14 INTEODUCTIOX. 



vibrate by rubbing the legs against sharp obUque ridges at the sides 

 of the abdomen or tbe edges of the elytra, and in Iscliiopsopha, a 

 Papuan and Australian genus of Cbtomix.e, at the sides of two or 

 three of the abdominal segments, which are scraped by riilges on 

 the inner face of the hind femora. 



The sound produced by these organs is generally a very high- 

 pitcbed and by no means loud musical note, sometimes only 

 audible by the human ear wlieu the insect is held within a few 

 inches. It is in no way comparable with that emitted by the vocal 

 ■organs of Crickets, Grasshoppers, or Cicadas; there is no contrivance 

 in the Coleoptera for increasing the volume of sound, nor is the 

 faculty, except in a very few highly exceptional instances, peculiar 

 to the males as in the former insects. The use of the faculty is 

 verv doubtful. Darwin expressed himself unable to conceive of any 

 purpose it could serve except communication between individuals of 

 the two sexes or emulation between those of the same sex. The 

 facts mentioned above, and indeed most of the results of recent 

 investigation, seem to me opposed to this explanation, especially as 

 no organ even probably auditory in function lias been found in any 

 beetle and no completely satisfactory evidence has been obtained 

 that an auditory sense exists. Unless this can be shown we must 

 look for the significance of the stridulaling organs in their effect 

 upon some other animals than those possessing them. Mr. Guy 

 Marshall has suggested (Trans. Ent. Soc. Lond. 1902, p. 403) that in 

 many instances, and especially when the habits are nocturnal, the 

 sounds may serve to protect the insects from enemies by indicating 

 nauseous qualities or in some cases by suggesting the buzzing of 

 sting-bearing species. Although it is very probable that the 

 origans may have in many cases acquired such uses, a survey of all 

 those groups in which stridulation is known respectively to occur 

 and not to occur seems to me to preclude the idea that the faculty 

 is to any large extent a concomitant of unpalatability. For 

 reasons which I propose to discuss elsewhere it seems to uie 

 possible to account for all the known phenomena and to explain 

 the evolution of the structures concerned upon the hypothesis that 

 stridulation is in itself an unpleasant property and a forui of 

 protection against insectivorous animals. If this view is correct 

 the sound is not the essential feature but only a bye-product of 

 the vibration, which in hard-shelled insects nuist be com- 

 municated to a large part of the surface, and I think may not 

 unreasonably be supposed to produce disagreeable sensations in the 

 mouth of a captor, as is recognised to be the effect of a panoply of 

 spines or bristles. This theory obviously involves the rejection of 

 t he term • vocal ' organs for the structures here described, at least as 

 a general designation. 



Several Lamellicorn beetles in which no striilulating surfaces 

 seem to exist have been described as producing hissing or piping 

 sounds. It has been suggested that this may be coiniected with 

 the spiracles, as in various JDiptera, but no precise observations are 

 yet forthcoming. 



