Sensory Discrimination: Hearing 121 



brought near without touching the web, the spider does not 

 reach for it, but drops down at the end of a thread. If the 

 fork touches the web again, the spider climbs the thread and 

 finds the spot very quickly (100). 



34. Hearing in Insects 



The sense of hearing in insects also is problematical. 

 When the insect makes a sound itself, which, as in the case of 

 crickets, is connected with the mating process, it would seem 

 a priori highly probable that it can hear. Various struc- 

 tures have been designated as auditory organs, the finely 

 branched antennae of mosquitos and gnats, on the same 

 doubtful evidence that they have been found to vibrate 

 in response to musical tones (479) ; and in the Orthoptera 

 certain very peculiar structures situated on the front legs of 

 grasshoppers and crickets, and in the first segment of the 

 abdomen in locusts. These structures Graber called chor- 

 dotonal organs, and he felt convinced from experimental 

 tests that they were auditory. The cockroach, Blatta, 

 while running about the room will stop, he says, for an 

 instant when the strings of a violin are struck. A blinded 

 specimen, hung by a thread, became violently agitated at a 

 sudden tone from a violin. A water insect, Corixa, al- 

 though undisturbed by the water vibrations produced by 

 pushing a bone disk toward it in the water, gave decided 

 reactions when the disk was connected with an electric bell. 

 Other water beetles were still more sensitive. That they 

 distinguished pitch differences Graber thought probable 

 from the fact that he observed reactions of different degrees 

 of violence to sounds of different pitch; and their dis- 

 crimination of intensity changes he thought demonstrated 

 by the fact that if a continuous tone, sounding while a water 



