Occasionally we found one about half the usual size, indicating a young spider, but no other sizes. My guide said they only emerge from their tunnel at night, and proved it by an ingenious mechanical device made of straws attached to the door. When the door was opened, the straws lifted up, but did not fall down when it was closed. Whenever he found the straw still up in the morning he knew the door had been opened in the night. As they are nocturnal in habits, they doubtless prey upon other insects, such as sow-bugs and crickets, which the night brings forth. Two bright specks upon the top of the head appear to be eyes, but they are so small they probably only serve to enable them to tell night from day. I think these spiders are mainly guided by a marvelously acute tactile sense. They probably feel the slightest vibration in the earth or air, unless they have a sixth sense of which we know nothing. All their work, the building and repairing of their nests, as well as all their hunting, is done by night. This habit, in connection with their extreme shyness, makes the task of getting at their life-histories a difficult one. The inside of the burrow seems coated with a finer and harder substance than the soil in which they are dug. It is made on the spot, the spider mixing some secretion of her own with the clay, and working it up into a finer product. The trap-door sooner or later wears out at the hinge, and is then discarded and a new door manufactured. We saw many nests with the old door lying near the entrance. The door is made of several layers of silk and clay, and is a substantial affair. The spider families all have the gift of genius. Of what ingenious devices and arts are they masters! How wide their range! They spin, they delve, they jump, they fly. They are the original spinners. They have probably been on their job since carboniferous times, many millions of years before man took up the art. And they can spin a thread so fine that science makes the astonishing statement that it would take four millions of them to make a thread the caliber of one of the hairs of our head--a degree of delicacy to which man can never hope to attain. Trap-doors usually mean surprises and stratagems, secrets and betrayals, and this species of the arachnids is proficient in all these things. The adobe soil on the Pacific coast is as well fitted to the purposes of this spider as if it had been made for her special use. But, as in all such cases, the soil was not made for her, but she is adapted to it. It is radically unlike any soil on the Atlantic coast--the soil for cañons and the rectangular watercourses, and for the trap-door spider. It is a tough, fine-grained homogeneous soil, and when dry does not crumble or disintegrate; the cohesion of particles is such that sun-dried brick are easily made from it.