244 On Aquatic Carnivorous Coleoptera or Dytiscide. 
traction is exercised on its base, of assistance in folding the wing. The median 
nervure is a broad one, and runs from the base of 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 (carpa of Lacordaire, Int. 4 l’Ent. 1., 
p. 365). The submedian vein is a double one ; its two portions starting from the 
base of the wing, 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 marks off 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 vein ; 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 off by the median, is connected to it by a very short transverse nervule. In 
Dytiscus marginalis this short nervule is present, but rudimentary. 
The wings of the Dytiscide vary a good deal in form, and also in their 
pigmentation: the 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 
