James T Costa. American Scientist. Volume 85, Issue 2. Mar/Apr 1997.
Of the many ingenious entomological studies undertaken by the l9thcentury French naturalist J. Henri Fabre, surely the most enduring must be his experiments with the gregarious pine processionary caterpillar. As their name implies, these caterpillars follow one another head-to-tail in processionary fashion as they forage for their daily meals. In an effort to determine how faithful the caterpillars were to their group and to their trail in the face of starvation, Fabre set them marching around the rim of a large circular palm planter with food mere inches to one side of their silk-coated path. Here they circled and circled day and night for a week in vain hopes of getting somewhere. Fabre concluded that the caterpillars, like so many other insects, are victims of a single-purpose instinctive response; focusing exclusively on their trails, the caterpillars “cling obstinately to [their] silk ribbon … because they lack the rudimentary glimmers of reason which would advise them to abandon it.”
Caterpillars and their kin may indeed lack reason, but in a natural context their hard-wired instinctive behaviors can be astoundingly complex-and effective. And Fabre, although seemingly dismissive of the problem-solving abilities of caterpillars in a predicament, was one of the earliest observers to recognize that some caterpillars may have something to say to each other. The caterpillars of many butterfly and moth species are, in this regard, social insects, although of a somewhat different stripe from those commonly considered to be social by many entomologists. When it comes to insect societies, most students of social behavior think in terms of the ants, bees, wasps and termites-the socalled “eusocial” insects. These are, after all, the premier social groups, long occupying the social-insect limelight by virtue of their near-ubiquity, remarkable division of labor, sophisticated behavioral interactions and impact on human society.
Indeed, human fascination with the eusocial species is many-faceted, arising from parallels-real and imagined-with human societies. Unfortunately the historical development of the study of social insects may have steered scientists in the wrong direction: Species that did not fit the eusocial mold were largely neglected. A paradigm that cannot account for the behavior of all social insects cannot claim to be complete, and increasing numbers of entomologists have come to appreciate the weaknesses of a conceptual framework based on the eusocial insects. Other models of social behavior are being developed in several quarters; at the heart of these is the nature and the role of the information that individuals of a species may exchange among themselves. Here I describe the current social-insect paradigm and how we might think about the behavior of insects, such as caterpillars, that do not quite fit in.
Social-Insect Family Values
On an individual basis, industrious ants and busy bees have long taught us moral lessons, and their colonies have been viewed as analogues of human polities writ small, complete with “queens,” “soldiers” and “workers.” Hence, Charles Butler entitled his 1609 treatise on bees The Feminine Monarchie. The fact that these societies are often families also appealed to some observers. Aristotle wrote in De Generatione Animalium that “The bees attend upon their kings because they are their offspring …,” viewing kinship as the underlying motivator for worker fealty. Kinship has in fact provided significant insights into potential mechanisms for the evolution of such societies, especially those species in which some colony members behave “altruistically,” either giving their lives in the defense of the group (such as honeybees) or forgoing personal reproduction in order to assist in rearing the offspring of colony mates or the queen.
In his Origin of Species, Darwin expressed concern over the power of natural selection-his evolutionary agentto produce individuals that give up reproduction, a trait that seems manifestly maladaptive. He termed worker bees “neuters” because they do not develop mature sexual organs, and wrote in his chapter on instinct that neuters posed a “special difficulty” for his theory. With characteristic insight, however, Darwin glimpsed a solution to this evolutionary dilemma in the fact that colonies of bees are families. This solution-that selection may act at the family level-was later explored in more detail by other evolutionary thinkers, including Sir Ronald Fisher and J. B. S. Haldane, two of the architects of the evolutionary Modern Synthesis of the early 20th century. In the early 1960s, this line of thinking was mathematically formalized by W. D. Hamilton into inclusive fitness theory, popularly called kin selection.
The advent of kin-selection theory did both service and disservice to the study of social insects. On one hand, Hamilton’s seminal work performed the valuable function of rigorously codifying the mathematical underpinnings of the kin-selection mechanism, showing how the total fitness of individuals was actually the sum of two joint sources: a direct component stemming from an individual’s personal reproductive effort and an indirect component stemming from the reproductive effort of relatives. The magnitude of the indirect component depends on the degree of the genetic relationship to those relatives, and there are thus conditions under which it may pay to forgo reproducing yourself and instead assist in the rearing and defense of relatives. This underlies Haldane’s famous comment that he would lay down his life for two brothers (or, someone added, eight cousins).
