830 On Aquatic Carnivorous Coleoptera or DytiscidcB. 



called forth by the scraping of the sharp upturned edge of the last ventral segment 

 against the two raised fde-like bands which I have already described as existing on 

 the inner face of the wing-cases. This apparatus is not of itself however sufficient to 

 produce a sound of such power as that given forth by this creature, and there is little 

 doubt that it is rendered much more effective by the pecuhar method of locking the 

 elytra to the sides of the hind body. At the spot where the ventral segments are 

 strongest (viz., the part of the side where a horny raised process traversing the in- 

 ternal face of the first ventral segment abuts) a prominence fits into a large narrow 

 cavity formed by the peculiar process previously described ; on the inner face of the 

 wing-case the coadaptation of these parts is of the most perfect nature, and a locking 

 together of the wing-cases and the ventral segments of a very remarkable character 

 is the result ; when sound is to be produced it is necessary that the insect should 

 greatly bend forward the terminal ventral segment, otherwise it could not reach the 

 file ; by this pressure a great strain or tension is produced at the base of the ventral 

 segments, and by means of the lock just described this is transmitted to the elytra, 

 which are thus thrown into a state of tension perfectly comparable with that of a 

 stretched violin string, and when the file on the stretched elytra is scratched a very 

 powerful sound is produced ; this structure is of the most perfect character ; the 

 very act of commencing to make a noise tunes the instrument that is to produce the 

 noise. 



The geographical distribution of Pelobius is interesting ; three species are known, 

 one occuring in Europe, but not known beyond its limits (except at such points as 

 Tangiers) the other two peculiar to Australia. The Australian species are structu- 

 rally, and in appearance, excessively similar to the European one, but are distin- 

 guished by a slight structural peculiarity, inasmuch as the scutellum is largely 

 exposed in the European species, nearly concealed in those of Australia. 



I do not think any more instructive animals exist than these Pelobii, and when 

 entomology receives from able men the full amount of attention it deserves, I 

 doubt not that highly important results will be obtained from their study. 



I think it would be natural that Pelobius should be classed with the Carabidse 

 rather than the Dytiscidse ; it is in fact purely a Carbideous insect with modifications 

 to enable it to live in water : I do not mean to say that this phrase would be inappli- 

 cable to true Dytiscidse ; but what I intentl is this — if we look upon all DytiscidaB 

 as modified Carabidse, then we may say that in true Dytiscidae the modifications for 

 aquatic existence are predominant in the whole external organization, whereas in 

 Pelobius the Carabideous structure is still predominant. 1 believe investigation 

 will show that the ancestors of Pelobius were formerly Carabidse not very difierent 

 to the Carabidee around it, and only became dwellers in water after the species had 

 reached a comparatively mature stage of adaptation for existence on land : whereas 

 true Dytiscidfe became dwellers in water at a much earlier period of their ancestral 



