On Aquatic Carmvorous Coleoptera or Dytiscide. 241 
between the second and third segments are more or less fine and indistinct, these 
segments being in fact soldered together and immovable, but the remaining ventral 
sutures are distinct, the plates being mobile : the fourth and fifth ventral plates are 
shorter than the others, but usually not very greatly so; the sixth or last plate is 
more elongate, more or less obtusely pointed, and with its hind edge more or less 
finely margined—very finely in Hydroporides, coarsely in Cybister ; in Ilybius 
this plate shows a considerable difference of form according to the sex, but this is 
arare exception. The outer or upper portion of the ventral segments, placed 
under the wing-cases, and marked off as I have already described by a raised 
carina, varies a good deal in its width, especially on the last three segments; it is 
broad in Hyphoporus and in the convex forms, such as Hydrovatus, and Hydro- 
canthus, and it 1s very narrow in Dytiscus ; in the Colymbetini this upper piece of 
the fourth and fifth segments is much narrower than it is in the Agabini. 
No important or constant character can be pointed out as distinctive of the 
Dytiscidze in opposition to the Carabidee as regards either the structure or con- 
nexions of the hind body. In the Dytiscidz owing to the great development of the 
hind coxze, the abdominal segments in the middle of the body are to all appearance 
widely separated from the metasternum, whereas in the Carabide it is quite 
frequently the case that the second ventral plate of the hind body, touches in the 
middle of the body the point of the metasternum ; but there are numerous Carabidee 
in which this is not the case (see especially Pseudomorphides and Trachypachys) ; 
the fact is not of any importance otherwise than as indicating the constant and 
complete apposition and union of the two internal lamine of the coxe in the 
Dytiscidee. 
Recollecting the aquatic life of the Dytiscidz, and their very peculiar method of 
obtaining a supply of air, we should expect to find some notable character in the 
breathing orifices or stigmata. Such is not the case however, and it is only in the 
few species constituting the group Dytiscini, that we meet with any peculiar 
development of the stigmata ; while in the larger portion of the family the stigmata 
show no character by which they can be distinguished from those of the Carabide. 
Enyrra or Wine-cases.—The elytra in the Dytiscidee play a very important part, 
as it is by their means that water is excluded from all the dorsal portion of the 
body behind the pronotum. They are always hard and never abbreviate, and they 
fit so accurately to one another along their suture, and at the sides of the body, 
that the insect is enabled to carry about under them in the water a supply of air 
for respiratory purposes. They are never soldered together at the suture as is so 
frequently the case in the Carabidse, and they are kept in the closed position by an 
extremely beautiful combination of adaptations of various parts ; they lock together 
at the suture by a kind of joint similar to that called a rabit by joiners: a raised 
line is developed on their inner face which serves as a stop for the upturned edges 
of the ventral plates to repose against, while the inflexed epipleura rests its edge 
