414 PROCEEDINGS OF THE ACADEMY OF [May, 



distinct. A process of the neurilemma (neu.) encloses the nerve 

 fiber (n. f.) which runs to the sense cell (s. c.) lying at the base of the 

 hypodermis. After the neurilemma passes around this sense cell it 

 can no longer be seen, but the delicate sensory fiber (s. f.) continues 

 to the socket. The sense cell at the right is much smaller and the 

 neurilemma seems to stop before it surrounds the cell. Fig. 6, from 

 the distal end of the femur of Th. tepidariorum six or seven hours 

 after moulting, shows the actual union of the sense cell (s. c.) with the 

 base of the movable hair. Here the sensory fiber (s. f.) ends in the 

 cavity of the hair very near the bottom. This sense cell is similar to 

 those of the lyriform organs, but is more difficult to find. In this 

 section only fragments of the two muscles (m.) were discernible, and 

 the hypodermis (hyp.) is pulled away from the newly formed cuticula 

 (cu.). Fig. S, from the same section as fig. 6, is a better representa- 

 tion of the socket and muscles, but the sense cell was not present. 

 Here the apparently smooth muscles are attached at one end to the 

 bottom of the flask (fl.) of the socket and at the other end to the 

 basement membrane (b. m.) of the hypodermis. The sensory fiber 

 is very indistinct here. The cytoplasmic substance (cyt. s.) exhibited 

 in the cavity of this hair was observed in another hair of the same 

 individual. It has the same color as has the sensory fiber, but instead 

 of its walls being smooth, they are corrugated. Thus this substance 

 must be dead cytoplasm and not a continuation of the fiber into the 

 cavity. In the cross sections of the smaller fixed tactile hairs the 

 sockets were always similar to those drawn, but in only one case did I 

 see the sense cell, and in no instance did I ever see any signs of the 

 muscles. 



One can easily illustrate that the largest hairs and particularly the 

 spines on the legs of any spider are movable by gently touching them 

 with a small object. Before being touched they form an angle of 

 about twenty degrees with the leg; at the instant of being irritated 

 they lie down almost flat on the leg, the spider either lifts its leg or 

 moves away and in only a few seconds the hairs are raised again to 

 their normal position. Perhaps the movable spines serve primarily 

 as a protective purpose in guarding off enemies and various kinds of 

 obstacles, while probably the movable tactile hairs are used primarily 

 as tactile instruments which receive the impulses of gusts of wind and 

 the heavy vibrations of the webs, and the various kinds of smaller 

 tactile hairs with their nervous connections receive and transmit the 

 more delicate tactile stimuli. 



Dahl (1883) called the slenderest and probably the most delicate 



