THE SENSES AND THEIR ORGANS. S13 



My own opinion is that all these various types of hairs may be re- 

 garded, generally speaking, as Tactile hairs, and that they serve to com- 

 municate to the spider the sensations which are included by more highly 

 organized animals in the distinct senses of touch and hearing, and, I 

 might add, of smell. 



It seems to me that there can be nothing contrary to this view in the 

 fact of differences in the forms of hairs, if we suppose that the several 

 types may indicate some differentiation in the character of touch sensa- 

 tions communicated by them, so that a spider may be able to distinguish 

 between the agitations of the air caused by ordinary movements of the 

 wind and the impressions of waves of sound, and those sensations which 

 result from touch proper, as the undulatory motion of surfaces on which 

 a spider rests, or the agitation of the web upon wliicli it hangs and the 

 trapping thread to which it holds as it lurks within its den. In other 

 words, there are differences in the sensations produced by the organs of 

 touch, but these have not been so far differentiated as to justify us in 

 distinguishing any of them as organs of hearing. 



Mr. Wagner calls attention to facts which may lead up, after wider 

 study, to important conclusions. He says that the Orbweavcrs (Epeiridte) 

 and Lineweavers (Therididai), for example, only possess these hairs upon 

 the tibia and metatarsus ; while the Wanderers have them not only more 

 numerously on the tibia and metatarsus, but also upon the tarsus. We 

 perhaps may not accept Mr. Wagner's opinion that the Wanderers are ex- 

 posed to far greater dangers than the Sedentaries, but certainly there is a 

 difference in the form in which the dangers approach them, as well as in 

 the character of the dangers. The greater number of Tactile hairs on the 

 legs of Wanderers may perhaps be associated with the fact that they do 

 not rest upon a web, but come in contact with the ground and the vari- 

 ous surfaces on which thoy lurk for prey. Their feet also are used, at 

 least in some cases, for digging holes in the earth and for other uses which 

 are not habitual to Orbweavcrs and Lineweavers. Moreover, Sedentary spi- 

 ders, hanging on their webs by their feet, need a concentration of sense 

 organs in the neighborhood of the claws or tips of the tarsus ; and it 

 seems to me that the Sedentaries are well provided in this respect, and 

 are thus able to detect the slightest motion that runs along the lines of 

 their snares when agitated by insects or by raiding enemies. However, 

 we must confess that here we are largely in the region of conjecture, but 

 the manner of life among Wanderers, one would suppose, naturally re- 

 quires a better physical organization, inasmuch as they are not provided 

 with the habit which constructs trapping instruments for the accession of 

 prey and the defense of their persons. In other words, it may be that the 

 presence of additional sensation hairs ui)on the Wanderers is a compensa- 

 tion for the lack of industrial facilities. 



Mr. Wagner has also found some interesting facts concerning the 



