SOCIAL INSECTS Social Wasps.—It will be convenient to discuss the social wasps first because they are of special interest in illustrating the evolution of social behaviour. Collectively wasps (q.v.) form an extensive group numbering over io,000 species, the vast majority being solitary in habit. The social wasps, which alone concern us here, include about Boo species and are all members of the family Vespidae of the superfamily Vespoidea. Wasps are pri marily predacious and feed upon other insects which they cap ture as their prey : they are also fond of nectar, over-ripe fruit, honey-dew, etc. Their mouth-parts have not attained the length and perfection found among bees, and hence wasps are unable to exploit the secretions of deeply seated nectaries. They con struct paper nests formed of fragments of dry wood chewed and mixed with saliva : such nests consist wholly, or in part, of combs, or regular hexagonal cells, in which the young are reared simul taneously.
In oriental wasps of the genus Stenogaster some of the species are solitary, while others betray elementary social habits. Accord ing to F. X. Williams the social members of this genus construct small nests of comparatively few cells and the colonies comprise but few individuals. The female parent feeds the larvae from day to day until they are fully grown, when the cells are then sealed over : on emergence from the pupae the daughter wasps share the nest with the parent. In primitive African wasps of the genus Belonogaster the colonies are of larger size and in the stronger nests the older females devote themselves to egg-laying and the younger to foraging for food and nest materials. There is thus an indication of division of labour, but without any differentiation in structure among the females. Belonogaster is termed a polygynous wasp because each nest contains a number of fecundated females, and when a colony has attained its full development they leave in parties and found new nests elsewhere. Among certain South American wasps of the subfamily Epiponinae there is found the beginnings of caste differentiation : certain of the females are workers, i.e., their ovaries are imperfectly developed and they are
either sterile or capable only of laying unfertilized male-producing eggs, while others are true females or queens with fully developed ovaries and capable of fertilization. Since many of these wasps have numerous queens and their colonies are perennial, their nests become extremely populous comprising thousands of individuals. This overcrowding, however, is relieved by their periodically emitting swarms of workers accompanied by a small number of queens. Such swarms are the forerunners of new colonies and are unknown among wasps outside the tropics.
The most familiar social wasps are those of the genus Vespa which includes all of the common wasps and hornets of the north temperate zone. Since they are inhabitants of cooler regions, their colonies last for a single season only : swarming is unknown for the reason that this habit is an adaptation to the continuously favourable conditions of food and temperature that prevail only in the tropics. Furthermore, the colonies of Vespa are mo nogynous, each being founded by a single fertilized queen. At the end of the season all members of a nest perish with the excep tion of an annual brood of queens which, after fertilization, hiber nate and each founds a new colony the next spring. A typical wasp's nest is composed of a fertilized female or queen, a large number of workers and a smaller number of males. The three forms of individuals are very alike in coloration, but the queens are considerably the largest : the males have seven evident ab dominal segments and 13 joints to the antennae, whereas only six abdominal segments and 12 antennal joints are found in the queens and workers. The latter are a very distinct caste, being much smaller than the queen and incapable of fertilization : such eggs as they occasionally produce give rise to males only.