Spiders often descend by letting out the thread to which they hang; and are able to control their speed, and to stop the flow of thread, at will. They sometimes hang down by a thread, and allow themselves to be swung by the wind to a considerable distance, letting out the thread when they feel they are going in the right direction. Spiders in confinement begin at once to spin, and never seem comfortable till they can go all over their box without stepping off their web. The running spiders, that make no other webs, when about to lay their eggs, find or dig out holes in sheltered places, and line them with silk. Species that live under stones or on plants all line their customary hiding-places with web, to which they hold when at rest. Several of the large running spiders dig holes in sand, and line them with web, so that the sand cannot fall in; and build around the mouth a ring of sticks and straws held together by threads. TRAP-DOOR NESTS. The building of tubular nests is carried to the greatest perfection by certain genera of the _Mygalidæ_. (See page 13.) _Atypus_, the most northern genus of this family, makes a strong silken tube, part of which forms the lining of a hole in the ground, and part lies above the surface, among stones and plants, Fig. 22, A. The mouth of the tube is almost always closed, at least when the spider is full grown. Another genus, which lives in warm countries, makes tubes lined with silk, and closed at the top by a trap-door. A common species, _Cteniza Californica_, lives in the southern part of California, and is often brought east by travellers. It digs its hole in a fine soil, that becomes, when dry, nearly as hard as a brick; but the spider probably works when the ground is wet. The holes are sometimes nearly an inch in diameter, and vary in depth from two or three inches to a foot. The mouth is a little enlarged, and closed by a thick cover that fits tightly into it, like a cork into a bottle. The cover is made of dirt fastened together with threads, and is lined, like the tube, with silk, and fastened by a thick hinge of silk at one side, Fig. 22, B. When the cover is closed, it looks exactly like the ground around it. The spider holds on the inside of the door with the mandibles and the two front pairs of feet; while the third and fourth pairs of legs are pressed out against the walls of the tube, and hold the spider down so firmly, that it is impossible to raise the cover without tearing it.