The termites, or Isoptera, differ from the social Hymenoptera in a number of ways. They derive from a much more primitive group of insects and have been described as little more than “social cockroaches.” Instead of the strong female bias characteristic of the ants, bees, and wasps, termites have regular sex determination; thus, workers have a fifty-fifty sex ratio. Additionally, termite development lacks complete metamorphosis. Rather, the young termites resemble adults in form from their earliest stages. As a consequence of these differences, immature forms can function as workers from an early age, and—at least among the lower termites—they regularly do so.

Termites also differ from Hymenoptera in their major mode of feeding. Instead of feeding on insects or flowers, all termites feed on plant material rich in cellulose. Cellulose is a structural carbohydrate held together by chemical bonds that most animals lack enzymes to digest. Termites have formed intimate evolutionary relationships with specialized microorganisms—predominantly flagellate protozoans and some spirochete bacteria—that have the enzymes necessary to degrade cellulose and release its food energy. The microorganisms live in the gut of the termite. Because these symbionts are lost with each molt, immature termites are dependent upon gaining new ones from their nestmates. They do this by feeding on fluids excreted or regurgitated by other individuals, a process known as trophallaxis. This essential exchange of materials also includes, along with food, certain nonfood substances known as pheromones.

Insect Communication

Pheromones, by definition, are chemicals produced by one individual of a species that affect the behavior or development of other individuals of the same species that come in contact with them. Pheromones are well documented throughout the insect world, and they play a key role in communication between members of nonsocial or subsocial species. Moth mating attractants provide a well-studied example. Pheromones are nowhere better developed than among the social insects. They not only appear to influence caste development in the Hymenoptera and termites but also permit immediate communication among individuals. Among workers of the fire ant (Solenopsis saevissima), chemical signals have been implicated in controlling the recognition of nestmates, grooming, clustering, digging, feeding, attraction or formation of aggregations, trail following, and alarm behavior. Nearly a dozen different glands have been identified which produce some chemicals in the Hymenoptera, although the exact function of many of these chemicals remains unknown.

In addition to chemical communication, social insects may share information in at least three other ways: by tactile contact, such as stroking or grasping; by producing sounds, including buzzing of wings; and by employing visual cues. Through combinations of these senses, individuals can communicate complex information to nestmates. Indeed, social insects epitomize the development of nonhuman language. One such language, the “dance” language of bees, which was unraveled by Karl von Frisch and his students, provides one of the best-studied examples of animal behavior. In the waggle dance, a returning forager communicates the location of a food resource by dancing on the comb in the midst of its nestmates. It can accurately indicate the direction of the flower patch by incorporating the relative angles between the sun, the hive, and the food. Information about distance, or more precisely the energy expended to reach the food source, is communicated in the length of the run. Workers following the dance are able to leave the nest and fly directly to the food source, for distances in excess of one thousand meters.

Benefits of Cooperation

Living in cooperative groups has provided social insects with opportunities not available to their solitary counterparts. Not only can more individuals cooperate in performing a given task, but also several quite different tasks may be carried out simultaneously. The benefits from such cooperation are considerable. For example, group foraging allows social insects to increase the range of foods they can exploit. By acting as a unit, species such as army ants can capture large insects and even fledgling birds.

A second benefit of group living is in nest building. Shelter is a primary need for all animals. Most solitary species use naturally occurring shelters or, at best, build simple nests. By cooperating and sharing the effort, social insects are able to build nests that are quite elaborate, containing several kinds of chambers. Wasps and bees build combs, or rows of special cells, for rearing brood and storing food. Subterranean termites can construct mounds more than six meters high, while others build intricate covered nests in trees. Mound-building ants may cover their nests with a thatch that resembles, in both form and function, the thatched roofs of old European dwellings. Colonial nesting provides two additional benefits. First, it enhances defense. By literally putting all of their eggs in one basket, social insects can centralize and share the guard duties. The effectiveness of this approach is attested by one’s hesitation to stir up a hornet’s nest. Nest construction also provides the potential to maintain homeostasis, the ability to regulate the environment within a desirable range. Virtually all living creatures maintain homeostasis within their bodies, but very few animals have evolved the ability to maintain a constant external living environment. In this respect, insect societies are similar to human societies. Workers adjust their activities to maintain the living environment within optimal limits. Bees, for example, can closely regulate the internal temperature of a hive. When temperatures fall below 18 degrees Celsius (64.5 degrees Fahrenheit), they begin to cluster together, forming a warm cover of living bees to protect the vulnerable brood stages. To cool the hive in hot weather, workers initially circulate air by beating their wings. If further cooling is needed, they resort to evaporative cooling by regurgitating water throughout the nest. This water evaporates with wing fanning and serves to cool the entire hive. Other social insects rely on different but equally effective methods. Some ants, and especially termites, build their nests as mounds in the ground, with different temperatures existing at different depths. The mound nests of the African termite, Macrotermes natalensis, are an impressive engineering feat. They are designed to regulate both temperature and airflow through complex passages and chambers, with the mound itself serving as a sophisticated cooling tower.

Finally, group living allows the coordination of the efforts of individuals to accomplish complex tasks normally restricted to the higher vertebrates. The similarities between insect societies and human society are striking. An insect society is often referred to as a superorganism, reflecting the remarkable degree of coordination between individual insects. Individual workers have been likened to cells in a body and castes to tissues or organs that perform specialized functions. Insect societies are not immortal; however, they often persist in a single location for periods similar to the life spans of much larger animals. The social insects have one of the most highly developed symbolic languages outside human cultures. Further, social insects have evolved complex and often mutually beneficial interactions with other species to a degree unknown except among human beings. Bees are inseparably linked with the flowers they feed upon and pollinate. Ants have actually developed agriculture of a sort with their fungus gardens and herds of tended aphids. On a more sobering note, ants are the only nonhuman animals that are known to wage war. There are also some species of ants called kidnapper ants that steal young ants from the colonies of other species and force them to work in their colony. These striking similarities with human societies have led researchers to study social insects to learn about the biological basis of social behavior and have led to the development of the branch of science known as sociobiology.