Kin selection theory not only showed that the evolution of seemingly altruistic traits was possible, it also identified specific variables that could be empirically measured: How closely related are colony mates? Can colony members discriminate kin from non-kin, thereby preserving the family structure of the colony or selectively aiding their closest relatives? The ability to quantify patterns of relatedness and empirically assess these patterns under the umbrella of Hamiltonian theory led to a veritable revolution in social-insect biology.
This revolution had its negative side, however. The ability to quantify patterns of relatedness, greatly augmented by technological advances such as protein electrophoresis and DNA technology for detecting genetic marker loci, together with development of a robust statistical methodology for estimating relatedness in a variety of contexts, led to empirical preoccupation with the genetic makeup of colonies and mechanisms preserving or undermining that makeup. Although a great deal was learned about the relationships and mating habits of social insects, the kin selection-driven paradigm for sociality also fostered a nearly exclusive focus on family structured societies and the genetic relationships of individuals within them. Nongenetic factors of potential importance in the evolution of sociality (including ecological determinants such as predation or thermal ecology) were overlooked, and social species that lacked the family structure of the Hymenoptera and Isoptera were neglected. In fact, one of the three defining criteria for eusociality became “overlap of generations” within colonies (the co-occurrence of one or both parents with their offspring), thus building family structure into the very concept of “true” sociality The other two criteria pertain to interactions within a familial context: reproductive division of labor and cooperative care of the young.
Preoccupation with the eusocial insects is evident in the terminology devised to describe the spectrum of social species, a family oriented and hierarchical classification. The scheme has its roots in a system developed by the American myrmecologist William Morton Wheeler, who, in his 1928 book The Social Insects: Their Origin and Evolution identified a seven-step evolutionary series characterized by “constantly increasing intimacy of the mother with her progeny” Unsurprisingly, Wheeler regarded ant societies, his passion, as “social forms sensu stricto,” a designation eventually replaced by the more succinct term “eusocial.” The historical development of the sociality lexicon follows what might be described as a “top-down” pattern, whereby features of the most complex systems were described first, and then categories lacking these features were devised. The “presocial,” “subsocial” and “quasisocial” categories were erected in reference to the conceptual framing of “eusocial” characteristics.
The lexicon of the social hierarchy quickly hardened into a monolithic construct with the publication of Harvard biologist E. O. Wilson’s 1971 book The Insect Societies, in which terms then gaining currency in the field were comprehensively synthesized, setting the conceptual framework for the social species subsequently discussed and studied. This classification system has played a valuable role in organizing the complex diversity of social species, especially when applied to the groups for which it was originally intended: societies of ants, bees, wasps and termites. In recent years, however, students of social evolution have become increasingly uncomfortable with this system, and many authors have recently recommended either expanding or narrowing the traditional concept of sociality, or even dismantling formal categories altogether.
Perceived problems have arisen on several fronts. One important issue is philosophical: Terms such as “presocial” and “subsocial” are, in an important sense, value-laden in that they unwittingly pack teleological overtones. Are presocial species somehow “evolving toward” eusociality? Do the terms presocial and subsocial imply that such species are not social at all? These problems suggest a certain sloppiness in evolutionary thinking by implying some groups are “less evolved” than others. On another front, the traditional classification has been strained by a veritable explosion in the number and diversity of species clearly social, but lacking the particular defining character traits of the complex hymenopteran and isopteran societies. Because the traditional system was developed with such societies in mind, traits such as overlapping generations, reproductive division of labor, cooperative brood care and the presence of behavioral or morphological castes became key criteria for sociality. Over the past few decades, however, representatives of such disparate taxa as thrips, aphids, beetles, spiders, mites and naked molerats have joined the social bestiary, and the character traits of ant and bee societies are increasingly seen as inappropriate standards for so diverse an array of taxa.
In response to these problems, it is likely that the concept of sociality will ultimately be expanded to explicitly accommodate social species of widely differing colony demography, life history and complexity. Space constraints preclude a detailed treatment of the varied alternative frameworks recently proposed, but much recent discussion has been aimed at conceptually unifying animal societies traditionally treated as very different phenomena, for example, cooperatively breeding birds, herding ungulates, and caste-divided ant or bee colonies. This is still very much a field in a state of flux, however, because while the bestiary has been growing the conceptual stable has not-many of the traditional categories are retained (albeit redefined) under most recent proposals.
