930 On Aquatie Carnivorous Coleoptera or Dytiscide. 
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. 
T 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 (Dytiscus duodecimpustulatus, &c.;); in 
others (such as Hydroporus 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 
ereater differentiation has occurred; thus inCcwlambus 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 organism, 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. Hydroporus 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 
theseregions are very few in number, and none are yet known with certainty to inhabit 
