On Aquatic Carnivorous Coleoptera or DytiscidcB. 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 lacinise, 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, is 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 Dytiscidae, although it is a nearly constant character of 

 the Carabidfe. 



The metathoracic episternum is a 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 coxae very largely developed, while in other groups it deviates but 

 little from a straight line, till it is near the intermediate coxae, 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 it. The 

 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 at a 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 



