The Adaptiveness of Social Wasp Nest Architecture

Abstract

In the tropics, where the majority of social wasp species and the greates diversity of nest architecture occur, predation by ants on the brood in the nest is believed to be a major force in nest evalution. Tropical social wasps can be divided into two major groups, according to the type of nest and type of defensive behavior evolved against ants. The first of these includes the genera Polistes, Mischocyttarus, Belonogaster, Parapolybia, and Ropalidia (except the subgenus Icarielia), all of which construct small uncovered nests consisting of a single comb of cells suspended from a petiole built up of a tough material of glandular origin. Mischocytraus, Polistes, and probably the other members of this complex smear the petiole with a glandular secretion that has ant-repellent properties. The species in this adaptive zone form small colonies that are founded by one or a few fecundated females (queens). This type of defense is well adapted to solitary founding because it is equally effective whether the queen is on the nest or off on a foraging trip. The second group includes the remining genera of tropical Vespidae, most of which build enclosed nests. It is proposed that the crucial advantage of the envelope enclosing the nest is that is greatly restricts access by ants to the brood, and thus facilitates detection and removal of approaching ants. Such active defense calls for large numbers of workers, some of which station themselves around the entrance where they serve as "guards." The genera in this complex found colonies as swarms consisting of several queens and many workers. The swarm in particularly adaptive during colony founding in that it provides the large numbers of worker necessary to guard the broad access to the young brood before the workers emerge. In smaller nests, selection for cryptic configurations, probably in response to vertebrate predations, is evedent, whereas in larger nests there will be stronger selection for more economical comb configuration. Several ways of evolving larger petiolate nests are considered. A unique method of stacking combs by treating the envelope of one comb as a fondation for the cells of the next occurs in several genera. This method of construction creates structural problems which have been solved by the evolution of new construction techniques. The arguments pointing to the strong role of ant predation in the evolution of tropical social wasps can be extended to account in part for certain major patterns in the geographical distribution of social wasps. The Vespinae appear to be adapted to temperate regions by virtue of their monogynous colony founding and their large fat-laden queens. Their apparent lack of effective means of defending the brood against ants before any workers emerge may be what keeps them out of the tropics, where ant predation is largely seasonal. In many tropical genera the swarm provides numerous "guards" which actively defend the brood during the vulnerable early stages of colony development. Later, the entrance hole in the envelope provides a narrow, easily guarded access. On the other hand, the swarm appears ill-adapted to surviving the harsh winters of temperate regions and may be what keeps these groups from ocurring much beyond the limits of the tropics. The remaining genera are primarily tropical but range well into temperate regions. Their chemical defense system permits them to succed in the tropics in the face of high ant predation pressure while esssentially retaining the monogynous type of founding that is adaptive where hibernation is necessary.