“That it should be thus slightly built is not surprising, for it is intended as a temporary structure for protecting the insect while it is excavating its hole, and as a pile of materials, well arranged and ready at hand, for the completion of the interior building,--in the same way that workmen make a regular pile of bricks near the spot where they are going to build. This seems, in fact, to be the main design of the tower, which is taken down as expeditiously as it has been reared. “Réaumur thinks, that by piling in the sand which has previously been dug out, the wasp intends to guard its progeny for a time from being exposed to the too violent heat of the sun; and he has sometimes even seen that there were not sufficient materials in the tower, in which case the wasp had recourse to the rubbish she had thrown out after the tower was completed. By raising a tower of the materials which she excavates, the wasp produces the same shelter from external heat as a human being would who chose to inhabit a deep cellar of a high house. “She further protects her progeny from the ichneumon-fly, as the engineer constructs an outwork to render more difficult the approach of an enemy to the citadel. Réaumur has seen this indefatigable enemy of the wasp peep into the mouth of the tower, and then retreat, apparently frightened at the depth of the cell which she was anxious to invade.” It is no wonder that the Sand-wasp should be so anxious to insure the safety of her nest, for her foes are multitudinous. Putting aside the ordinary Ichneumon-flies, we have the predatory Tachinæ, which are always hovering over such nests, and trying to deposit eggs therein. For many years I have been in the habit of receiving letters from novices in entomology, wanting to know whether I am aware that the common Housefly is in the habit of acting as a parasite. Of course, the writer has mistaken the Tachina for a house-fly, but I cannot regret the fact that some one has really begun to observe Nature, and not only to read books. DOORS AND HINGES. Having seen that both in Nature and Art the entrances to dwellings are guarded by tunnel-like approaches, we come naturally to another mode of guarding the entrance, namely, by a door moving on hinges. As to the multitudinous examples of doors and hinges in modern civilisation, we need hardly discuss them, except to show the exact analogies which occur in Art and Nature. Doors moving on hinges are very plentiful in Nature, even where we should least expect them. Take, for example, an egg, especially the egg of an insect, and we shall see that it is just about the last object in which we should expect to find a hinged door. Yet, if the reader will refer to the illustration on page 7, he will see that the tiny eggs of the common Gnat, numerous as they may be, are each furnished with a door which opens as soon as the inmate is hatched, and allows the little larva to escape into the water.