The thing is obvious: the Wasp does not improve the mud to make it into mortar; she uses it as it is. It is no less obvious that nests of this sort are not made for out-of-doors, even if the larva were not of such a chilly humour. A shelter that keeps them under cover is indispensable, otherwise they would go to pieces at the first shower of rain. This explains, apart altogether from questions of temperature, why the Pelopæus has a preference for human habitations, which afford the best protection against damp. Under the mantels of our chimneys she finds at one and the same time the heat required by the larvæ and the necessary dryness for the nests. Before receiving its final coating, which conceals the structural details, the Pelopæus’ edifice does not lack elegance. It consists of a cluster of cells, sometimes arranged side by side in one row—which gives the fabric something of the look of a mouth-organ with reeds all short and all alike in size—but more often grouped in a varying number of layers placed one above the other. In the most populous nests I count as many as fifteen cells; others contain only about ten; others again are reduced to three or four, or even to one alone. The first appear to me to represent a mother’s whole output of eggs; the second signify incomplete layings, deposited here and there, perhaps because better sites were found elsewhere. The cells are not far removed from the cylindrical shape, with a diameter increasing slightly from the mouth to the base. They measure three centimetres [29] in length, their breadth where they are widest being about fifteen millimetres. [30] Their delicate surface, carefully polished, shows a series of stringy projections, running obliquely, not altogether unlike the twisted cords of certain kinds of gold-lace. Each of these strings is a layer of the edifice; it comes from the clod of mud employed on the coping of the part already built. By numbering them one can tell how many journeys the Pelopæus has taken to her mortar. I count between fifteen and twenty. For one cell, therefore, the industrious builder fetches materials something like twenty times and perhaps even oftener, for one of these cushions of mud is not always, so it seems to me, completed in a single spell of work.