On Aquatic Carnivorous Coleoptera or DijtiscidcB. 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 

 a rare 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 is 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 

 Dytiscidte in opposition to tlie Carabidre as regards either the structure or con- 

 nexions of the hind body. In the Dytiscidre owing to the great development of the 

 hind coxte, the abdominal segments in the middle of the body are to all ajDpearance 

 widely separated from the metasternum, whereas in the Carabidse 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 Carabidse 

 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 lamince of the coxas in the 

 Dytiscidse. 



Recollecting the aquatic life of the Dytiscidte, 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 Carabidee. 



Elytra or Wing-cases. — The elytra in the Dytiscidoe 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 abbi'eviate, 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 



