94G On Aquatic Carnivorous Coleoptera or Dytiscidce. 



epirftenium, and very small in width at the posterior angle, which itself is obtuse and 

 little prominent. This though true of all Noterides is not untrue of all other 

 Dy tiscidse, for such a description applies also to Huxelhydrus of the Bidessini. The 

 fork of the mesosternum always is highly developed, and accurately coadapted with 

 the inter-coxal process of the metasternum, and one of the most striking charac- 

 teristics of the Noterides is. the accurate coadaptation of the three parts, prosternal 

 process, mesosternal fork, and inter-coxal process of metasternum ; it is by this that 

 the immobility of the prothorax is secured ; and the perfect continuity of the 

 prothorax and after body is thus rendered possible. 



The middle legs in the higher Noterides are a good deal thickened and flattened 

 out (see especially Synchortus). and their coxse are small and globular ; they play 

 no doubt, as above observed, a considerable part in the locomotion of the individual, 

 and have undergone a considerable change to facilitate this. This is quite in 

 opposition to the higher forms of other groups of Dytiscida;, for in them the middle 

 legs do not share in a correlative manner the development of the hind legs into 

 S'wdmming organs. 



The metasternum is peculiarly formed in the higher Noterides ; its middle part 

 is prominent, and very closely soldered with the prominent coxal processes, so that 

 the suture is more or less nearly obliterated ; it is short in the middle, and forms 

 only a very obtuse point there ; extending laterally this suture has very little 

 anterior direction, but owing to the brevity of the metasternum, it is never far 

 separated from the middle coxa, and sometimes indeed almost touches the coxal 

 cavity near its middle {vide Noterus), it then proceeds almost directly outwards 

 with a slight slope in the backward direction till it attains the episternum ; its 

 anterior part in the middle always forms a very distinct and rather broad inter-coxal 

 process, and on the outside of the coxal cavity extends a good deal forwards ; a 

 lateral wing of the metasternum is thus formed, having a very peculiar shape, this 

 shape depends on the middle coxa having, as it were, by the unusual growth of the 

 mesosternum, been forced backwards so that its cavity intrudes greatly on the 

 metasternum, and as the metasternum is short, and has suffered compression from 

 behind in the middle, by the growth of the peculiar and enormously developed coxal 

 processes, the consequence is that on the space between these two forces it has 

 become very reduced, and the lateral wing is almost cut off from the middle part. 

 This peculiar structure is best studied in Noterus {vide fig. 39) and Synchortus, but 

 where the swimming powers of the middle and hind legs are but little developed, 

 as in the Suphisini, all that can be said is that there is such a form of the parts as 

 might be developed into the peculiar structure described, by such changes as are 

 necessary to improve the swimming powers. Thus in Suphis dilformis the middle 

 coxa intrudes comparatively little on the metasternum, and the hind femur when 

 flexed for the stroke of propulsion can be brought comparatively little towards the 

 middle, so that its stroke is nmch less powerful than it is in Noterus and 



