Page:Popular Science Monthly Volume 62.djvu/166

160 sand is loose and desert-like for about two inches in depth, after which it meets sand finer in grain and baked hard. The larger grit lies on the surface. If you look carefully at these patches, you may see a delicate outline, shaped like a butterfly's wings, traced on the red sand, one pair of 'wings' being generally larger than the ether, as is the case with a real butterfly. The impression is very beautiful and fairy-like. These 'wings,' which are covered with gritty red sand and lie flush with the surface of the ground, are generally about an inch and a quarter square, the longest measurement being across the front pair. If you insert the blade of your knife under the smaller wings, you may turn the lid over its hinge, which is on the side of the larger pair. This needs to be done carefully, for the lid is limp and delicate and easily doubles up and loses it shape; the operation is somewhat suggestive of tossing a pancake. You expose a smooth bare spot, shaped like the double wings, nearly in the middle of which is the hole.

Though this butterfly lid is about an inch and a quarter square, the hole is hardly, if anything, more than an eighth of an inch in diameter. It is situated almost under the middle of the lid, just below what corresponds to the thorax of the butterfly. The lid is attached to the side of the hole towards the head of the butterfly, and the attachment is restricted to the width of the thorax, leaving the whole outline of the wings free. The hole, which is from two to three inches deep and beautifully brown-webbed throughout, runs straight into the ground at an acute angle under the hinge. The side of the opening opposite the hinge slants, and on the slant, attached to the web lining of the hole, lies a small, loose, felt-like flap.

It must be remembered that the large 'butterfly' lid is never raised—indeed, cannot be raised—by the spider; for she is not strong enough, and, if she were, it would double up and lose its shape. So it always remains flat on the ground, covering the hole and the ground for more than half an inch all around it. The spider creeps in and out underneath, as from under a blanket; you can trace its passage by a little wave rippling along under the sand-covered wings. The flap, I think, has two purposes. First, it affords the little spider a means of easy ingress and egress to the hole under the lid; it is firm and felt-like to give the spider a good hold for her feet over the loose shifting sand, and it is slanting to enable her gradually