Where do social caterpillars fit into this discussion? Although traditionally shoehorned into the old category of “communal” with a diversity of other social groups lacking various traits used to define eusocial insects, a large body of research on social caterpillars over the past two decades has shown these insects to be remarkably complex.
What are social caterpillars? A thumbnail sketch might describe them as gregarious cohorts of larvae that cooperate in defense, foraging and nest building. Social caterpillars differ from most social insects in a number of key respects, most of which can be understood in terms of demography Eusocial societies are by definition complete families, with one or both reproductive parents and their progeny. Caterpillars, in contrast, are juveniles; they may be families in the sense that members are siblings, but they are orphans-the adults typically die within a few days or weeks of mating and depositing their eggs. This simplifies caterpillar societies considerably since there are no parent-offspring interactions, and there is no need to control reproduction within the colony or to rear the brood.
Taking the consequences of demography a step further, there are at least two other important characteristics of caterpillar societies shaped by their juvenile makeup, their longevity and their foraging behavior. Unlike the societies of most (but not all) eusocial species, which may persist for many years in species with queens that continually produce offspring, caterpillar societies are single-generation cohorts that dissolve when the larvae mature. Second, in contrast to many insect societies that bring food back for the larvae, social caterpillars do not retrieve food to their nest.
Much of the early work with eusocial insects focused on how individuals in a colony communicate with each other and the function of the information they exchange. It turns out that these insects have a great deal to say to one another (usually by pheromones) about maintaining the colony. Topics of discussion range from finding and gathering food to feeding the brood and cleaning and defending the colony
What concerns might dominate the communications of caterpillars? Aside from maintaining group cohesion, caterpillars have the same needs as vulnerable juveniles of all species. They must grow as quickly as possible because they are tempting morsels to an array of predators, and they must grow as large as possible because their body mass at maturation is directly related to their fecundity as adults. Accordingly, defense and foraging are subjects of overriding importance to a caterpillar.
Foraging behavior has proved to be a convenient framework for categorizing caterpillar societies. The American entomologists T. D. Fitzgerald and S. C. Peterson first classified social caterpillars as either patch-restricted, nomadic or central-place foragers depending on whether they feed only in their immediate vicinity, travel between feeding patches or launch periodic food searches from a nesting site. These foraging modes also relate to other social characters: Patch-restricted and centralplace foragers often build nests or tents, whereas nomads do not, and nomads more often sport bright warning coloration and chemical defenses than other foraging types.
Much of what has been learned of caterpillar communication has been derived from studies of pheromones. In nearly all such cases the use of pheromones has been experimentally established, but the pheromones themselves have not been identified. It is likely that visual and tactile signals are used by various species as well, and a few have been shown to use substrateborne vibrational cues.
Herding and Defensive Displays
Many social caterpillars can be likened to herding mammals, especially nomadic species that travel en masse from feeding site to feeding site without returning to a permanent nest or resting site. Herding behavior involves the expression of group-cohesion signals and is thought to fill a defensive role by, for example, diluting the probability that any one individual will be taken by a predator and making it more likely that a predator will be detected in a timely fashion. Such signals effectively function as “boundary markers,” defining the spatial bounds of the group to keep individuals from being separated, or simply to orient individuals to the group. Nomadic forest tent caterpillars (Malacosoma disstria), for example, are known to overmark their silk trails with a marker pheromone in such a way as to draw colony mates from a local feeding site to a nearby group resting area. Silk-borne trail pheromones functioning as boundary markers have been demonstrated in a variety of social lepidopterans, including the fall webworm (Hyphantria cunea), the small ermine moth (Yponomeuta cagnagella), and the range caterpillar (Hemileuca oliviae). One uncommon mode of maintaining group cohesion is found in the sawfly Hemichroa crocea, which reportedly signals colony mates with vibrations generated by rubbing against the substrate beneath it. Sawflies belong to the order Hymenoptera, not Lepidoptera, but their caterpillar-like immatures are commonly treated as “ecological analogues” of butterfly and moth larvae; many sawfly species are social, foraging in either patch-restricted or nomadic fashion.
