There is in fact no line of demarcation between worker and queen in the wasps as there is in the honey-bee. Von Siebold long since drew attention to the existence of parthenogenesis in certain species of wasps, and it appears probable that it is of common occurrence. Our knowledge of the social life of European wasps has recently been much increased by the observations of two French naturalists, P. Marchal and C. Janet. The latter has given an elaborate history of a nest of the hornet, showing the rate and variations of increase in numbers. His observations on this and other species indicate that warmth is of the utmost importance to wasps; the Insects themselves create a considerable amount of heat, so that the temperature of their abodes is much greater than that of the air. He considers that in Europe an elevated temperature is essential for the development of the individual,[43] and that the chief object of the various wrappers of paper with which the Insects surround their nests is to keep up this high temperature. These envelopes give a great deal of trouble to the Insects, for they have to be repeatedly {86}destroyed and reformed, as the combs they contain increase in size. Marchal's observations[44] relate chiefly to the production of the sexes and worker-forms, in the subterranean species, _Vespa germanica_ and _V. vulgaris_. The layers of comb include cells of two sizes. The upper layers, which are the first formed, consist of small cells only: the lower combs are constructed (at Paris) early in August, and consist of larger cells from which males and large females are reared. The males are, however, reared also in large numbers in the small cells. If the queen be removed, the workers become fertile, and produce parthenogenetically many eggs, but all of the male sex. He entertains no doubt that even when the queen is in full vigour the workers produce males if there is an abundant food supply. The social wasps at present known number 500 or 600 species. _Polistes_ is a very extensive genus, and it has also a very wide geographical distribution; some of the species—and those found in widely-distant parts of the world—are remarkable on account of their excessive variation in colour, and it is worthy of note that the extreme forms have been more than once taken from the same nest.