212 On Aquatic Carnivorous Coleoptera or Dytiscide. 
The small amount of variation exhibited by the trophi throughout the family is 
perhaps largely connected with the diferent parts being thus closely packed 
together. However this may be, the absence of any conspicuous developments of 
the trophi is remarkable, especially when we recall the fact that these parts in the 
Dytiscidee are extremely similar, except in some minor details, to those of the 
Carabidz, and remember the great modifications of these parts that the latter family 
displays ; if the labrum of Carabus, where the lhgula is merely a small semi-mem- 
branous piece connecting the anterior parts of the large paraglosse, be contrasted 
with that of Anthia, where the ligula is a very large, elongate, horny lobe, at the 
base of which oneach side is attached the isignificant paraglossa, the great 
difference here displayed will render very striking the uniformity of the correspond- 
ing parts throughout the Dytiscide. 
The parts of the mouth show no important differences from those of the Carabidee, 
but there are two or three comparatively unimportant details as to which a suffi- 
ciently constant diierence exists to make them worthy of mention ; these are—first, 
the complete absence of outstanding sete from the palpi or other parts of the 
mouth : second, the fact that the second joint of the maxillary palpi is only about 
equal in length to the third, whereas in the Carabide in the vast majority of 
cases the second joint is considerably longer than the third; and, third, the deep 
emargination of the extremity of the lower face of the supports of the labial palpi. 
These characters are not, it will be admitted, of great importance, and as regards the 
second of them it may be mentioned that, in the Pseudomorphini, the second joint of 
the maxillary palpus remains, as in the Dytiscide, scarcely longer then the third 
joint. The undivided external maxillary lobe seen in Amphizoa is also paralleled 
among the Carabidee (Callistus, and one or two other genera). 
The Ayrenn# of the Dytiscide are always eleven-jointed, and are usually slender 
with the joints elongate, the second being, however, frequently shorter than the 
others, and the joints from the third to the apex each a little shorter and more 
slender thanits predecessor. Although this is the structure in the larger and higher 
forms, there is a considerable diversity in other parts of the family. The Noterides 
are especially peculiar in this respect, their antenne, being short and more or less 
distinctly incrassate in the middle; in this group even when the antennz are 
apparently slender and simple as in Hydrocanthus, it will be found that the middle 
joints (joints 6 to 8) are a little broader and longer than the following or than the 
preceding ones. In the Hydroporides the rule is that the second joint is as long as, 
or longer than the third, and the antenne are only as long as, or shorter than the 
head and thorax, and not remarkably slender, while in the Hydrovatini and 
Hyphidrini and Bidessini it is not unusual that they should be feebly serrate 
internally. In the Colymbetides the antenne are slender and filiform or setaceous, 
but never become very remarkable on account of elongation and tenuity, and in 
the earlier genera of the group, are usually comparatively thick ; it is in the 