Many of the most spectacular examples of defensive communication are manifested in group display behaviors. Virtually all caterpillar (and sawfly) species, solitary and social alike, have a repertoire of defensive behaviors ranging from escape to attack, but in social species such behaviors are often expressed in group displays, which may enhance the effectiveness of the defense. Species of tent caterpillars and their relatives, for example, are known to engage in synchronous rearing and flicking of the anterior half of their bodies when threatened by parasitoids or other predators. Parasitoids such as tachinid flies and braconid wasps seek to deposit eggs on or in the body of a caterpillar; upon hatching, the larval parasitoid feeds on the caterpillar from within, eventually killing it. Flicking the body may make egg deposition difficult for some parasitoids, and it is possible that mass flicking displays are more effective than individual or asynchronous displays. Individual caterpillars or small subgroups can be incited to initiate a flicking display, but the fact that these displays quickly become synchronized throughout the colony indicates that there is exchange of information.
Synchrony is also a characteristic of other well-known caterpillar defensive displays. Many social species of the moth family Notodontidae, for example, bend their bodies en masse into a defensive U-shaped posture by simultaneously raising and bending both the head and posterior end of the abdomen back over the body. Similarly, many social sawflies synchronously raise the entire body behind the head into a threatening S-shaped posture. These group displays can be startlingly effective to a visually hunting predator, and they are often augmented with chemical defenses.
Although group defense is ubiquitous among social caterpillars, relatively few engage in cooperative foraging. This is because caterpillars living near their food (patch-restricted and nomadic foragers) have no need to assist one another in finding something to eat when it is at their tarsi-tips, so to speak. As local patches are exhausted, patch-restricted foragers may simply expand the spatial bounds of their colony to adjacent patches, whereas nomadic foragers typically move en masse to a new patch. Such colonies may exhibit group-cohesive communication but do not cooperate in foraging per se. In contrast to these foraging modes, the ecology of central-place foragers lends itself to coordinated orientation toward food patches.
The combination of a predictable home base and dispersed resources underlies some of the most sophisticated forms of communication known among social insects, including caterpillars. This arrangement sets the stage for recruitment communication, whereby colony members are actively recruited to a food find by successful foragers. The premier example of recruitment communication is undoubtedly the waggle dance of the honeybee, in which scouting bees communicate the location of flowers by an elaborate display that conveys profitability, distance, direction and perhaps content. Many ants and termites recruit nest mates as well, typically through the use of trail-based pheromonal markers.
Trail-based recruitment communication also typifies cooperatively foraging caterpillars, the best studied of which are the tent caterpillars of the genus Malacosoma. T. D. Fitzgerald of Cortland College has, over the past two decades, led a systematic series of studies on the intricacies of the behavioral ecology and social biology of the eastern tent caterpillar (M. americanum), which is exceedingly abundant east of the Mississippi River. These caterpillars specialize on black cherry and apple trees, on which they construct silken tents in the early spring. As the colony develops, the caterpillars contribute both to building the tent, which must continually expand to accommodate the growing larvae, and to locating young foliage at the tips of tree twigs. Rather than foraging as a coherent mass, these caterpillars fan out to search for acceptable food. On finding a patch, individuals feed and then return to the tent, depositing a recruitment pheromone from the underside of the abdomen. Unsuccessful caterpillars periodically return to the tent, where they may pick up a recruitment signal from a colony mate that has fared better than they. In this way the tent functions as a predictable information center-the essence of recruitment communication.
There are several interesting elements to the foraging communication of eastern tent caterpillars. For one, recruitment in this species is elective. Foraging caterpillars evaluate the profitability of a patch and only recruit others if the patch exceeds a certain threshold. Another is the exquisite sensitivity of the tent caterpillars to their trail pheromone. The active components of the pheromone have been identified as mono- and di-ketones of the steroid cholestane, the only caterpillar trail pheromone characterized to date. Fitzgerald has empirically shown in trail-choice studies that caterpillars will respond to this material at concentrations of 10-11 grams per millimeter of trail-astonishingly small quantities. Moreover, these caterpillars employ their pheromone in a two-tiered trail system with exploratory and recruitment components. The difference between these trails appears to be quantitative rather than qualitative: Leaving the tent and searching for food, the larvae mark the substrate weakly and intermittently, whereas caterpillars returning from a successful foraging bout heavily overmark the exploratory trail with a recruitment trail. An abundance of data show that, given a choice between a trail deposited by a fed colony mate versus an unfed colony mate, hungry eastern tent caterpillars will nearly always choose the fed larva’s trail. This choosiness permits the encoding of information in the trail, as opposed to simply using it as a group cohesive device. The nomadic relatives of eastern tent caterpillars, forest tent caterpillars, also mark trails (with the same or a similar pheromone) to different degrees before and after finding food, but this species shows no preference given a choice between these trail types; no foraging information is encoded.
