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R Ropalidia Independent-Founding Species Raghavendra Gadagkar In common with most Belonogaster, ▶ Mis- Centre for Ecological Sciences, Indian Institute of chocyttarus, ▶ Parapolybia, and ▶ Polistes, inde- Science, Bangalore, India pendent-founding Ropalidia are characterized by small colonies, simple, small, open nests often suspended by a narrow petiole, relatively little Ropalidia is a large genus of social wasps in the queen-worker dimorphism, behavioral control of subfamily Polistinae. It comprises about 186 workers by the queen, and a well-developed Van known species distributed in tropical Africa, and der Vecht’s gland on the sixth gastral sternum for southern and eastern Asia to Australia [7]. It is defense against ants. As their name implies, new considered the sister-genus of Parapolybia, and colonies are founded by queens or potential these two genera together are considered the sis- queens, unaccompanied by workers. This descrip- ter-group of Polybioides + Belonogaster. The four tion of “independent-founding” is only to contrast genera together comprise the Old World tribe them with swarm-founding species, because, in Ropalidiini. However, research on taxonomy and the absence of queen-worker dimorphism, the systematics of the genus has since stagnated, question of workers accompanying queens or depriving us of a satisfactory understanding of not, does not arise [7, 13]. the diversity and taxonomic organization of the ▶ Nest Structure. Ropalidia nests may be ori- genus and even more seriously, of any reasonable ented in the vertical or horizontal plane and may phylogenetic analysis. This is a pity, because be of varying sizes. Most often, the comb is cir- Ropalidia has long been considered a crucial cular or oval in shape with many rows and col- genus for understanding social evolution in social umns of cells, but in some species there may be as wasps, especially on account of its variation in few as two long columns of cells (Fig. 1). Some- ▶ colony cycle. It is the only genus that includes times, several small combs with their individual both independent-founding and swarm-founding petioles may be clustered together to serve as a species and shows many other variations in single nest [7]. Such multicombed nests at the nesting pattern. extreme may reach thousands of combs, as in © Springer Nature Switzerland AG 2019 C. Starr (ed.), Encyclopedia of Social Insects, https://doi.org/10.1007/978-3-319-90306-4_102-1 2 Ropalidia Ropalidia, Fig. 1 Typical nests of Ropalidia marginata (left) and R. jacobsoni (right) in Bangalore, India. (Photos by Thresiamma Varghese) Ropalidia, Fig. 2 A photograph of an aggregation of R. plebeiana colonies (aptly called a “town”) hanging from a large concrete pipe (left) and a close-up photo of founding female wasps that cutting a nest into two different parts (right). (Photos by Soichi Yamane) Ropalidia plebiana in Australia (Fig. 2), although can turn-over in the tropics, nests can be occupied the adaptive significance of such massive dense by generations of wasps and are potentially aggregations is unclear [12]. immortal. Nesting Cycle. The nesting cycle depends on New nests may be initiated by one female the climate, with annual colonies in the higher (single-foundress or haplometrotic nests) or by latitudes and perennial indeterminate cycles in small groups of females (multiple-foundress or the tropics. In an indeterminate cycle, in addition pleometrotic nests). In single-foundress nests, to being initiated at any time of the year, the the lone female performs all the tasks involved duration of the cycle can be extremely variable. in building and maintaining the nest, as well as Many nests fail without producing any adult off- foraging and feeding the larvae, laying eggs, and spring, but some can last several months and defending the nest. In a multiple-foundress asso- probably years. Since both workers and queens ciation, usually only one female lays eggs, while Ropalidia 3 the remaining co-foundresses function as sterile more food and are more likely to become egg workers that perform all other tasks. However, layers. Poorly fed larvae develop into adults that one of them may sometimes replace the queen eat less even when provided ad libitum food and and take over as the next queen [7, 12]. are more likely to become non-egg laying workers The egg layer, whether in a single- or multiple- [8]. Such pre-imaginal caste bias, however, is only foundress nest, is usually inseminated and has a bias. Actual caste differentiation remains subject stored sperm in her spermatheca although she to social interactions. Dominance-subordinate may occasionally not mate until after assuming interactions before or after colony initiation help the role of principal egg layer. In long-lived nests, make the dominants into egg layers and subordi- such as those of R. marginata, queens are period- nates into workers. Even such differentiation is ically replaced by their daughters, sisters, nieces often reversible, making it possible for queens to or cousins while the rest of the individuals con- be replaced and workers to develop