Some conceptual issues in the origin of eusociality.

Certain issues arising in connection with the evolutionary origins of eusociality are discussed. Previous results about when natural selection favours helping behaviour are generlised, and the differing viewpoints of both parents and offspring are considered. Particular attention is given to the evolutionary implications of different patterns of overlapping generations observed in bivoltine insects. As argued by Seger (1983), these patterns imply different conditions under which a daughter is selected to help her mother rear additional siblings in haplodiploid populations. Other factors that can alter the selective advantages of helping behaviour under haplodiploidy are also discussed, including the possibility of sex ratio manipulation and the novel result that helping behaviour may be locally favoured in populations that are spatially patchy with respect to sex-specific fitness. A new hypothesis is also presented: The fact that sisters are selected to aid their mother to parasitise other sisters may have played an important role in the origins of eusociality. A given offspring benefits from having maternally parasitised siblings because such siblings rear additional siblings (to which the given offspring is more closely related) instead of nieces and nephews. Finally, the importance of haploidiploidy in the origins of eusociality is discounted; the virtually unique biology of aculeate Hymenoptera would seem to be of much greater importance.

Temporal, spatial, sex-ratio and body-size heterogeneity of prey species taken by the beewolf Philanthus sanbornii (Hymenoptera: Sphecidae).

More than 3000 prey representing 108 species of bees and wasps were identified from exoskeletal remains taken from nests of the beewolf Philanthus sanbornii at a site in eastern Massachusetts over a period of five years. Quantitative reference samples totalling more than 4000 items were collected from flowers at the same site over a period of four years. These data give a uniquely detailed view of the way in which a generalist predator exploits a diverse prey community. Most species show striking year-to-year variation in relative abundance, in both the prey and reference collections, but the overall abundances of species tend to be similar in the two collections, as do their sex ratios. This shows: (i) that P. sanbornii takes virtually every bee and wasp species found at flowers during its flight season (except for the relatively small number of species too large to handle); (ii) that prey are taken at rates roughly proportional to their local abundances (with a few exceptions); and (iii) that the local bee and wasp communities have lively dynamics (at least on spatial scales equivalent to the flight ranges of P. sanbornii females). Prey species are non-randomly distributed among nests of individual females within years, and among cells within nests, in a pattern suggesting that females often return repeatedly to hunting sites at which they have had success; the pattern does not suggest that individual females develop preferences for particular prey taxa. The size-abundance distributions of female bees appear to be trimodal at both the individual and species levels, most strongly so when cleptoparasitic species are removed from the sample. Such patterns are seen weakly or not at all in the size-abundance distributions of male bees, male wasps, and female wasps. Bees and wasps of both sexes visit flowers for nectar, but only non-parasitic female bees harvest pollen; this suggests that the multimodality may be caused by aspects of pollen collection that tend to scale with size. The seven Philanthus species of eastern North America vary greatly in size, but they share a common set of relatively small prey species. As in other parts of North America, larger species of Philanthus tend to have relatively broad diets because they also take larger prey that are not available to their smaller congeners. However, long-tongued bees appear to be under-represented in the diets of most North American Philanthus outside the zebratus species group.