[Illustration: FIG. 101.--Spider and its web in a rose-bush. (Photograph from life by Cherry Kearton; from "Wild Life at Home," by permission of Cassell & Co.).] The habits and instincts of spiders in connection with the care of the young, the building of webs and nests, ballooning by means of silken lines, the active stalking and catching of prey, etc., are very interesting and offer a good field for independent observation and study by the student. [Illustration: FIG. 102.--The triangle spider, _Hyptiotes_ sp. (California), with its web; the spider rests on the taut guy-line, with a loop of the line held between its fore and hind legs; when an insect gets into the web the spider loosens the hold of its hind feet on the guy-line, thus allowing the web to spring forward sharply and further entangle the prey. (From Jenkins and Kellogg.)] FOOTNOTES: [9] There are in many forms a few internal projections from the exterior cuticle which act as internal skeletal pieces. [10] The labrum differs from the other mouth-parts in not being composed of a pair of body appendages; it is simply a fold or flap of the skin of the head. [11] A Text-book of Zoology, Parker & Haswell, 1897. [12] The Cambridge Natural History, vol. V, 1895, vol. VI, 1899. [13] A Manual for the Study of Insects, J. H. and A. B. Comstock, 1897. [14] It has been shown by experiment that the winged individuals, which are able to leave the old food-plant and scatter over new plants, do not appear until the food-supply begins to run short. At the insectary of Cornell University ninety-four successive generations of wingless individuals were bred, by taking care to provide a constantly abundant supply of food. This experiment was continued for more than four years. CHAPTER XXII MOLLUSCA: THE MOLLUSCS THE FRESH-WATER MUSSEL (_Unio_ sp.) =Structure= (fig. 103).--TECHNICAL NOTE.--The fresh-water or river mussel lives commonly in the streams and lakes or ponds in the United States. It frequents muddy or sandy bottoms. Specimens can often be secured with a long-handled rake from the shore or picked up in shallow streams with the hand. If possible to keep the animals alive until ready for use, some of their habits may be observed. Place them in a tub or trough with water and mud; when they have settled themselves put some powdered carmine, starch, or similar substance in the water near them, and note the water-currents.