There is another, and just as vital a condition governing its construction—the necessity for strict economy in material. If there were any natural substance having the qualities of tenacity, lightness, ductility, and strength which the bees could obtain out of doors instead of wax, no doubt they would use it for comb-building, and they would not spend hours of precious time and consume large quantities of hard-won stores in the manufacture of their own material. But it seems there is nothing in nature possessing the needful properties. Bees collect a resinous substance, notably from the buds of the poplar, which they use for stopping up crevices. They dilute this also into a varnish, with which they paint the finished combs, and sometimes even combine it with wax to form a rough filling; but it appears to be useless in cell-construction. The whole city must needs be made of wax, and wax alone; and the bees are as careful of this precious substance as a miser of his gold. Starting with these conditions—efficient house-accommodation for the colony secured at the least cost in time, labour, and material—the bee tackles the problem before her with an ingenuity that is little short of astounding. She appears to begin with the central dominant unit of the difficulty, and to work outward, vanquishing subsidiary problems as she goes. Her line of reasoning seems to run somewhat in this way. To raise the young, and store the honey, there is needed some kind of cell or receptacle. The young larvæ being cylindrical in form, a cylindrical cell is indicated; and this shape will serve also for the honey-barrels. Not a few, however, but many thousands of these vessels will be required: they must therefore be placed close together, as well for economy of space as for natural warmth. The cells could be grouped together mouth upwards in horizontal planes, storey above storey; but such a method of construction would be economically unsound. To prevent sagging in the heat of the hive, and under the weight they will be called to bear, the cell-bases would have to be thickened collectively into a substantial floor, which would need shoring-up at intervals—after the manner of the wasps. But in this, much valuable material would be diverted from its proper use. Obviously, a better plan would be to lay all the cells on their sides, and pile them up into a vertical wall. And, just as obviously, if two walls of these superimposed cells were placed back to back, so that one central vertical sheet of wax would serve to stop the ends of all the cells, right and left, a saving of half the material used for the cell-bottoms would at once be effected.