Castes In The Worker Force Morphological Castes

discrete worker morphology In all termites and in approximately 10% of ant species, workers exhibit developmental allometry resulting in body shape variation within the worker caste. Interestingly, this type of morphological caste has not been documented in social insects with flying workers, such as bees and wasps. There is typically some association between a worker's body form and the tasks that she performs. One of the most common types of morphological specialization is the assignment of large workers, called soldiers, to the special role of colony defense. When the colony is threatened by an animal, the soldiers advance and attack, while other workers flee. Often the soldiers uniquely possess heavily armored exoskeletons and some type of weaponry, including enlarged muscular heads, long, piercing mouthparts, or glands that produce defensive chemicals. In other cases, worker body shape variation affects the performance of more mundane tasks such as food collection. In army ants (Eciton spp.), longer-legged workers select larger food items to carry back to their colonies. In leafcutter ants (Atta spp.), the largest workers are soldiers, the medium-sized workers cut and transport leaves, and the smaller workers usually remain in the nest to tend the colony's fungus garden. An ant worker's body size and shape are fixed upon adult emergence; further growth is not possible. In contrast, some termite workers (Zootermopsis spp.) exhibit considerable caste plasticity, potentially molting among different body forms, and even switching from soldier to nonwinged reproductive castes under certain conditions.

body size effects Even in monomorphic species, body size differences can influence the tasks that workers perform. In some species, larger-bodied workers dominate their smaller nestmates (Polistes metricus, P. fuscatus, and P dominulus), and dominance status in turn affects the tasks a worker performs. In some bumble bees (Bombus spp.), however, larger workers are more likely to perform certain tasks such as foraging to collect food for the colony, independently of obvious worker aggression.

Behavioral Castes

Workers can be assigned to behavioral castes when they specialize on a subset of the tasks that the colony needs. In some eusocial insect species such as Bombus and Polistes, workers exhibit a great deal of flexibility, switching among tasks often, and behavioral castes are weakly defined. In honey bees (Apis mellifera) and swarm-founding paper wasps (Polybia spp.), on the other hand, workers specialize more consistently.

AGE OR TEMPORAL POLYETHISM Changes in task specialization as workers age are among the best-studied factors that influence workers' behavioral caste. "Age" or "temporal polyethism" refers to an ordered, predictable sequence of task specializations through which an adult worker passes as it ages. Typically, species with temporal polyethism exhibit centripetal development: workers begin by working deep inside the nest, close to the queen(s) and brood; they later perform tasks at the nest periphery; and they finally move further out to perform risky tasks such as foraging and nest defense (Fig. 1). This centripetal pattern of development is remarkably similar among the diverse eusocial insect species that exhibit well-developed temporal polyethism. Workers usually follow the same sequence of task specializations, but individuals vary in their rate of passage

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