The mason wasps, or mud-daubers, build their compartment houses generally in places easily accessible to the investigator; therefore the experiments and observations which I am about to detail can be duplicated and verified without difficulty. These interesting members of the Hymenoptera, the _avant-couriers_ of the social insects, can be seen any bright day in August or September busily engaged on the margins of ponds, ditches, and puddles in the procurement of building materials. They will alight close to the water's edge, and, vibrating their wings rapidly, will run hither and thither over the moist clay until they arrive at a spot which, in their opinion, will furnish suitable mortar. Quickly biting up a pellet of mud, they moisten it with saliva, all the while kneading it and rolling it between maxillæ and palpi. When it has reached the proper consistency they bear it away to some dry, warm place, such as the rafters of an outhouse or a garret, and there use it in the construction of their adobe or mud nests. There may be dozens of these nests in the process of construction, and arranged on the rafters, side by side, yet these busy little masons never make the mistake of confounding the houses; after securing mortar they invariably return, each to her own structure. This statement can be easily verified. While the insect is engaged in applying the mortar, take a camel's-hair brush and quickly paint a small spot on her shoulders with a mixture of zinc oxide and gum arabic; then mark the nest. The marked wasp will always return to the marked nest. As soon as the cells are completed, the wasp deposits an egg in each, and immediately begins to busy herself about the future welfare of the coming baby wasps. Just here these remarkable creatures show that they possess a mental faculty which far transcends any like act of human intelligence; they are able to tell which of the eggs will produce males and which females. Not only are they able to do this, but, seemingly fully aware of the fact that it takes a longer time for the female larvæ to pupate than it does the male larvæ, they provide for this emergency by depositing in the cells containing female eggs a larger amount of food. It is in the procurement and storage of this food-supply that these insects give unmistakable evidence of the possession by them of the faculty of computing. The knowing little mother is well aware of the fact that as soon as the egg hatches the young grub will need food, and an abundance of food at that; so, before closing the orifice of the cell, she packs away in it the favorite food of her offspring, which is spiders. She knows that in the close, hot cell the spiders, if dead, would soon become putrid and unfit for food: therefore, she does not kill them outright, but simply anæsthetizes them by instilling a small amount of poison through that sharp and efficacious hypodermic needle, her sting.[85]