930 On Aquatic Carnivorous Coleopfera or DytiscidcB. 



Another character which I have much used for the definition of the genera, being 

 also one which has not hitherto been observed or studied, requires some comment. 

 I allude to the development of a ligula or tongue on the inner face of the elytra 

 near the extremity. In all the Hydroporini there exists a raised line on the inner 

 face of the wing-case, approximately parallel to, but at some little distance from, the 

 outer edge. The function of this ridge is to ensure adaptation of the wing-case to the 

 sides of the body and to give increased power of resistance to strains or pressures 

 that would tend to force the wing-cases open and thus allow the access of water to the 

 soft dorsal portions of the insect, where the breathing apertures are placed, an event 

 which would speedily prove fatal. In certain species this ridge is but little developed 

 and its effectiveness correspondingly small (Dy tiscus duodecimpustulatus, &c. ,) ; in 

 others (such as Hj^droporus gigas, Boh., No. 427) it becomes, a short distance from 

 its terminal portion, highly developed, and folded over so as to form a groove into 

 which the sharp upper edges of the side pieces of the penultimate ventral segments 

 are received ; and thus an effective mechanism results. In other forms however a still 

 greater differentiation has occurred; thus inCi3?lambus we find that although the ridge 

 is but little developed, yet it shows at one point near the extremity an abruptly pro- 

 jecting ligula or tongue bent back very close to the face of the wing-case, and forming 

 a very effective holdfast, and allowing at the same time a considerable vertical 

 motion of the ventral segments, without any danger of their being displaced ; a 

 very effectual mechanism is thus formed by the use of a very small amount of 

 material ; this then may be looked on as at present the highest development of 

 this part of the oi-ganism, and its maximum appears to be attained by Hyphydrus 

 of the Hyphydrini. 



In connection with this I may remark in passing that the Coelambi show a diminu- 

 tion of the epipleura, and the wing-cases are thin and the material necessary for 

 their formation is comparatively limited. It would thus appear that these insects 

 have been developed under circumstances where economy of chitinous substances 

 was of much advantage to the organism ; the species are widely distributed and 

 some are especially fond of brackish waters, others of chalky waters, and it is 

 probable we have in these facts the clue for ascertaining at what periods of the 

 world's history and under what circumstances it was that these creatures appeared 

 over a large portion of the surface of the earth, flydroporus enneagrammus is the 

 species where the elytra contain the least material, and this species has been found 

 only in very salt water. 



In the Australian genus Paroster, a like feebleness of the wing-cases exists, and 

 I anticipate they will also be found to be inhabitants of similar waters. 



The Hydroporini are specially characteristic of the European and Mediterranean, 

 the North American, and the Australian regions : and one small genus (Herophydrus) 

 appears to belong to Madagascar and tropical Africa. The species found outside 

 these regions are very few in number, and none are yet known with certainty to inhabit 



