Encyclopedia-C.qxd 11/9/02 5:25 PM Page 151 C in which the concept has been used and the array of important Caddisfly phenomena caste encompasses in different insect societies. see Trichoptera REPRODUCTIVE CASTES Differences in Reproductive Function A fully social or eusocial group is generally understood to exhibit reproductive division of labor. This means that eusocial groups must include some individuals that forgo direct Caste reproduction and instead aid the rearing of the offspring of others in their group. In eusocial insects, the helpers comprise Sean O’Donnell the worker caste and reproductive females are referred to as University of Washington queens. Termite colonies possess long-lived royal couples (a queen and a king), whereas in eusocial Hymenoptera, males he term “caste” refers to long-term, stable differences among are sometimes referred to as drones. Males in the order Tinsect colony members that affect the roles played by Hymenoptera (bees, ants, and wasps) rarely work for their individuals in their social group. It was the existence of distinct colonies and typically die soon after mating. In contrast, male morphological castes in insect colonies that led Charles Darwin eusocial thrips (Thysanoptera) and termites (Isoptera) comprise to identify social insects as a major challengePROOF to his theory of part of the worker force and participate fully in colony labor. evolution by natural selection. Few topics are more central to Social insect species vary according to whether the group’s the study of social insect biology than caste. It is ironic, then, members are permanently relegated to reproductive versus that few topics have generated more controversy and debate worker roles and in the degree of fecundity differences between among social insect biologists. Beyond the deceptively simple reproducers and workers. There is a general evolutionary definition just offered, there is little agreement on how the trend toward increased reproductive caste specialization as term should be defined, or on what characteristics should be more complex, larger societies evolve from smaller, simpler used to identify the castes of individual insects. In part, the ones. In some ants, workers lack reproductive organs and are controversy exists because published definitions of caste are permanently sterile. In most species, however, workers can often not operational. For example, some definitions do not achieve limited direct reproduction under some conditions. specify whether caste differences must be developmentally fixed and permanent or, alternatively, whether individuals can Morphological Differences exhibit caste flexibility. As a result, “caste” has been applied to a wide array of physiological and behavioral phenomena. Some species are reproductively monomorphic, and repro- The diversity of caste systems between and within evolu- ductives do not differ significantly in body structure from tionary lineages of social insects may preclude a simple, uni- workers. Many sweat bees and bumble bees, some paper wasps, versal definition of caste. Rather than advocate a single defi- and even some primitive ants are examples of reproductively nitional point of view, this article explores the diversity of ways monomorphic species. Workers in monomorphic species are 151 Encyclopedia-C.qxd 11/9/02 5:25 PM Page 152 152 Caste often smaller than reproductives, but there can be considerable as the sole reproductive. Subordinate Polistes females function overlap in body size distributions among the reproductive castes. as workers. In some cases, clear physiological differences distinguish workers from reproductives when morphology does not. For example, temperate Polistes paper wasp colonies produce gynes (potential CASTES IN THE WORKER FORCE female reproductives) at the end of the summer. Gynes possess Morphological Castes enlarged, nutrient-laden fat bodies, not present in female workers, that permit them to overwinter in a quiescent state. Discrete Worker Morphology In all termites and in In contrast, consistent reproductive caste differences in approximately 10% of ant species, workers exhibit body size and shape have evolved in several lineages of social developmental allometry resulting in body shape variation insects. Most eusocial insects with wingless workers, such as within the worker caste. Interestingly, this type of ants and termites, retain a morphologically distinct morphological caste has not been documented in social reproductive caste with wings. In species with flying workers, insects with flying workers, such as bees and wasps. There is developmental allometry can still result in the production of typically some association between a worker’s body form and distinct, nonoverlapping body forms for reproductives and the tasks that she performs. One of the most common types workers. Morphologically discrete reproductive castes are of morphological specialization is the assignment of large found among honey bees, stingless bees, and some paper workers, called soldiers, to the special role of colony defense. wasps. Reproductives are often larger than workers, but also When the colony is threatened by an animal, the soldiers differ in body proportions (hence shape) in ways that suggest advance and attack, while other workers flee. Often the specialization in egg laying, such as relatively enlarged soldiers uniquely possess heavily armored exoskeletons and abdomens. The degree of morphological differentiation some type of weaponry, including enlarged muscular heads, between reproductive castes probably evolves in response to a long, piercing mouthparts, or glands that produce defensive complex array of natural selection pressures. For example, the chemicals. In other cases, worker body shape variation affects degree to which the colony occupies a defensible, long- the performance of more mundane tasks such as food lasting nest site may in part determine the whether queens collection. In army ants (Eciton spp.), longer-legged workers can afford to adopt relatively immobile body forms. 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 Caste Determination: Immature Development and smaller workers usually remain in the nest to tend the Adult Interactions colony’s fungus garden. An ant worker’s body size and shape Other than an interesting exception in the ant Harpagoxenus are fixed upon adult emergence; further growth is not sublaevis, there are no well-documented cases of genetic possible. In contrast, some termite workers (Zootermopsis differences that affect reproductive caste differentiation. spp.) exhibit considerable caste plasticity, potentially molting Often caste differentiation must depend in part on among different body forms, and even switching from soldier differential patterns of gene expression during development, to nonwinged reproductive castes under certain conditions. particularly in species with distinct caste morphology. Differences in environmental conditions during immature BODY SIZE EFFECTS Even in monomorphic species, development can have strong effects on an individual’sPROOF caste. body size differences can influence the tasks that workers Nutritional effects on reproductive caste have been docu- perform. In some species, larger-bodied workers dominate mented in numerous taxa and appear to be widespread, if not their smaller nestmates (Polistes metricus, P. fuscatus, and P. universal, among eusocial insects. Differences in the amount dominulus), and dominance status in turn affects the tasks a of food provided to larvae may underlie many of the dif- worker performs. In some bumble bees (Bombus spp.), ferences between reproductives and workers, especially in however, larger workers are more likely to perform certain species exhibiting the common pattern of larger body size for tasks such as foraging to collect food for the colony, reproductives. However, differences in food quality, possibly independently of obvious worker aggression. including the addition of glandular secretions and pheromones, cannot be ruled out. Especially interesting in Behavioral Castes this regard are eusocial wasps, whose reproductives are smaller than workers (genus Apoica) or identical in size but Workers can be assigned to behavioral castes when they different in shape (genus Pseudopolybia). specialize on a subset of the tasks that the colony needs. In Social interactions among adults may also influence some eusocial insect species such as Bombus and Polistes, reproductive caste, particularly in species without apparent workers exhibit a great deal of flexibility, switching among morphological caste differences. For example, dominance tasks often, and behavioral castes are weakly defined. In honey interactions among paper wasp (Polistes) females, which often bees (Apis mellifera) and swarm-founding paper wasps (Polybia cooperate to start new colonies, determine which female acts spp.), on the other hand, workers specialize more consistently. Encyclopedia-C.qxd 11/9/02 5:25 PM Page 153 Caste 153 AGE OR TEMPORAL POLYETHISM Changes in task through the sequence. Changes in hormone titers, such as specialization as workers age are among the best-studied juvenile hormone, have been implicated in determining the factors that influence workers’ behavioral caste. “Age” or rate of temporal polyethism in Apis and Polybia. “temporal polyethism” refers to an ordered, predictable Workers’ relative age influences