Division of labour and organization of work in the primitively eusocial wasp Ropalidia marginata

Abstract

Ropalidia marginata is a primitively eusocial polistine wasp with the expected lack of morphological caste differentiation between queens and workers. The lack of morphological caste differentiation appears to be compensated by a system of behavioural caste differentiation. The wasps in a colony may be classified into three behavioural castes which we have called Sitters, Fighters and Foragers and the queens are almost always in the Sitters caste. Consistent with this and unlike in most other primitively eusocial species studied, R. marginata queens are relatively inactive, behaviourally subordinate individuals. There is no evidence that they regulate activities of their workers. The workers continue to remain active, bring food and feed the larvae, even if the queen is removed. Worker activity appears to be regulated by the workers themselves through the use of dominance behaviours' which are hypothesized to have come to represent larval and adult hunger signals, to the foragers. In undisturbed colonies, intranidal workers who also unload food and pulp bearing foragers, appear to regulate foraging rates. In the absence of unloaders, the foragers themselves feed the larvae and apparently obtain first-hand information about larval hunger levels. In spite of its primitively eusocial status, R. marginata has a well developed age polyethism. Workers show strong preferencest o feed larvae, build the nest, bring pulp and bring food, in that order, as they age. However, the relative position of a wasp in the age distribution of the colony, rather than her absolute age, is a stronger predictor of her task performance. Soliciting behaviour (a form of trophallaxis) provides a plausible mechanism for the wasps to assess their relative ages. A computer simulation model, adapting the verbal activator-inhibitor model proposed for honey bees, demonstrates that a relative-age based rule for division of labour provides the necessaryfl exibility for coloniest o respond adaptivelyt o changing colony demography or varying demands for food. Thus, morphologically identical individuals, and in spite of retaining some reproductive options,h ave accessto a variety of mechanismsto efficiently divide labour and organize work.