94 ENTOMOLOGY 



Professor A. F. Shull, who has made precise observations on the 

 stridulation of (Ecanthus, finds that there are numerous variations of 

 rate that cannot be accounted for by differences of temperature; that 

 Dolbear's formula cannot be applied without a possible error of 6.65 

 F. ; that humidity seems to affect the rate of chirping and that crickets 

 show a certain individuality in their manner of chirping under the same 

 external conditions. 



Hearing. There is no doubt that insects can hear. The presence 

 of sound-making organs is strong presumptive evidence that the sense of 

 hearing is present. Female grasshoppers and beetles make locomotor 

 and other responses to the sounds of the males, and male grasshoppers 

 will answer the counterfeit chirping made with a quill and a file. 



Auditory organs are not restricted to any one region of an insect, but 

 occur, according to the species, on antennae, abdomen, legs, or elsewhere. 



The antennae of some insects are evidently stimulated by certain 

 notes, particularly those made by their own kind. Thus the antennae 

 of the male mosquito are auditory, as proved by the well-known experi- 

 ments of Mayer. He fastened a male Culex to a microscope slide and 

 sounded various tuning forks. Certain tones caused certain of the 

 antennal hairs to vibrate sympathetically, and the greatest amount of 

 vibration occurred in response to 512 vibrations per second, or the note 

 C" ', which is approximately the note upon which the female hums. 

 The male probably turns his head until the two antennae are equally 

 affected by the note of the female, when, by going straight ahead, he is 

 able to locate her with great precision. 



In the lack of experimental evidence, other organs are inferred to be 

 auditory on account of their structure. Locustidae bear on each side of 

 the first abdominal segment a tympanal sense organ the subject of 

 Graber's well-known figure (Fig. 139) . This organ is admirably adapted 

 to receive and transmit sound-waves. The tympanum, or membrane, 

 is tense, and can vibrate freely, as the air pressure against the two sur- 

 faces of the membrane is equalized by means of an adjacent spiracle, 

 which admits air to the inner surface. Resting against the inner face 

 of the tympanum are two processes (Fig. 139, p, p), which serve proba- 

 bly to transfer the vibrations, and there is also a delicate vesicle con- 

 nected by means of an intervening ganglion with the auditory nerve, 

 which in this case comes from the metathoracic ganglion. The nerve 

 terminations consist of delicate bristle-like processes which are probably 

 affected by the oscillations of the fluid contained in the vesicle just 

 referred to. 



