Behavioral homeostasis refers to mechanisms of behavior that allow an insect or group of insects to maintain conditions within a certain range of values. These conditions may be the temperature of the body or the environment, internal water balance or environmental humidity, nutritional state or food stores, the balance between different activities of the individual or of the group, or the number and composition of individuals in a group. Behavioral mechanisms of homeostasis are important to individual insects, whether solitary individuals or part of a group, and include such nearly universal behaviors as feeding and drinking, as well as behavior concerned with thermoregulation and habitat choice. This article, though, is mostly concerned with homeostasis in groups of insects, such as the colonies of bees, wasps, ants, and termites. Individual behavioral homeostasis in physiological regulation, thermoregula-tion, and habitat choice are covered elsewhere in this encyclopedia.
ENVIRONMENTAL REGULATION BY GROUPS OF INSECTS
Insects are relatively small animals, with high surface-to-volume ratios. Because of this, they readily lose body heat or water to the environment (or gain heat if the ambient temperature is high). However, a few species of insects form large groups that are able to exert some control over these processes. The most striking examples of this come from the social insects (the wasps, ants, bees, and termites), but some other insects also form groups that enhance homeostasis (Fig. 1 ).
The control of groups of insects over heat exchange may take two forms. First, they may form a cluster that effectively makes them more similar collectively to larger organisms. If the surface-to-volume ratio is of a cluster of insects rather than an individual, it has a smaller value, and heat exchange is slower. Second, most social insects construct nests, and the architecture of these nests can result in the interior environment being substantially different from the ambient environment outside the nest.
FIGURE 1 Honey bees (Apis mellifera, blue line) and yellow-jacket wasps (Vespula vulgaris, red line) both maintain their nests at temperatures that fluctuate less than outside air temperatures (black line). At cool outside temperatures, as here, the nests are kept warmer than ambient. Note that the honey bee colony, with tens of thousands of workers, achieves more precise temperature homeosta-sis than the wasp colony, with only hundreds of workers. (Data from Kemper, H., and Dohring, E.
Honey bees exhibit both of the above strategies. Honey bees (Apis spp.) arose in the tropics, but A. mellifera and A. cerana have colonized much of the temperate zone as well. These honey bees are unique among temperate insects in maintaining a high temperature in their nests throughout the winter, even when environmental temperatures are dramatically lower. For example, an A. mellifera colony can maintain a temperature in the center of its winter cluster inside the nest of 35°C, even when the temperature outside the nest is —40°C. The bees accomplish this by clustering together tightly so that the bees themselves, as well as the nest structure (often a hollow tree or wooden beehive) serve as insulation. The bees consume honey as metabolic fuel and contract their large flight muscles to create heat. Bees on the outside do get chilled, but they trade places with bees in the warm interior from time to time. Even when a bee colony is not in a nest, as when they are moving as a swarm to a new homesite, they maintain warm temperatures inside the cluster of thousands of bees.
The environment is not always cold, so temperature homeosta-sis for a bee colony sometimes involves cooling the nest. Honey bees fan their wings to move outside air through a colony to remove excess metabolic heat (and carbon dioxide). When this does not cool the colony enough, the bees begin to collect water and evaporate it within the nest to provide cooling. Also, when the nest becomes too warm, many bees leave the cavity and cluster outside the nest, reducing the heat input from their metabolism.
Many species of termites, like honey bees, live in large groups. Indeed, the largest colonies of social insects occur among the termites, some species of which may have several million individuals in a nest. Unlike honey bees, termite workers do not have wings, and so they cannot move air by fanning. Instead, some species of termites rely on the structure of the nest to regulate temperature and humidity. Macrotermes subhyalinus colonies, for example, construct tall “chimneys” on their nests. These chimneys are thought to increase airflow in two ways. As the metabolic heat of the termite colony (and the fungus gardens that they cultivate in the nest) warms the air in the chimneys, it rises and is replaced by cooler air from passages near the ground. Also, when wind blows across the open tops of the chimneys, the Bernoulli effect causes lower air pressure at the chimney top and draws air upward.
Other species of Macrotermes build nests with a closed-circuit air circulation system. In these nests gas exchange takes place by diffusion through the relatively thin walls of air channels in ribs on the outer edges of the nest. As air moves through the channels by convection of warm air, carbon dioxide diffuses out, and oxygen diffuses inward in, while limiting loss of moisture. The refreshed air is pulled back to the central nest to replace the air drawn off by convection. In the nest, respiration of termites themselves and the fungus they cultivate uses oxygen and produces carbon dioxide. The cycle repeats over and over, insuring oxygenated air with less loss of water than would occur with an open-circuit system.
In Australia, Amitermes meridionalis nests are constructed as flat towers, always oriented with their long axis north and south. The result is that they are warmed by sun as it rises in the east early in the morning and strikes their broad side, but they receive relatively little sunshine at midday when the sun is in the north and strikes the nest edge on. These termites are known to sense the earth’s magnetic field and use it to coordinate the nest-building activity of the colony’s many workers to achieve this striking geographic orientation of the nest.
Although less organized in their social behavior than most social insects, tent caterpillars use some of the same thermal strategies to get a jump on the warm season. The larvae of tent caterpillars cluster together and form tents from silk that they produce. A group of caterpillars clusters together inside the tent during the night, where both the tent and the presence of many clustered insects reduce heat losses. The higher temperatures that the caterpillars experience allow them to develop more quickly than they would if they were isolated and exposed to the low temperatures that are common, especially at night, in their environment. Tent caterpillar behavior is adapted to keeping with the group. They find their way back to the tent by trails of odors and silk that are laid down as the caterpillars move from the tent to the foliage on which they feed during the day.
COMMUNICATION AND GROUP ACTIVITIES
An individual organism must allocate its time and resources between food collection, reproduction, habitat selection, and other activities. In the social insects, one sees similar behavioral adaptations. There is added complexity, though, because in social insects they occur at both the level of the individual and the level of the group. Group-level adaptations include the regulation of numbers of individuals in the colony, the allocation of reproduction between workers and sexual forms, the division of labor among individuals (e.g., caste), and the social organization of food collection (e.g., recruitment). All of these homeostatic activities by colonies of insects require mechanisms of communication to coordinate the activities of multiple individuals. It is for this reason that the social insects provide so many examples of communication among insects, because in nonsocial insects, communication is largely restricted to behavior associated with mating or defense. As in other insect groups, much of the communication in social insects is carried on chemically, by means of pheromones.
Homeostasis is fundamental to the survival of organisms, because the processes of life occur in a well-regulated manner only within a certain range of conditions. The same could be said about the processes conferring advantages of group living on those insects that live in groups. If a colony is too large, or fails to coordinate its activities in foraging, reproduction, or defense, it may perish. It is the function of behavioral mechanisms of homeostasis to regulate both the group environment and the properties of the group itself in a manner that preserves its efficient functioning.