On Aquatic Carnivorous Coleoptera or Di/tiscidce. 891 



The compression of the prosternal process is greater than in any group of A.gabus 

 except the 22nd group. 



I have not been able to observe the tomeutum on the interior of the elytra 

 in all the species, but I have found it trustworthy in all such cases as I have 

 examined. 



Man}^ of the species are very similar to one another, and require an examination 

 before they can be distinguished with certainty. 



Considerable variety prevails in i-egard to nearly all the characters of the genus. 

 The characteristic shape or form is scarcely present in certain species, especially 

 the smaller ones : the lobing of the hind tarsi, very conspicuous in certain species, 

 (Dytiscus ater, e. g.,) is much less developed in others (I. oblitus for example). 

 The swimming legs are better developed in I. apicalis than in any other species, and 

 they are in it much thicker, shorter and more powerful than they are in I. disce- 

 dens ; these two species being the extremes of the genus in this respect. The 

 hind coxae and the wings of the metasternum, also show considerable variety, in 

 fact there are scarcely two species which agree exactly in the size and form of thes3 

 parts. 



The external sexual differences are confined to the legs and last ventral segment; 

 difference in sculpture between the two sexes being quite absent ; the male tarsi are 

 never highly developed, the incrassation of their joints being at most only moderate; 

 the clothing of their undersurface however attains a considerable length in some 

 species {cf. Dytiscus ater), and as it spreads out in a divergent manner increases 

 very much the apparent size of the tarsus ; these hairs bear very small paper-like 

 palettes at their extremity. The last ventral segment is usually coarsely strigose 

 in the male (though to a variable extent) on its apical portion, and very frequently 

 has a fine carina on the middle of this part ; in the female this segment has its 

 apical portion compressed on each side in a peculiar manner, so as to form a notch 

 or groove which varies in apparent shape in accordance with the point from which 

 it is viewed. 



The genus is characteristic of the northern portions of the two hemispheres, and 

 there is no doubt, that the north of America will prove to be its metropolis ; I, 

 discedens must be considered the most primitive of the species yet known ; it has 

 poor swimming legs with the setse at the outer angle of the femur ver}'- small, 

 rather small coxse, feeble development of the male feet and their clothing, the 

 claws of its hind tarsi are less differentiated than in other species, the lobing of 

 the joints of the hind tarsi is small, and the characteristic facies of the genus is but 

 little developed in it. The few species knoAvn from the eastern portions of the 

 Old Woi'ld and Japan are aberrant, as is also the I. cinctus of Thibet. 



I have arranged the species in two groups according to whether the hind tarsi 

 of the males have on the outside the joints margined beneath, or quite unmargined. 



TRANS. ROy. UUB. SOC, U.S., VOL. II. 5 Y 



