255 INVESTIGATIONS INTO THE REGULATION OF DOMINANCE BEHAVIOUR AND OF THE DIVISION OF LABOUR IN BUMBLEBEE COLONIES (BOMBUS TERRESTRIS) by ADRIAAN VAN DOORN (ZoologicalInstitute II, Röntgenring10, 8700 Würzburg, West-Germany. Laboratory of ComparativePhysiology, Jan van Galenstraat40, 3572 LA Utrecht, The Netherlands*) SUMMARY During the first part of colony life Bombusterrestris queens have a strong regulating in- fluence on worker dominance. Dominant workers from queenless groups which are introduced into the colony are immediately dominated by the queen. They drop to a low position in the dominance hierarchy of the colony and may start foraging. The queen's dominance signal decreases at a certain, queen-specific time after she has switched to the laying of unfertilized eggs. Then an introduced dominant worker will supersede her and become the 'false-queen'. The false-queen apparently does not pro- duce the complete dominance signal since she usually has to carry out attacks on the workers to establish and maintain her dominance. The workers' flexibility with respect to the tasks they perform (foraging or nest duties) decreases with their age and in the course of colony development. House bees, especially those which have achieved a high position in the dominance hierarchy, are less inclined to change their tasks after removal of the foragers than foragers after removal of the house bees. But, in both cases, most of the work is taken over by young workers (less than 10 days old). Foragers which change to nest duties may substantial- ly increase their dominance and may become egglayers. Juvenile hormone (JH) treatment does not affect the division of labour, but it does influence the activity of the workers. The influence of JH on worker dominance, as observed in queenless workers, seems to be overridden by colony factors such as domination by the queen. JH treatment stimulates oogenesis, but ovary development decreases again (by degeneration of the developed oocytes) down to the level that cor- responds to the worker's dominance status as soon as the exogenous JH has been broken down and excreted. Ovariectomy influences neither the division of labour nor the establishment of a dominance hierarchy. Dominant ovariectomized workers perform all behaviours nor- mally displayed by egglaying workers (including among others egg eating and ag- gressiveness), except constructing and testing of egg cells. The latter two behaviours seem to depend both upon dominance and upon the presence of ripe eggs in the ovaries. It is concluded that a Bombusterrestris colony is a stable social structure, in which the dominance of the queen and colony needs for food determine the differentiation in foragers or house bees, some of which will become egglayer. The physiology of a worker reflects this social context rather than autonomous developments at the in- dividual level. * Present address, also for requests for reprints. 256 INTRODUCTION Among the members of bumblebee colonies there is a hierarchical order (based upon who gives way to whom), which is connected with the division of labour. Oviposition is performed by dominant females: at the beginning of colony development only by the queen, and later on, once the queen has lost control over her workers, it is also per- formed by dominant workers. Foraging is done by subordinate workers (FREE, 1955a; ROSELER & ROSELER, 1977; VAN HONK & HOGEWEG, 1981; VAN DOORN & HERINGA, 1986). The division of labour contrasts with that among honeybees and stingless bees. Whereas in these bees tasks are age dependent (LINDAUER, 1952; SOM- MEIJER, 1984), in bumblebees task allocation to individuals is of rather a permanent nature. Division of labour in bumblebees is connected not only with dominance but also with body size. The largest workers tend to become foragers, the smallest ones never leave the nest. Some of the workers may change their occupations in accordance with the current requirements of their colony. Usually ovarian activity is low in foragers (RICHARDS, 1946; CUMBER, 1949; BRIAN, 1952; FREE, 1955b; ROSELER, 1974; GAR6FALO, 1978; VAN DOORN & HOGEWEG, 1985). The queen of Bombus terrestris (VAN HONK et al., 1980), like the queen of several other bumblebee species (POMEROY, 1981; FISHER, 1983), dominates her workers partly by pheromones. The pheromone in- hibits corpora allata activity [Juvenile hormone (JH) synthesis] and thus ovary development (RosELER et al., 1981). In queenless workers JH treatment influences dominance behaviour. Ovariectomy does not affect the establishment of a dominance order in queenless groups, but ovarian activity is positively correlated with the dominance of workers (VAN DOORN, submitted). As mentioned before, the queen loses control over the workers at a certain stage of colony development. Worker oviposition always starts after the queen has switched to the laying of unfertilized eggs (VAN DER BLOM, 1986; VAN DOORN & HERINGA, 1986). The timing of the switch is determined solely by properties of the queen (POMEROY & PLOWRIGHT, 1982; DUCHATEAU & VELTHUIS, in press). The queen's loss of control seems to be due to changes in her pheromonal output. It is still unclear whether or to what extent the queen's switch and the changes in her pheromonal output are connected (VAN DER BLOM, 1986; DUCHATEAU & VELTHUIS, in press; VAN DOORN & CHRAMBACH, submitted). Several experiments were performed to obtain more insight into: 1. The relation between the queen's ontogeny and her ability to reduce .