On Aquatic Carnivorous Coleoptera or Dytiscide. 231 
in front of them, and their form therefore depends largely on the shape of these 
adjacent parts: when the hind coxa, has become very large it forms an arch in front 
and thus encroaches on the lacinia, which suffers a corresponding diminution in area, 
but as its external extremity is fixed to the outer side of the coxa, and this does’ 
not grow forward so much as the middle does, consequently the outer portion of the 
lacinia extends backwards outside the arch of the coxa as a more or less slender 
band according as the coxa is more or less extremely developed ; the extreme forms 
thus assumed are very remarkable, (see Hyphydrus, Eretes, Laccophilus) ; in the 
genus Agabus great variation exists as to the size of these laciniz, and in most 
other genera considerable diversity exists on this point. Usually the outer 
extremity of the lacinia terminates as a more or less acute point, but in the 
Thermonectini and Eretes the lacinia becomes a little broader near its apex, thus 
giving rise to a peculiar shape, which in Eretes where it is most extreme, may 
almost be described as clavate, the extremity of the wing appearing obtuse or 
almost rounded. In the Noterides the wing of the metasternum assumes a peculiar 
form characteristic of the tribe, but as this as well as some other variations, 1s cor- 
relative with the shape and development of the hind coxa, it will be better spoken 
of when those organs are described. In Pelobius and Amphizoa, the metasternum 
has near its hind margin in the middle a transverse line or suture, which is not 
present in any other of the Dytiscide, although it is a nearly constant character of 
the Carabidee. 
The metathoracic episternum isa large and conspicuous piece forming the shoulder 
of the metasternum; it is nearly triangular in form, its front margin being in 
apposition with the hind margin of the mesothoracic epimeron ; the base of the 
triangle is adapted to the wing of the metasternum, and is curved in the 
Thermonectini and Eretes, Hyphydrus, Laccophilini, and in such other forms as 
have the hind coxe very largely developed, while in other groups it deviates but 
little from a straight line, till it is near the intermediate coxw, where it is always 
curved, sometimes (Hydaticus) abruptly so; its inner angle penetrates to the 
middle coxal cavity in the Dytisci Complicati—a point of structure which has not 
yet been detected in any other Coleoptera—but in the Dytisci Fragmentati, it fails 
to reach the cavity, stopping short in the Vatellini at a considerable distance from 
the outside of the cavity, but in Laccophilus very nearly reaching to its Uhe 
posterior angle of the triangle extends as far backwards as, or even slightly 
farther than, the apex of the metasternal wing, and forms there a blunt, or 
rounded truncate angle, touching the margin of the elytron, or ata little distance 
from this: its third or humeral angle is covered by the wing-case ; the upper 
edge of the episternum just behind this angle shews a groove into which the 
inflexed edge of the epipleura fits. The anterior edge of the episternum is 
thickened so as to form a margin along its front, and when this margin is very 
large and much exposed as it is in Hydrovatus, and numerous others, it has 
