498 



Comparative Animal Physiology 



Minnich ^^' ^^ that caterpillars are sensitive to sound, and that this sensitivity 

 seems to be localized in hair sensilla. In the hairy caterpillar Vanessa antiopa 

 sound waves of frequencies below 1000 cycles cause a contraction of longi- 

 tudinal dorsal muscles which raise the anterior end of the animal. The lower 

 frequency limit of response was below 32 cycles and was undetermined. The 

 response could be reversibly abolished by application of a water spray or of 

 flour to the hairs. These experiments were repeated with various species, 

 and it was found that relatively hairless caterpillars also responded but at a 

 higher intensity. 



Pumphrey and Rawdon-Smith ^^ studied the electrical changes of the 

 nerve from the anal cerci of various adult Orthoptera in response to sound 

 waves. The threshold curve for the cricket is given in Fig. 172; curves for 



100 500 



Frequency (c./sec.'* 



Fig. 173. Curves indicating the frequency response of a cercus preparation of Gryllus 

 (a) and the human ear (b). The points are experimentally determined and the line (a) 

 corresponds to a constant displacement of 56 m/j. of the distal end of the hair. (Pum- 

 phrey.^') 



locusts and cockroaches are similar. The end-organs are the long hair 

 sensilla, and the responses can be inhibited by coating the cerci with dust or 

 petroleum jelly. At low frequencies there was a synchronous response in 

 the nerve, i.e., one impulse for each sound wave. This synchrony was pres- 

 ent initially up to 800 cycles, but, as in the mammalian cochlea, "alterna- 

 tion" may be shown at higher frequencies. The lower response limit was 

 not determined, but it probably is well below 50 cycles. 



Movement of the hair sensillum produces a displacement of the dendrite 

 of the sense cell. The threshold displacement of the dendrite calculated 



