AXATOMY AND PHYSIOLOGY 85 



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

 GEcanthus, 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 in- 

 dividuality in their manner of chirping under the same external condi- 

 tions. 



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 experiments 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 oc- 

 curred in response to 512 vibrations per second, or the note C" , which is 

 approximately the note upon which the female hums. The male prob- 

 ably 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. Acridiidae bear on each side of 

 the first abdominal segment a tympanal sense organ the subject of 

 Graber's well-known figure (Fig. 137). 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. 137, p, />), which serve probably 

 to transfer the vibrations, and there is also a delicate vesicle connected 

 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. 



Other tympanal organs, doubtless auditory, are found on the fore 



