On Aquatic Carnivorous Coleoptera or Dytiscide. 223 
Hydrovatus, and the scutum is then very inconspicuous; in Laccophilus especially 
it appears as a short band in front of the scutellum without any division into two 
lobes. I have already remarked that the hind margin of the scutum is impinged on 
in the middle by what appears to be an angular prolongation from the front of the 
scutellum ; this prolongation is however probably a distinct piece of the scutum, as 
it certainly is in the Carabidze, in some members of which family (Harpalus 
caliginosus e.g.) it appears as a large piece, nearly as large as the scutellum, reaching 
quite to the front of the mesonotum, and thus widely separating the two lobes of 
the scutum. The scutellum always comes to the surface of the body at the base of 
the suture of the elytra, but is very frequently invisible (Laccophilus, Noterides, 
Hydroporides and Methles) owing to its being covered by the middle of the base of the 
prothorax; but in the Macro-Dytiscidz the scutellum is always visible. In the groups 
where it is concealed it varies in form and size, thus in Noterus it forms a very short 
broad triangle the apex of which is directed to the hind part of the body ; in 
Laccophilus also it forms a short, broad triangle with rounded sides ; in Hyphydrus 
it forms an irregular four-sided figure, broadest behind, the hind margin straight and 
with projecting lateral angles; in Hydrovatus it reaches its extreme diminution, 
and can only be detected as a minute transverse raised band : in the other Dytiscidee 
it has the form of a more or less elongate triangle, with the base in front attached to 
the scutum, but the sides forming a free edge, under which lock the anterior parts of 
the sutural edges of the wing cases. The post-scutellum is a transverse band on a 
lower level than the scutellum ; on it rests the base of the wing case when closed, 
and to its extremity is attached a small membrane ; this membrane 1s sometimes 
ciliate at its edges (Dytiscus, Cybister) and is always present in the Dytiscidee, and 
also in some Carabide, though it has been supposed by Lacordaire (Introduction 
? Entomologie, I., p. 847) to be peculiar to Dytiscus. 
The most important piece of the mesosternum—the mesosternum proper or 
medisternum—consists of a middle portion, giving off above a lateral branch on 
each side, so that it may be roughly described as being somewhat T shaped; the 
middle portion is the lowest part of the mesosternum, and has more or less the form 
(when disarticulated) of a short six-sided column, as seen in Dytiscus, Cybister, 
and most of the larger Dytiscide, but in the case of such of the smaller Dytiscidee 
as have only a very slight extension of the mesothorax in the longitudinal direction, 
the columnar appearance is quite lost (Laccophilus, Hydrovatus, Hyphydrus, &c.), 
and it is also but little conspicuous when the depressed form of the insect causes 
the column to be very short (as in Copelatus). Like the prosternum proper, the 
medisternum appears to consist of two pieces placed one in front of the other, and 
distinguished by a very prominent line of junction. The front of the column is 
placed immediately behind the anterior coxee, and is on each side usually more or 
less hollowed for their accommodation, so that in fact the column bears two facettes 
in front which are sometimes separated by a channel (Cybister); in Dytiscus 
