K. Chandrashekara

Evidence for two distinct movement-detecting mechanisms in insect vision

terns could contribute to synchronization of the stridulatory and ventilatory rhythms. Furthermore, induced changes in the neuronal discharge modulate stridulation and the coupled abdominal expiratory movement or even bring the song to an end. The effects of such a neuron would be enhanced during the song, owing to the increase in its spike activity, and by the simultaneous action of all the neurons in this class. Given that spiking of the A neurons (in G. bimaculatus) is also suppressed during walking [18], it is conceivable that a function of the SD-AE neurons is to influence motor activities in general. Possibly, the class of SD-AE neurons also exists in grasshoppers [19] and is of overall importance.

Queen Succession in the Primitively Eusocial Tropical Wasp Ropalidia marginata (Lep.)(Hymenoptera: Vespidae)

Females of the primitively eusocial wasp Ropalidia marginata can be classified into three behavioral groups: Sitters, Fighters, and Foragers. It has been speculated that both Sitters and Fighters may be hopeful queens and that the Foragers may have little or no opportunities for direct reproduction. Here we show that in 9 of 12 queen-removal experiments where such a behavioral differentiation could be discerned, the individual that became a queen (the potential queen) was a Sitter in 6 cases, a Fighter in 2 cases, and a Forager in only 1 case. Although potential queens spent significantly more time absent from the nest and showed significantly higher rates of dominance behavior compared to the mean values for nonqueens in their colonies, they were intermediate with respect to all behaviors and age when compared to the range of values for nonqueens in their colonies. Potential queens were not necessarily the highest-ranking individuals among the nonqueens. The pattern of queen succession in this species appears to be quite different from the temperate pattern, where an old and active forager of high dominance rank is the potential queen. Although somewhat similar to the tropical pattern of a relatively younger female that has performed relatively little foraging being the potential queen, it is perhaps more accurate to describe the potential queens of R. marginata as “unspecialized intermediates”.

The role of larval nutrition in pre-imaginal biasing of caste in the primitively eusocial wasp Ropalidia marginata (Hymenoptera : Vespidae)

Abstract. 1 When freshly eclosed females of the primitively eusocial wasp Ropalidia marginata (Lep.) are isolated, only about 50% of them build nests and lay eggs thereby suggesting a pre‐imaginal biasing of caste. 2 Wasps that lay eggs take a very variable amount of time after eclosion to start doing so. 3 Females eclosing from nests where larvae are fed at a relatively higher rate are more likely to become egg‐layers and are likely to take less time after eclosion to begin to lay eggs. 4 Thus, both forms of pre‐imaginal biasing of caste, namely, differences in egg laying capacity and differences in the time taken to attain reproductive maturity, appear to be influenced by larval nutrition.

Worker-brood genetic relatedness in a primitively eusocial wasp

A striking feature of eusocial insects is the differentiation of colony members into a fertile reproductive caste and a sterile worker caste [1, 2]. High workerbrood genetic relatedness is the most widely accepted explanation for the apparent altruism on the part of workers that is implied by such reproductive caste differentiation. The haplodiploid genetic system leads to high genetic relatedness between full sisters (r = 0.75) in the Hymenoptera, an insect order with multiple origins of eusociality [1, 3]. However, polyandry (multiple mating by queens) and polygyny (the simultaneous presence of more than one queen in a colony) reduce worker-brood genetic relatedness [4]. Here, we show, by pedigree analysis in a primitively eusocial wasp, that even when there is only one queen at any given time, serial polygyny (the frequent replacement of queens) leads by itself, and even more potently in combination with polyandry, to a substantial reduction in worker-brood genetic relatedness.

Ropalidia rufoplagiata: a polistine wasp society probably lacking permanent reproductive division of labour

SummaryRopalidia rufoplagiata Cameron (Hymenoptera: Vespidae), a polistine species from penisular India, appears to be unique among all known primitively eusocial wasps. A total of 33 out of 46 identified females from an observed colony were found to oviposit on 1–17 occasions. No single predominant egg-layer could be identified during the 45-day period. Of the 17 dissected egg-layers, 12 were mated. All egg-layers showed several oviposition-related behavioural patterns including systematic, but indiscriminate, cannibalism of eggs and larvae, cleaning of empty cells, and guarding of freshly-laid eggs. There was no correlation between the egg-laying activity of the females (whether mated or not), oophagy, and their position in the dominance hierarchy. All nest-maintenance activities were performed exclusively by the egg-layers, while the non-egg-layers were mainly involved in the extranidal task of foraging. No significant morphometric differences between egg-layers and foragers could be discerned. Almost all the older individuals in the colony were egg-layers, while foragers were mainly younger animals. Such a temporal differentiation in reproductive labour suggests the absence of a permanent reproductive caste in this species.

Unmated Queens in the primitively eusocial wasp Ropalidia marginata (Lep.) (Hymenoptera: Vespidae

SummaryIn the primitively eusocial tropical waspRopalidia marginata, five out of eleven colonies studied had an unmated female as their queen. In two colonies this was the case despite the presence of another mated individual in the colony. We found no detectable differences between colonies with unmated queens and those with mated queens. We argue that in species such asR. marginata, where intracolony relatedness is expected to be low and where sociality is likely to be maintained because several individuals have opportunities for direct reproduction in the future, individual selection is likely to override “the good of the colony” and lead to such phenomena as that of unmated queens.

Queen success is correlated with worker-brood genetic relatedness in a primitively eusocial wasp (Ropalidia marginata)

Ropalidia marginata is a primitively eusocial polistine wasp in which, although there is only one queen at any given time, frequent queen replacements lead to a system of serial polygyny. One of the most striking features of this system, is the enormous variation in the success of different queens. Measuring queen success as queen tenure, total number of offspring produced, number of offspring produced per day of tenure, and proportion of eggs laid that develop into adults, we show here that each measure of queen success is correlated with worker-brood genetic relatedness and not correlated with worker: brood ratio or the age of the queen at takeover. We interpret these results as meaning that queens are better able to obtain the cooperation of workers when worker-brood genetic relatedness is high.