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in which the concept has been used and the array of important Caddisfly phenomena caste encompasses in different 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 , 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 , 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

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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 , the soldiers differ in body proportions (hence shape) in ways that suggest advance and attack, while other workers flee. Often the specialization in 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

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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 social status and task sequence of task specializations through which an adult performance in some species. In the paper wasps P. Au: as she ages? worker passes as it ages. Typically, species with temporal exclamans, the first-emerging (and consequently the oldest) polyethism exhibit centripetal development: workers begin workers in the nest tend to socially dominate their nestmates, by working deep inside the nest, close to the queen(s) and a pattern referred to as gerontocracy, which is independent of brood; they later perform tasks at the nest periphery; and body size variation. In this case, age influences workers’ they finally move further out to perform risky tasks such as behavioral caste in a static way, rather than in a dynamic way foraging and nest defense (Fig. 1). This centripetal pattern of as in temporal polyethism. development is remarkably similar among the diverse eusocial insect species that exhibit well-developed temporal INDIVIDUAL DIFFERENCES AND SPECIALIZATION polyethism. Workers usually follow the same sequence of task Superimposed on broader patterns of division of labor, such specializations, but individuals vary in their rate of passage as body size or age effects, workers sometimes exhibit idiosyncratic specialization on tasks. For example, Apis and Polybia foragers often specialize by collecting one of the several materials their colony needs to function. Such specialization may be benefit the colony by increasing the efficiency or reliability of task performance. Genetic Effects Genetic relatedness among the offspring in the worker force is highest when the workers are born to a single reproductive female, which has mated with a single male. Some social insect species exhibit mating behavior or social structure that decreases the genetic relatedness among the offspring workers within colonies. When queens mate with several different, unrelated males (polyandry), or when several reproductive females are present in the colony (polygyny), workers can find themselves sharing a nest with a combina- tion of more closely and more distantly related individuals. In a number of polyandrous and polygynous species, including Apis spp. and stingless bees (genus Partamona), several species of ants, and Polybia spp., workers that are more genetically similar have been found to specialize on similar tasks. Experience and Learning A predicted benefit of task specialization is that workers can improve performance as they gain experience. There is evidence that some insect PROOFworkers learn to perform tasks more effectively with experience. Bumble bees collecting nectar and pollen from complex flowers learn to do so more rapidly after repeated attempts to handle a given type of flower. Polybia foragers are less likely to return from foraging trips empty-handed as they gain foraging experience.

COLONY-LEVEL INVESTMENT Investment in Growth and Maintenance versus Reproduction FIGURE 1 Temporal or age polyethism in the paper wasp Polybia aequatorialis. Data were collected on 130 individually marked, known-age One of the major challenges that faces growing organisms is workers. Two measures of worker activity at three task sets are plotted against the developmental decision of how many resources to invest worker age (solid lines: mean rate of task performance; dashed lines: number in growth and how many to invest in reproduction. Insect of workers performing the task). Note the typical centripetal developmental sequence: in-nest tasks (mostly nest cleaning) are followed by building on colonies can be treated as organisms in this sense, since each the nest exterior, and later by foraging (leaving the nest and returning with colony must decide how much it will invest in different castes food and building materials). (i.e., in workers vs reproductives). To the extent that colonies Encyclopedia-C.qxd 11/9/02 5:25 PM Page 154

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are reproductive units, optimality theory predicts that natural selection will favor colonies that allocate their limited resources efficiently into different castes. Many insect societies segregate the production of workers (early in colony development) from the production of new queens and males (later in colony development).

Worker Caste Ratios Insect colonies appear to behave in an adaptive manner by adjusting their worker caste ratios to meet current colony needs. Production of different worker castes reflects a trade-off between the costs and benefits of producing and maintaining workers of different kinds. As ant colonies with morphologically specialized workers grow in size, their amount of investment in large-bodied workers increases, and many eusocial insects produce tiny nanitic workers early in colony development. Colonies of the ant Pheidole pallidula increase their rate of production of soldiers when exposed to potential competitors. Similar colony flexibility is apparent in age–caste distributions. In honey bees and paper wasps, if the level-of-colony need for foragers changes, some workers accelerate or reverse their FIGURE 1 Caterpillar of the polyphemus , Antheraea polyphemus, showing the five pairs of prolegs bearing crochets (hooks). (Photograph by behavioral development, performing the age-atypical tasks that Joseph L. Spencer, Illinois Natural History Survey.) are in greatest demand. Identifying the mechanisms that link individuals’ developmental plasticity with the level of colony need remains as a central challenge in the study of caste. common form has a conspicuous head, a thorax with three See Also the Following Articles pairs of legs, and an abdomen with five pairs of prolegs that Colonies Division of Labor Juvenile Hormone bear crochets (hooks) (Fig. 1) that enable the caterpillar to cling tightly to or wedge itself between materials. In fact, Further Reading some of the giant moth caterpillars (Saturniidae) can Evans, J. D., and Wheeler, D. E. (2001). Gene expression and the evolution cling so tightly to a twig that a proleg can be ripped from the of insect polyphenisms. Bioessays 23, 62–68. body if they are pulled too hard. A few other orders of insects Hölldobler, B., and Wilson, E. O. (1990). “The Ants.” Harvard University Press, Cambridge, MA. contain larvae that are caterpillar-like, but only the larvae of Hunt, J. H. (1994). Nourishment and social evolution in wasps sensu lato. the leaf-feeding sawflies (Hymenoptera) are commonly In “Nourishment and Evolution in Insect Societies” (J. H. Hunt and C. encountered. They are easily mistaken for caterpillars, but A. Nalepa, eds.), pp. 221–254. Westview Press, Boulder, CO. they usually feed in groups (as do some caterpillars), rear up O’Donnell, S. (1998). Reproductive caste determination in eusocial wasps when disturbed, have more than five pairs of prolegs on the (Hymenoptera: Vespidae). Annu. Rev. Entomol. 43,PROOF323–346. Oster, G. W., and Wilson, E. O. (1978). “Caste and Ecology in the Social abdomen, and never have crochets on the prolegs. Insects”. Princeton University Press, Princeton, NJ. Peters, C., and Ito, F. (2001). Colony dispersal and the evolution of queen morphology in social Hymenoptera. Annu. Rev. Entomol. 46, 601–630.

Caterpillars Frederick W. Stehr Michigan State University

he larvae of butterflies, skippers, and of the order FIGURE 2 Twig-mimic inchworm caterpillar of a moth (Geometridae). TLepidoptera are generally known as caterpillars. Caterpillars (Photograph by Fred Stehr, Department of , Michigan State come in a diversity of sizes, shapes, and colors. The most University.)