55S PROCEEDINGS OF THE ACADEMY OF [Dec, 



the antero-dorsal aspect of the abdomen and is a thickened chitinous 

 plate, slightly protuberant, of oval form. Now these species all show 

 a peculiar movement of the abdomen, such as I have never seen so 

 pronounced in any other spiders; they raise and lower the abdomen, 

 slightly rotating it on its pedicel, and continue this movement for 

 several seconds at a time. This movement is exhibited generally just 

 when the spider comes to rest after a run, and is exhibited equally by 

 both sexes. An individual frequently rotates the abdomen imme- 

 diately before starting on a run, on meeting another unexpectedly or 

 on coming into contact with a large living insect. It is sometimes seen 

 when the spider is quietly cleansing itself, but not when the cage is 

 given a sudden rap. The hardened abdominal plate at such times rubs 

 against the posterior border of the cephalothorax; accordingly, it is 

 a stridulation movement. But if any sound is produced it is quite 

 inaudible to the human ear, and if one individual stridulates in the 

 close vicinity of another the latter does not respond by any movement 

 whatsoever and therefore does not appear to be affected by any sound. 

 It is in no way a sexual call, for the male hunts the female and finds 

 her by touch, and neither he nor she stridulate during the mating. 

 The ants they resemble do not show this movement. 



This genus then exhibits a good case of stridulation, but if any 

 sound is produced thereby it seems to cause no effect on other individ- 

 uals and, therefore, is probably not perceived by them. The presence 

 of a stridulation apparatus need not imply the power of hearing in the 

 species concerned, and indeed the studies of Wagner and Pritchett seem 

 to have definitely determined that spiders do not possess hearing — 

 they possess in its place a most acute sense of touch. 



These spiders run with perfect ease on smooth glass. 



6. Notes on Modes of Copulation. 



Phidippus purpuratus K. 



The pairing was observed in 26 instances. The male stands over 

 the female, his ventral surface against her dorsal, their heads in oppo- 

 site directions, their body axes not in the same line, but his bent 

 obliquely toward one side of her abdomen. Only one palpus is inserted 

 at a time, and one alone may be employed through the act, or the two 

 may be alternated one or more times. The female remains motionless. 

 In the cases timed the act lasted less than one hour in 5 instances, and 

 more than one hour in 9 instances, the longest noted being more than 

 8 J hours continuously. It may occur in the morning (7 cases), after- 

 noon (11 cases), or after 6 P.M. (8 cases). The same pair have bee- 



