222 On Aquatic Carnivorous CoJeoptera or Dytiscidce. 



Thus in the very different Pelobius, Amphizoa, Dytiscidse, and Haliplidoe, we find 

 a similarity of structure of the prothorax existing, not because of community of 

 descent, but because water ha\dng to be excluded from the interior of the body, the 

 simplest and best method of accomijlishing that end has been used in all thesedifferent 

 forms. As regards the peculiar Carabidse, Trachypachys and Systolosoma, we do 

 not know enough of their mode of life to explain tlie structure : but as regards 

 Silphomorpha and their allies we know that they are dwellers under bark, and we 

 can feel sure that if this has been the case throughout a very long period of the 

 ancestral record it must evolve a structural condition very different from that of 

 the cursorial Carabidse. Omophron has a peculiar mode of life, concealing itself 

 for long periods in fine wet sand, and we find that in it tlie articulation between 

 the pro- and meso-thoraces is well fitted to guard against the entry of fine sand, 

 although it would not be suflficient to keep water from gaining access to the interior. 



The prothoracic stigma is placed high up, near the junction of the epimeron with 

 the pronotum, and is protected by a hollow in the front edge of the uiesothoracic 

 episternum ; the orifice is short and small, but the stigma is perfectly fitted for 

 respiration, although it is not used for inspiration during the aquatic life of its 

 possessor. 



The Mesothoeax in the Dytiscidse might, when the under surface of the body is 

 looked at, be supposed to be absent, for it is not exposed in front of the metasternum, 

 but is, as it were, turned inwards, so as to be placed more or less at right angles with 

 the longitudinal axis of the body ; the result of this is, that a kind of cavity or 

 fissure, roofed above by the sides of the prothorax, exists in front of the metasternum, 

 and in this hollow the flexed front and middle legs are packed during the process 

 of swimming. This position of the mesosternum is however by no means invariable 

 in the family, for though carried to an extreme extent in Hyphydrus, Hydrovatus, 

 Cybister, and other higher forms, yet in the Vatellini, Sternopriscus and others the 

 position differs but little from that of the Carabidte. The mesonotum has been hither- 

 to but little studied although the visibihty or invisibility of the scutellum has largely 

 determined the classification of the family. It offers however considerable modifica- 

 tions which should not be neglected. The scutum appears in the form of two 

 lateral lobes joined together along the middle line ; their base is nearly straight 

 except that in the middle it is frequently (Dytiscus, Cybister, Graphoderes, &c.) 

 intruded on by an angular projection from the middle of the front of the scutellum ; 

 taken together the two lobes appear in front always more or less emarginate, being 

 longest at the outer side, shortest at their point of junction in the mesial line ; this 

 emargination may be very slight (Dytiscus bipustulatus No. 751 gen. Agabus e.g.,) 

 or very deep (Dytiscus roeselii No. 1169, Cybister e.g.) In Noterus the base of the 

 scutum is deeply transversely impressed, and the impression divided in the middle by 

 a raised line, but in general the lobes are slightly convex, and without any definite 

 impression. The mesonotum is very small in Eyphydrus, Laccophilus and 



