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Show 1889.] REV. O. P. CAMBRIDGE ON NEW ARANEIDEA. 39 lichen (?) growth that is common on dry soils at this time of year. As seen from above, the door is a square, with its two anterior angles rounded off, the straight side or base forming the hinge-end. But if the door is opened and viewed from below, it has a circular appearance, reminding one of a gun-wad, which is due to its white lining having been worked into a circular form to fit the mouth of the cylindrical burrow. In cross section the door is conical, like a plug, with its lower surface convex, like the bottom of a decanter-stopper. This surface is beautifully worked over with a network of fine, tough fibres, into which the spider (as I have seen) inserts its claws to keep the door closed against the entrance of an enemy. In no other part of the burrow-lining is this network of fibres to be seen. The door is always made thin at the hinge-end, and thick at the forward end, the average respective thicknesses being one sixteenth of an inch and one quarter of an inch ; so that its section is wedge-like. The result of this construction is that the door will always on release after being opened fall down by its own weight, fitting with exceeding and surprising accuracy into the mouth of the burrow. So preponderating is the weight of the door at its forward end, and so instantaneously does the action of gravity cause it to fall when released after being held up, that the shutting of the door closely simulates the action of a spring, and it is very difficult at first sight to realize that no elastic force exists in the action. Thus, when a spider sitting at the mouth of its hole, with the door ajar, resting on its back, darts down its burrow when startled, the door seems to snap down with the action of a suddenly released spring; but that this is really caused by gravity anyone may convince himself by cutting the mouth of a burrow out of the ground, and noticing the action of the door and its hinge when held upside down. " The hinge, which is beautifully flexible, is formed by a prolongation and local thickening of the lining of the burrow, which is also carried over the lower surface and round the edges of the door. The part of the lining forming the hinge is thick and tough, and of the same colour outside as the ground, but there is nothing special or mechanical in its structure as a hinge. It may be mentioned here that the lining of the tube is thinned off at the mouth of the burrow to receive the door, a distinct rim being usually observable at the commencement of the burrow proper. There are generally a few withered grass-blades worked into the edge of the door, or into the edge of the mouth of the burrow, so as to form a kind of semicircular fringe, which often catches a practiced eye and leads to the detection of the hole. The grass-blades are probably inserted to aid in assimilating the outside of the burrow to its surroundings, a purpose in which they certainly fail, so far as the human animal is concerned. In a few cases I have also noticed grass-blades worked into the general surface of the door, and at this season, when the grass is everywhere withered, these certainly aid in its concealment; but during the rains, when the adjacent grass is green, one would think that yellow withered grass-blades on or near the burrow-mouth would tend to make it conspicuous. |
