938 Oil Aquatic Carnivorous Coleoptera or Di/tiscidce. 



on the outer face of the joints of their tarsi are not ciHate ; the hind coxoe are 

 never very large : the coxal jirocesses are elongate and narrow, and the coxal notch 

 is very deep. The circular outline of the eyes is not interrupted by the side of the 

 head over the insertion of the antennas. The stigmatic rugse on the side piece of 

 the first ventral segment are present and highly developed. The stigmata of 

 the two last ventral segments are large and highly developed. The front tarsi of 

 the males are greatly dilated so as to form a large circular plate. 



These characters are amply sufficient to distinguish the Dytisciui in a certain 

 manner from all the other secondary aggregates. They have usually been classified 

 near Acilius and Cybister on account of the large, circular, front tarsi of the male, 

 but this has been a mistake, for the only real ajiproximation they make to any 

 insect outside of their own aggregate is to Colymbetes of the Colymbetides. In 

 that genus we find as in Dytiscini, stigmatic rugse j^resent, and the posterior stig- 

 mata more or less enlarged, whereas in Acilius and Cybister both these important 

 peculiarities of the Dytiscini are entirely absent. Even as regards the male tarsi, 

 the approximation of Dytiscus to Colymbetes is quite as decided as it is to C}' bister. 

 It is true that the circular margin of the eye is common to Dytiscini and Acilius 

 and Cybister, and not to Colymbetes ; if however Dytiscus be carefully examined 

 as to this point it will be seen that certain species ( Vide D. lajjponicus and D. 

 marginalis) have the outline of the eye distinctly, if slightly, infringed on by 

 the side of the head over the insertion of the antenna, after the manner of 

 Colymbetes, so that this point justifies the classification of the Dytiscini in the 

 neighbourhood of the Colymbetini, certain members of which have the eyes not 

 greatly emarginate. 



The enlargement of the terminal abdominal stigmata is a character of considerable 

 importance and interest, and it seems remarkable that the few species constituting 

 this group Dytiscini, should be the only ones of the whole family Dytiscida;, or 

 carnivorous water beetles, that have developed to a great extent this respiratory 

 structure : of which we find as it were the preliminary stages in some of the 

 Colymbetini. 



The stigmatic rugae of the first abdominal segment are highly developed in tlie 

 Dytiscini so that the group stands far higher than any other of the Dytiscidae in 

 the development of its external respiratory apparatus. On the other hand in its 

 powers of locomotion as displayed by the perfection of the swimming legs, the 

 Dytiscini remain far inferior to the Thermonectini and Cybistrini, and even to some 

 Colymbetini. The present habits and past history of the species of the group will 

 no doubt be found to be correlative with these peculiarities of their organization. 



One of the most remarkable facts in this group is the existence, in numerous 

 species, of two forms of the female sex. It is in this group that the dimorphism 

 of the females so remarkable in the Dytiscidfe is carried to its gi'eatest extent ; and 

 it is of especial interest tu notice that the two genera composing the group are 



