| Ropalidia marginata | |
|---|---|
| Ropalidia marginata | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Insecta |
| Order: | Hymenoptera |
| Family: | Vespidae |
| Subfamily: | Polistinae |
| Tribe: | Ropalidiini |
| Genus: | Ropalidia |
| Species: | R. marginata |
| Binomial name | |
| Ropalidia marginata Lepeletier, 1836 |
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| Subspecies | |
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Ropalidia marginata is an Old World species of paper wasp found in Pakistan, peninsular India, Sri Lanka, South-east Asia and Australia.[1][2][3] It was originally described by Fabricius in 1793 under the name Vespa ferruginea, but that name was preoccupied, so the oldest available name for the species is that given to it later by Lepeletier in 1836. One of its subspecies, R. marginata jucunda, occurring in New Guinea and Australia, was described in 1898, and two others (R. marginata rufitarsis from Myanmar and R. marginata sundaica from Indonesia and the Malay Peninsula) were described in 1941.
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The nominate subspecies (R. m. marginata) has been studied extensively in India, though little is known of its biology elsewhere within its range, or the biology of any of the other subspecies. In India, it has an aseasonal, indeterminate and perennial colony cycle, which means that nest initiation occurs round the year, and nests are active throughout the year.[4][5][6]
These wasps make gymnodomous nests (open, not covered by an envelope) with one or more petioles, which they coat with ant-repellant chemicals. The nests are usually found in closed spaces with small openings, inside bushes and within various man-made structures like electric poles, broken pillars, crevices of buildings, electric cable boxes, switch boards, tube light holders, the bottom of park benches, and even from within dustbins and letter boxes. Nests that can be accessed only through very small openings are well protected from the hornets Vespa tropica, which are the prime predators of these wasps. Nest sizes range from 0 to 722 cells (mean ± s.d.: 133.7 ± 119.2), with 1 to 200 females (mean ± s.d.: 21.9 ± 22.3) and 0 to 33 males (mean ± s.d.: 2.4 ± 5.4).[6]
Males are produced aseasonally, and are thus found throughout the year in a subset of nests in the population. There is only a single queen in any nest of R. marginata, and she is not morphologically distinguishable from the workers. The nests are made of paper, which is produced by masticating cellulose (collected usually from plant sources) and mixing it with saliva.
The queen lays a single egg per cell, and the larvae grow inside the cells, being fed by the workers. The largest larvae spin a cap of silk on their cells and pupate inside. R. marginata nests can be founded either by solitary or multiple foundresses, and nest usurpation, adoption and joining are also quite commonly observed.[4]
A female wasp eclosing on a nest at any time of the year has several options open to her. She can (i) leave her natal nest and start her own single foundress colony; (ii) leave her natal nest and initiate or join other multiple foundress colonies along with her nestmates or with wasps from other nests; (iii) stay in her natal nest as a worker; (iv) stay in the natal nest as a worker for some time and then drive away the queen and take over as the new queen of the colony.[7]
It is clear that this versatility in the options for the workers provides a platform for potential conflict in the R. marginata society, and the fact that the queen is able to maintain complete reproductive monopoly makes this an excellent model for studying the evolution of co-operation in a primitive society.
Primitively eusocial societies are typically headed by behaviourally aggressive queens, who use aggression to suppress worker reproduction. The queen in R. marginata, however, is a "docile sitter" who does not use physical aggression to maintain her reproductive monopoly in the colony.[8][9]
The queen is also not responsible for maintaining worker activity in her colony, as in similar species. However, the queen in R. marginata is able to maintain complete reproductive monopoly in the colony. It has been argued for several years that the R. marginata queen uses a pheromone to signal her presence and fecundity to her workers, and this signal is perceived by the workers who refrain from reproducing; however, it has been shown that any such pheromone is non-volatile. [10]
The queen interacts very rarely with her workers, and direct or indirect physical interactions are not used by the workers to perceive their queen. The queen probably uses abdomen-rubbing behaviour to apply her pheromone on the nest material, through which the workers perceive her presence in the colony.[8]
When the queen is removed from the colony, the pheromone decays, and eventually the workers no longer perceive the queen signal. One of the workers then increases her aggression drastically; this individual, the "potential queen" (PQ), develops her ovaries within a few days and assumes the role of the queen. [11]
Primitive wasp societies are known to have distinguishable succession hierarchies, i.e., the loss of the queen results in her successor becoming the next egg-layer. Typically, such hierarchies are based on dominance rank, age or in some cases, body size.
However, in R. marginata, the potential queen, or the individual who steps up her aggression immediately after queen removal and eventually becomes the queen, seems to be an unspecialized individual in the presence of the queen. She is not unique in her dominance rank, behavioural repertoire, age, body size or ovarian condition. However, within minutes of queen removal, the potential queen becomes obvious to an observer due to her heightened aggression. Interestingly, the potential queen maintains this high aggression for only a few days, and gradually reduces the levels of aggression over a week or so, while she develops her ovaries.
Contrary to popular belief among scientists, the potential queen seems to require this heightened aggression, not to suppress the ovarian development in her nestmates, but to boost her own development.[12]
Typically, this heightened aggression is one-way, and shown by a single individual. She hardly even receives any aggression from the others in the colony. It has now been established that though the identity of the potential queen is cryptic to the observer in the presence of the queen, the wasps "know" who their successor is, and hence she does not face any challenge from her nestmates.[9]