their ovaries tinue to work for their new queens without appar- and become queens. In some species, dominance ent disruption to the overall social organization. In hierarchies persist throughout the colony cycle the extreme case, such serial polygyny may lead and make it relatively easy to predict the identity to highly variable genetic relationships within of successive queens based on their position in the colonies. Colonies in one study were shown by hierarchy. However, the actual frequencies of pedigree analysis to consist of a complex of dominance-subordinate interactions and, indeed, mothers and daughters, sisters, nieces, cousins, the mechanisms of queen succession may vary cousins’ offspring, mother’s cousins’ offspring greatly in different species [8, 9, 12, 18, 24]. and grand offspring. The lifespans of workers Queens of at least one species (R. marginata) can be quite variable although they typically live become meek and docile within a week of assum- for a few weeks. The tenure of queens can be even ing the role of sole egg-layer and appear to regu- more variable, ranging in one study from seven late worker reproduction through pheromones. days to 236 days. Correspondingly, the productiv- There is a reproductive queue in this species, ity of queens can also be highly variable, so that even though successors cannot be identified they may have from zero to hundreds of offspring before the death or loss of the reigning queen. and lay up to ten eggs per day [7, 8]. As far as can be told, potential queens are ▶ Caste Differentiation. Independent- not different from typical workers in morphology, founding Ropalidia are classified as primitively behavior, dominance rank, residual ovarian devel- ▶ eusocial, the main implication of which is that opment, or pheromone profile. Nor is the potential there is no morphological distinction between queen predicted by her genetic relatedness to queens and workers. Thus, caste differentiation the colony members. However, there is into fertile queens and sterile workers takes place intriguing evidence that the wasps themselves after the adults eclose. Such post-imaginal caste seem to “know” the identity of their next queen, differentiation is mediated through behavioral resulting in a conflict-free succession. Older indi- interactions. This social regulation of reproduc- viduals are more likely to become future queens, tion is in stark contrast to developmental regula- but it is by no means a simple age-based suc- tion seen in highly eusocial species, presumably cession [2, 5, 8, 9, 16]. including swarm-founding Ropalidia. In most Even in the absence of morphological differ- independent-founding species, all or most ences between colony members, there may be eclosing adults may be totipotent and capable of further behavioral differentiation even among the developing their ovaries and laying eggs under workers. Using multivariate statistical analysis of appropriate social conditions, at least in the trop- the time-activity budgets of individually identified ical species [7, 8, 12]. wasps, members of colonies of R. marginata and Nevertheless, at least R. marginata appears R. cyathiformis have been classified into three to have some pre-imaginal caste bias such that behavioral castes, namely, sitters, fighters, and well-fed larvae develop into adults who consume foragers (Fig. 3). Such behavioral caste 4 Ropalidia Ropalidia, Fig. 3 Left: Behavioral castes in R. R. marginata. Fitted polynomial regression lines for vari- marginata. Twenty wasps are shown as points in the coor- ation of the probability of task performance as a function of dinate space of the amplitudes associated with the first two absolute age. Thin broken line refers to Feed Larva, thick principal components. The points fall into three clusters (or broken line to Build, thin solid line to Bring Pulp and thick castes) by the criterion of nearest centroid. Circled dot = solid line to Bring Food. (Reprinted from [17]) centroid. (Reprinted from [10]). Right: Age polyethism in differentiation appears to go beyond mere classi- through all of these successive tasks before they fication into intranidal and extranidal workers, die. Both the frequency of performance of differ- because some of the sitters can be quite old and ent tasks as well as the probability of performing a may transition to become new queens without task in relation to other tasks is significantly ever becoming foragers. Because queens are influenced by age. The frequency and probability meek and docile and use pheromones in R. of intranidal tasks, such as feeding larva and marginata, it has been postulated that domi- building, decrease with age, while those of extra- nance-subordinate interactions have been co- nidal tasks such as bringing building material and opted for the decentralized self-organization of food increase with age. nonreproductive labor. There is a positive corre- The age-dependent performance of tasks is lation between the rate at which individuals are better explained as a function of relative age rather subject to domination and their rates of foraging.

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