In most insect societies there also is a division of labor among the workers for tasks related to colony growth and maintenance. The evolution of a highly structured worker force is generally seen as an evolutionary consequence of the developmental divergence between queens and workers. Once workers were limited to serving largely as helpers, their characteristics could be shaped further by natural selection acting at the level of the colony to increase colony fitness. This perspective is consistent with the observation that the most intricate systems of division of labor among workers are found in species with the strongest division of labor for reproduction.
Age-related division of labor is the most common form of worker organization. Workers typically work inside the nest when they are young and shift to defending the nest and foraging outside when they are older. In the more elaborate forms of age-related division of labor, such as in honey bee colonies, workers perform a sequence of jobs in the nest before they mature into foragers. Physiological changes accompany this behavioral development to increase the efficiency with which particular tasks are performed. Among these are changes in metabolism, diet, and glandular secretions.
A less common but more extreme form of division of labor among workers is based on differences in worker morphology. This is seen in a minority of ant species and nearly all termites. Morphological differences among workers result from processes similar to worker—queen caste determination and morphologically distinct worker castes are recognized. For example, small ant workers (minors) typically labor in the nest, whereas bigger individuals (majors) defend and forage. Sometimes this form of division of labor also involves dramatic morphological adaptations in some worker castes, such as soldiers with huge and powerful mandibles and the ability to release a variety of potent defensive compounds.
A third form of division of labor among workers involves individual variability independent of age or morphology that results in an even finer grained social system. There are differences in the rate at which individual workers grow; some show precocious behavioral development, while others mature more slowly. There also are differences between individuals in the degree of task specialization. For example, foragers may specialize in the collection of a particular resource, such as some honey bees that collect only nectar or only pollen. It also has been found that some workers simply work harder than others.
The prevailing behavioral explanation for these three forms of division of labor among workers involves the application of the stimulus—response concept. Workers are thought to differ in behavior because of differences in exposure to, perception of, or response thresholds to stimuli that evoke the performance of a specific task. These differences can result from differences in worker genotype, age, experience, or morphological caste. There is some behavioral evidence for differences among workers in stimulus perception and response thresholds; challenges for the future are to more precisely define the nature of the stimuli and extend these analyses to the neural levels.
Some endocrine and neural mechanisms regulating age-related division of labor have been discovered, primarily in honey bees. Changes in hemolymph titers of JH act to influence the rate and timing of behavioral development, but JH is not required for a worker to mature into a forager. Evidence for a similar role for JH has been found in the advanced eusocial tropical wasp Polybia occidentalis. JH also affects the activity of exocrine glands that produce brood food and alarm pheromones in honey bees, apparently acting to ensure that physiological changes are coordinated with behavioral development. As JH receptors have not yet been identified in any insect, it is not known whether JH exerts its effects on division of labor directly in the brain, on other target tissues, or at a variety of sites. Octopamine acts as a neuromodulator in honey bees. Higher levels of octopamine, particularly in the antennal lobes of the brain, increase the likelihood of foraging. Changes in brain structure also occur as a worker bee matures into a forager, particularly in the antennal lobes and mushroom bodies, but the functional significance of these changes is unknown. As with caste determination, molecular analyses of behavioral development have only recently been initiated. Differences in the expression of several genes have been detected in the brains of younger and older honey bee workers. The orchestration of the neural and behavioral plasticity that underlies age-related division of labor is undoubtedly based on changes in the expression of many genes in the brain and other tissues as well.
Mechanisms underlying morphologically based systems of worker division of labor also have been studied. Morphological differences among adult workers have their origin in pathways of development that diverge during the larval stage. Information on worker caste differentiation, drawn largely from studies of Pheidole ants, suggests mechanisms similar to those involved in queen—worker caste determination. Both larval nutrition and JH have been implicated in the differentiation of Pheidole minors and soldiers.
Genetics is one factor influencing the third form of division of labor among workers, individual variability among workers that is independent of age or morphology. Genotypic variation within colonies arises as a consequence of multiple mating by queens or multiple queens in a colony. This genotypic variation is strongly associated with behavioral differences between individuals within a colony. Genotypic variation in honey bee colonies is known to influence how specialized a worker becomes on a particular task or the age at which it shifts from nest work to foraging. For example, quantitative trait loci have been found that are associated with variation in the tendency of honey bees to collect either nectar or pollen. These findings can lead to the identification of differences in specific genes that contribute to individual differences in behavioral specialization. Genotypic effects on division of labor also have been documented in several ant and wasp species.
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