938 On Aquatic Carnivorous Coleoptera or Dytiscide. 
on the outer face of the joints of their tarsi are not ciliate; the hind coxz are 
never very large: the coxal processes 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 antenne. The stigmatic rugze on the side piece of 
the first ventral segment are present and highly developed. The stigmata of 
the two last ventral segments are Jarge 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 Dytiscini 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 approximation 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 rugze present, 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 1s to Cybister. 
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. lapponicus and D. 
marginalis) have the outline of the eye distinctly, if shghtly, infringed on by 
the side of the head over the insertion of the antenna, after the manner of 
Colymbetes, so that this point justities the classification of the Dytiscini in the 
neighbourhood of the Colymbetini, certain members of which have the eyes not 
ereatly 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 Dytiscide, 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 rugze of the first abdominal segment are highly developed in the 
Dytiscini so that the group stands far higher than any other of the Dytiscidee 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 isin this group that the dimorphism 
of the females so remarkable in the Dytiscidze is carried to its greatest extent ; and. 
it is of especial interest to notice that the two genera composing the group are 
