226 On Aquatic Carnivorous Coleoptera or Dytiscidve. 



in Bidessus, where the episternum and epiraeron are each small in area ; the 

 Noterides are therefore distinguished by the sublinear epimeron and the fact that 

 this is very small in comparison with the episternum. Although the epimeron is 

 always smaller than the e2:)isternum, yet in the higher forms it increases so much 

 in size that it becomes almost equal to the episternum in ai-ea {vide Laccophilus, 

 Acilius, and others). Tlie upper extremity of the epimeron, extends the whole 

 length of the mesosternal flank, terminating, at the upper articulation of the wing- 

 case, behind the extremity of the episternum ; its upper edge is grooved to receive 

 the margin of the inflexed base of the wing-case. 



There are some points specially characteristic of the mesothorax of the Dytiscidse, 

 and a comparison of these with the Carabidte is of interest; they are — 1. The 

 direction of the mesosternum ; 2. The magnitude of the anterior sternal pieces ; 

 3. The magnitude of the posterior piece ; 4. The separation of the middle coxee ; 

 and 5. The penetration of the epimeron to the coxal cavity. 1. As regards the 

 first of these, there does not exist to my knowledge any Carabid in which the 

 direction of the mesosternum is so remarkably divergent from that of the meta- 

 sternum as it is in most of the Dytiscidaj, and in the great majority of the members 

 of the two families this difference is very considerable ; there are, however, some 

 Dytiscidse — the Vatellini and Sternoprisciis, (and even a sjiecies of the genus 

 Hydroporus, Dytiscus dorsalis No. 630) — in which the mesosternum has only to a 

 comparatively slight extent assumed the diaphragmatic position it possesses in 

 their allies ; and on the other hand in Systolosoma, Trachypachys and Cyclosomus 

 of the Carabidfe, the direction of the mesosternum becomes to a great extent that 

 of the DytiscidjB ; no absolute distinction exists therefore between the two families 

 in this respect. 2. The medisternum, and the episternum are very much larger in 

 the Carabidae than they are in the Dytiscidne (tlie head and prothorax must be 

 separated from the afterbody before this can be appreciated). This distinction is 

 remarkable and is no doubt correlative with, if not dependent on the fact that the 

 presternum has a connexion with tliQ metasternum in the Dytiscidse, this being 

 of course much facilitated by the abbreviation of the intervening mesosternum ; 

 while as another explanation of the curtailment, we have the fact that the anterior 

 part alluded to is prolonged to allow of a greater or less extent of rotation and 

 nutation of the prothorax in the Carabidi:e, while in the Dytiscidse this power has 

 been held in abeyance in consequence of the more supreme necessity for fixing and 

 securing these parts to prevent the admission of water to the interior of the insect. 

 Thus it is in the Carabidie where the head and prothorax are quite free and mobile 

 that we find the greatest difference from the Dytiscidse in the point alluded to, 

 while where there is a prosternal process in the Carabidge the medisternum and 

 episternum become sliorter : Cyclosomus and Trachypachys, Systolosoma, with 

 others of the fragmentary series of Carabidse (in which series it is that we find a 

 prosternal process of not unfrequent occurrence) have therefore shorter anterior 