The trail preferences of eastern tent caterpillars underlie the remarkably effective recruitment abilities of this species, enabling colonies numbering hundreds of individuals to quickly aggregate at a feeding site following the discovery of food, and shortly thereafter to return to the relative safety of the tent to metabolize their meal. To draw parallels with more complex social species, it has been pointed out that the foraging success of honeybees depends on two characteristics: The ability of foraging workers to evaluate patch profitability and recruit only to the highest quality patch available, and second, the ability of other colony members to respond to forager recruitment efforts. Foraging eastern tent caterpillars evaluate patch quality and communicate the location of only the best available patch. In turn, their colony mates exhibit clear receptivity to the information inherent in the trails.
Eastern tent caterpillars are among the earliest insects to emerge in the spring, and their rapid growth rate of approximately a centimeter per week is not entirely because of cooperative foraging. Like many other social species, these caterpillars modify their behavior to enhance the efficiency and the rate of their metabolism. Thermal ecology is especially important to tent caterpillars, which are subject to widely fluctuating temperatures. Individuals are capable of elevating their body temperatures by 30 degrees Celsius or more above the ambient levels when the sun is shining. This degree of temperature excess is comparable to that reported from other basking social caterpillars. These species not only raise body temperatures by basking, but also cool themselves when temperatures are too high by hanging from one or two pairs of abdominal legs, exposing greater body surface area to the effects of radiative cooling. This behavior points to the likely importance of aggregation in reducing heat loss when social caterpillars bask as a group.
Social species that do not engage in cooperative foraging may nonetheless exhibit cooperative feeding. In some species of caterpillars the weak mandibles of early instar larvae cannot break through the tough cuticle of their host plant to the nutritional mesophyll below. Such larvae would starve if it were not for older or larger larvae. Once the cuticle is penetrated in one spot, the remainder of the colony concentrate on the area, widening the breach such that all caterpillars can feed. This socially facilitated feeding is found in the jackpine sawfly Neodiprion pratti banksianae, a North American species that feeds on tough pine needles. Survivorship of the colony cohort is dependent on the successful penetration of the needle cuticle by one or a few individuals. Similarly, Asian burnet moths, Pryeria sinica, feed as a group, and successful feeding is a function of the number of individuals attacking host-plant leaves.
Evolution of Caterpillar Sociality
The eastern tent caterpillar is by far the best-known social caterpillar, but its remarkable behavior is unlikely to be unique. The emerging pattern is that trail marking is quite common in lepidopterans, even by caterpillars of some solitary species such as the territorial swallowtail butterfly, Iphiclides podalirius, a central-place forager. The most common mode of marking trails appears to be through imbuing the silk with a pheromone as the silk is drawn from the spinneret. Caterpillar spinnerets are modified labial palps, structures of the mouth. The silk glands are modified salivary glands, which often impart a chemical tag to the silk. Aside from the eastern tent caterpillar, only the social Lasiocampidae and its relatives have been shown to overlay a trail pheromone on deposited silk. Many of these lasiocampids are also central-place foragers that build communal silken tents, including species in the genera Eriogaster, Gloveria and Eutachyptera. Similar life histories are found in the New World butterfly genera Brassolis (Nymphalidae), Eucheira and Neophasia (both Pieridae), as well as the Old World moth genus Thaumetopoea (Notodontidae)the subject of Fabre’s classic studies. At least some of these species are likely to engage in recruitment communication in much the same way as the eastern tent caterpillar, and several are currently under investigation to address that very question.
