Fig. 17.
Fig. 18.
The shape of the spinnerets, and size and arrangement of the tubes, vary in different species. [Fig. 19] is a spinneret of Prosthesima, where there are a few large tubes in place of many small ones. In Agalena the two hinder spinnerets are long, and have spinning-tubes along the under side of the last joint, [Fig. 20].
When the spider begins a thread, it presses the spinnerets against some object, and forces out enough of the secretion from each tube to adhere to it. Then it moves the spinnerets away; and the viscid liquid is drawn out, and hardens at once into threads,—one from each tube. If the spinnerets are kept apart, a band of threads is formed; but, if they are closed together, the fine threads unite into one or more larger ones. If a spider is allowed to attach its thread to glass, the end can be seen spread out over a surface as large as the ends of the spinnerets, covered with very fine threads pointing toward the middle, where they unite, [Fig. 21].
Fig. 19.
Fig. 20.