12.2.1. The primitively eusocial hymenopterans

Hymenopterans exhibiting primitive eusociality include polistine vespids (paper wasps of the genus Polistes), stenogastrine wasps, and even one sphecid (Fig. 12.2). In these wasps, all individuals are morphologically similar and live in colonies that seldom last more than one year. The colony is often founded by more than one gyne, but rapidly becomes monogynous, i.e. dominated by one queen with other foundresses either departing the nest or remaining but reverting to a worker-like state. The queen establishes a dominance hierarchy physically by biting, chasing, and begging for food, with the winning queen gaining monopoly rights to egg-laying and initiation of cell construction. Dominance may be incomplete, with non-queens laying some eggs: the dominant queen may eat these eggs or allow them to develop as workers to assist the colony. The first brood of females produced by the colony is of small workers, but subsequent workers increase in size as nutrition improves and as worker assistance in rearing increases. Sexual retardation in subordinates is reversible: if the queen dies (or is removed experimentally) either a subordinate found-ress takes over, or if none is present, a high-ranking worker can mate (if males are present) and lay fertile eggs. Some other species of primitively eusocial wasps are polygynous, retaining several functional queens throughout the duration of the colony; whereas others are serially polygynous, with a succession of functional queens.

Primitively eusocial bees, such as certain species of Halictinae (Fig. 12.2), have a similar breadth of behaviors. In female castes, differences in size between queens and workers range from little or none to no overlap in their sizes. Bumble bees (Apidae: Bombus spp.) found colonies through a single gyne, often after a fight to the death between gynes vying for a nest site. The first brood consists only of workers that are dominated by the queen physically, by aggression and by eating of any worker eggs, and by means of pheromones that modify the behavior of the workers. In the absence of the queen, or late in the season as the queen’s physical and chemical influence wanes, workers can undergo ovarian development. The queen eventually fails to maintain dominance over those workers that have commenced ovarian development, and the queen either is killed or driven from the nest. When this happens workers are unmated, but they can produce male offspring from their haploid eggs. Gynes are thus derived solely from the fertilized eggs of the queen.

Cladogram showing probable relationships among selected aculeate Hymenoptera to depict the multiple origins of sociality (SOL, solitary; SUB, subsocial; EU, eusocial).
Figures 12.2. Cladogram showing probable relationships among selected aculeate Hymenoptera to depict the multiple origins of sociality (SOL, solitary; SUB, subsocial; EU, eusocial).

The superfamily Apoidea includes the Sphecidae sensu stricto, the Crabronidae (formerly part of a broader Sphecidae), the Ampulicidae (not shown), and all bees, here treated as one family, the Apidae, with several subfamilies (e.g. Apinae, Colletinae, Halictinae; not all solitary groups are shown) of uncertain relationships. Traditionally, bees have been classified in several families, a ranking that is unjustified phylogenetically. Probable relationships within non-social aculeate wasps (e.g. Ampulicidae, Pompilidae, and Rhopalosomatidae) and bees are not depicted. (Adapted from several sources including Gauld & Bolton 1988; Alexander 1992; Brothers 1999; B.N. Danforth, pers. comm.)

Chapter 12