ON INSECT SOUNDS. 23I 



animals consist generally of noises, or of compounds of noise and 

 tone, or of compounded tones. 



Noises are sounds produced by the vibrations of bodies which 

 do not swing with equal periodic movement. Tone is recognised 

 when the sounding body moves with regular periodicity. Vibra- 

 tions are periodic when in given intervals of time, a body swings 

 with exactly the same number of to-and-fro movements {e.g, 

 pendulum beat.) 



The strength of a tone is conditioned by the amplitude of 

 vibration 3 the pitch by the number of vibrations 3 the quality 

 (timbre, colour) by the particular form of the periodic vibration. 



The analysis of a compound tone produced by animals requires 

 special experimental treatment, which caiuiot always be applied, 

 and the distinctions of voice sounds has been from the time of 

 Aristotle down to the present moment based upon the character 

 of the instrument by which the sound is caused, rather than its 

 physical character. 



The most imperfect form (i insect sound is the single beat, as 

 in the case of the Anohiut/f, or death tick, or the rustling sound or 

 crackle of powerfully worked jaws of caterpillars and many insect 

 larvae when feeding. In these instances no respiratory organ is 

 concerned, nor is the sound a musical tone. Many animal sounds 

 which at lirst appear to be mere noise can be resolved by special 

 physical apparatus into tones musically distinct, so that the dis- 

 tinction between noise and tone is reduced to this— that certain 

 noises which can be so resolved are a complex of vibrations not 

 resolved by the human ear, except with the aid of acoustic 

 apparatus, and a portion not resolved by the insect ear. 



For us sounds constitute tone when they are so distinguished by 

 the human ear, whether this tone be produced by organs within 

 or outside the body, and with or without co-operation of the 

 respiratory organs. For example, the shrilling of the cricket, 

 grasshopper, water bug, kc; the chirp of a crioceris, the hum 

 of bees, gnats, &c. 



