244 On Aquatic Carnivorous Coleoptera or Dytiscidce. 



traction is exercised on its base, of assistance in folding the wing. The median 

 nervure is a broad one, and runs from the base oi the wing obliquely across it 

 towards the hind margin in such a manner as to divide it into two approximately 

 equal parts ; it does not however extend so far as the hind margin of the wing, 

 but its extremity is connected with the hind part of a closed, oval cell extending 

 forwards and connecting with the carpal cell (carj^a of Lacordaire, Int. a I'Ent. 1., 

 p. 365). The submedian vein is a double one ; its two portions starting from the 

 base of the Aving, run nearly parallel to one another about half across the wing, 

 and are then connected by a short oblique nervule, and then some distance behind 

 this converge, so as to form a narrow, elongate cell, from whose point a single 

 nervure is continued to the hind margin of the wing ; near the base of the wing 

 the inner portion of the submedian vein sends off an elongate accessory nervule, 

 which extends nearly or quite to the hind margin of the wing. The anal nervure 

 is simple and descends in a gentle curve to the hind margin, and mai'ks oflF very 

 distinctly an anal area. At the place where the wing is transversely folded, there 

 is situated a large cell, whose hinder part is connected with the cell placed on the 

 extremity of the median vem ; and from the outer portion of this cell are sent 

 off some veinlets towards the apex of the wing ; the anterior of these veinlets are 

 more or less indefinite and tend towards the front margin, the lower one of them 

 indeed runs more or less parallel with the apical portion of the costa and reaches 

 (or very nearly) the apex of the wing : from the lower part of the carpal cell, a 

 curved nervule proceeds outwards to the hind margin of the wing ; the lower of 

 the ultra-carpal nervules is connected with the submedian cell, and is short and 

 very curved. Between the subcostal and the median veins there is a more or less 

 distinct accessory vein which runs from near the base of the wing to the inner 

 angle of the carpal cell. The median vein gives off about its middle a nervule 

 which starting at first towards the inner margin of the wing, is quickly curved 

 towards the hind margin and bifurcates before reaching it, and thus makes with 

 the margin a triangular space in which is placed obliquely an oblong patch of 

 pigment near the middle of the posterior edge of the wing. The inner margin 

 of the wing at the base is bordered by a more or less distinct fine vein; the 

 median and submedian nervures are connected near their base, by a short, some- 

 what indefinite transverse nervule; and rarely (in Noterus) the cell on the 

 submedian vein at the point where it approaches very closely to the flexuous vein 

 given oft' by the median, is connected to it by a very short transverse nervule. In 

 Dytiscus marginalis this short nervule is present, but rudimentary. 



The wino-s of the Dytiscidse vary a good deal in form, and also in their 

 pigmentation : tlie wing of Pachydrus is short and broad, and very rounded at 

 the apex, while in Cybister the apex is acuminate. The pigmentation differs 

 much even in the same genus ; in Eretes the membrane of the wing is quite 

 colourless, while in Hydaticus flavolineatus (No. 1024) it is suffused with a smoky 