Studying Insect Society

Because of the diversity of questions that investigators have addressed regarding insect societies, many methods of scientific inquiry have been employed. In Karl von Frisch’s experiments, for example, basic behavioral observations were coupled with simple but elegant experimental design to unravel the dance language of bees. The bees were raised in an observation colony. This was essentially a large hive housed between plates of glass so that an observer could watch the behavior of individual bees. Researchers followed specific workers by marking them with small numbers placed on the abdomen or thorax. Sometimes, the entire observation hive was placed within a small, darkened shed to simulate the conditions within a natural hive more closely.

Bees learned to find an artificial “flower”—a glass dish filled with a sugar solution. Brightly colored backgrounds and odors, such as peppermint oil, were added to the sugars to provide specific cues for the bees to associate with the reward. Feeding stations were set up at fixed distances; observers could follow the exact movements of known individuals both at the feeder and at the hive. In this way, von Frisch was able to describe several types of dances (the round dance for near food sources, the waggle dance for feeding stations that were farther from the hive) and show that a returning bee could share information regarding the location and quality of a source with her nestmates. Scientists subsequently have developed robot bees that can be operated by remote control to perform different combinations of dance behaviors. This allows them to determine which parts of the dance actually convey the coded information.

The investigation of forms of chemical communication requires the application of a variety of techniques. Chromatography is useful for identifying the minute amounts of chemical pheromones with which insects communicate. Chromatography (which literally means “writing with color”) is particularly suitable for separating mixtures of similar materials. A solution of the mixture is allowed to flow over the surface of a porous solid material. Since each component of the mixture will flow at a slightly different rate, eventually, they will become separated or spaced out on the solid material. Once the components of the pheromone have been separated and identified, their activity is assessed separately and in combination using living insects. Such bioassays allow researchers to determine exactly which fractions of the chemical generate the highest response.

Other biochemical techniques, such as electrophoresis, have been used to determine subtle behavioral differences, such as kin discrimination among hive mates. Each individual carries a complement of enzymes or proteins that catalyze biological reactions in the body. The structure of such enzymes is determined by the genetic makeup of the individual, and it varies among individuals. Because enzyme structure is inheritable, however, much as eye color is, the degree of similarities between the enzymes can be used as a measure of how closely related two individuals are. The amino acids composing the enzyme differ in their electrical charges, so different forms can be separated using the technique of electrophoresis. When a liquid containing its enzymes is subjected to an electrical field, the proteins with the highest negative charge will move farthest toward the positive pole. This provides a tool to distinguish close genetic relatives for use in conjunction with behavioral observations to test, for example, whether workers can discriminate full sisters from half-sisters or relatives from nonrelatives, as kin selection theory would predict.

The Success of Social Insects

Social insects are among the most successful groups of animals throughout the world, especially in the tropics. Although the number of species is low when compared to all insects (twelve thousand out of more than a million species), their relative contribution to the community may be unduly large. In Peru, for example, ants may make up more than 50 percent of the individual insects collected at any site.

The study of social insects has provided scientists with new ways of looking at social behavior in all animals. Charles Darwin described the evolution of sterile workers in the social insects as the greatest obstacle to his theory of evolution by natural selection. In an attempt to explain this paradox, William D. Hamilton closely examined the social Hymenoptera, where sociality had evolved eleven separate times. Realizing that the haplodiploid form of sex determination led to sisters being more closely related to one another than they would be to their own young, Hamilton developed a far-reaching new theory of social evolution: kin selection, or selection acting on groups of closely related individuals. This theory, which provides insights into the evolution of many kinds of seemingly altruistic behaviors, arose primarily from his perceptions regarding the asymmetrical relatedness of nestmates in the social Hymenoptera. These insects, then, should be credited with providing the model system that has led to a subdiscipline of behavioral ecology known as sociobiology, the study of the biological basis of social behavior. Moreover, given their central roles in critical ecological processes such as nutrient cycling and pollination, it would be hard to imagine life without them.

Many twenty-first-century studies have shown that insect populations have steeply declined since the 1970s. One study examined decades worth of data from more than 1,600 locations worldwide and concluded that insect populations on land have declined by an average rate of 1 percent per year. The study attributed this decline to a combination of climate change, increased use of pesticides, and habitat destruction, among other issues. The study also found that freshwater insect populations have increased by 1 percent annually, attributed largely to clean water efforts. Insects are keystone species in our ecosystem, and without them, the food chain would fail. Conservation efforts that restore and preserve insect habitats will be increasingly important if the population decline continues.

Principal Terms

Brood: all the immature insects within a colony; these include eggs, larvae, and, in the Hymenoptera, the pupal stage

Caste: one of the recognizable types of individuals within a colony, usually physically and behaviorally adapted to perform specific tasks

Eusocial: referring to any of the truly social species characterized by division of labor, with a sterile caste, overlapping generations, and cooperative brood care

Haplodiploidy: sex determination found in the Hymenoptera, where males arise from unfertilized eggs and females from fertilized eggs

Metamorphosis (complete): a transformation that occurs during the development of higher insects, in which a grublike immature form enters a resting (pupal) stage for major tissue reorganization; after pupation, the adult, which bears no resemblance to the larval form, emerges

Pheromone: a chemical produced by one member of a species that influences the behavior or physiology of another member of the same species

Trophallaxis: the exchange of bodily fluids between nestmates, either by regurgitation or by feeding on secreted or excreted material

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