Although trail marking appears to be common in lepidopterans, trail-based cooperative foraging is restricted to a handful of species in scattered families. This taxonomic distribution suggests that social behavior may have evolved independently several times, with varying patterns of gain and loss of particular social characteristics (nest building and recruitment) in different lineages. It is not possible to draw firm conclusions at this point because the evolutionary relationships among many lepidopteran groups are poorly understood, but it may be that social traits such as group defense, trail-based communication and group nest building are often group elaborations of behaviors expressed by solitary ancestors. The taxonomic distribution and frequency of social traits relative to solitariness, as well as known life histories of extant “primitive” Lepidoptera, argues strongly for a solitary ancestral condition. Many solitary caterpillars exhibit the posturing, flicking and defensive regurgitation found in social species, as well as “personal” trail marking, perhaps to keep track of recent food finds or to mark territories, and individual shelter building, commonly by rolling or tying leaves with silk or spinning mini-tents. One important ingredient for the evolution of social behavior is batch oviposition, or egg clustering. Many lepidopteran species lay their eggs in clusters, but in most cases the larvae disperse upon hatching. Prolonged associations and cooperation among the larvae in a cohort may have been favored in ancestral solitary species that experienced improved survivorship by virtue of grouping.
Although being social may improve an individual’s defense and foraging success, socializing has its costs. Groups may be more prone to predator attack through conspicuousness, and disease transmission may occur at greater rates among closely associated individuals. There is also the problem of food availability and competition for resources. In a world of ever-shifting costs and benefits, it is likely that solitariness secondarily evolved from social ancestors in some lineages.
Regardless of the direction of social evolution in a particular lineage, it is likely that the selective pressures favoring gregariousness and cooperation are greatest in the early caterpillar instars. This is the period of greatest mortality, and thus the period when traits such as collective security, efficient food location and rapid shelter construction might be of greatest significance. Interestingly, the majority of socal caterpillars show a distinct waning in group fidelity as the larvae age, supporting the idea that the greatest benefits of grouping and cooperating are experienced early on. Waning group fidelity is gradual in some species, leading to inexorable dissolution of the group by late instars. More typically, the larvae stick together until the ultimate or penultimate instar, whereupon they are likely to abandon the group, perhaps as a consequence of exhausting the local food supply or to seek a pupation site.
Lessons from Caterpillars
In The Insect Societies, Wilson pointed out that “the terms society and social must be defined quite broadly in order to prevent the arbitrary exclusion of many interesting phenomena.” Although Wilson’s passion is ants, he recognizes that sociality is manifested in many ways in the insect world. The flurry of papers appearing in recent years seeking to amend the social lexicon and its conceptual context suggests that many students of social biology have maintained an exceedingly restrictive conception of sociality. Social caterpillars, as “nontraditional” social insects, are helping to restructure the conceptual edifice of sociality studies by showing that sophisticated behaviors can evolve in otherwise simple groupings of individuals. In terms of suites of such sophisticated behaviors, the highly complex societies of ants, termites, bees and wasps are truly evolutionary marvels, but it may be more valuable to focus on traits that diverse social systems hold in common rather than on those that separate them.
The study of insect social evolution is a field in flux, and it is not clear which of the recently proposed alternatives for framing sociality might be an improvement. It is possible that a focus on communication, in terms of its modes and contexts, might provide the first step for a new framework. “Reciprocal communication” was identified by Wilson as the “essential intuitive criterion of sociality …,” and has proved useful in framing the relative complexity of caterpillar societies and in pointing out the ecological factors of significance for caterpillar cooperation. These factors and the communicative solutions to the problems they pose underlie all social species.
Beyond their utility as foils for comparative studies of insect societies, social caterpillars offer lessons in the marvelous complexity of insect behavior. More than mere aggregates of individuals, these species appear to have exploited the possibilities inherent in grouping-potentials for building shelters, behaviorally thermoregulating, warding off enemies and finding food. Social caterpillars are in many ways very different social organisms than their better-known relatives, but this difference underlies an important lesson about what it means to be social, ironically pointing the way to commonalities across the sociality spectrum.
The author gratefully acknowledges the National Science Foundation, the Alfred P. Sloan Foundation and Sigma Xi for financial support of his studies of tent caterpillar sociality. Terry Fitzgerald, Geoff Morse, Naomi Pierce and Kathrin Sommer improved the manuscript with their suggestions, comments and criticisms. The author is especially grateful to Leslie Costa for her extensive assistance in the field and in the lab.