830 On Aquatic Carnivorous Coleoptera or Dytiscide. 
galled forth by the scraping of the sharp upturned edge of the last ventral segment 
against the two raised file-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 peculiar 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 n- 
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 pertectly 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 oxeat 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 Pelobil, 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 Carabidee — 
rather than the Dytiscide ; 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 Dytiscide ; but what I intend is this—if we look upon all Dytiscidee 
as modified Carabidee, then we may say that in true Dytiscide the modifications for 
aquatic existence are prodominant in the whole external organization, whereas in 
Pelobius the Carabideous structure is still predominant. I believe investigation 
will show that the ancestors of Pelobius were formerly Carabidze not very different 
to the Carabide 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 Dytiscidee became dwellers in water at a much earlier period of their ancestral 
