206 On Aquatic Carnivorous Coleoptera or Dytiscide. 
the antenna is situated on it, and in front of the cavity the side of the clypeus is 
likewise a little infolded ; in the case of Eretes (and in a less striking manner in 
the other Hydaticides) where the eyes are large, and the anterior parts of the head 
are very much reduced in size, the antenne are inserted rather on the upper than 
the under surface, the antennal cavity being quite visible from the front and above, 
owing to the diminution in size and the comparatively slight folding ofthe front 
angle of the epicranium ; on the other hand where the front of the clypeus is deflexed 
or inflexed, asin Hyphydrus, Queda, Hydrovatus, Pachydrus, the antennal cavity is 
situated quite on the undersurface of the head, the point of insertion being invisible 
when the insect is looked at from the front and above. The surface of the eye 
likewise is curved downwards, and continued on to the undersurface of the head so 
as to form a considerable portion of its area; between its inner termination and the 
elongate maxillary cleft of the buccal cavity there intervenes a narrow space, 
with a very polished surface, which serves as a groove for the lodgment of the base of 
each antenna when these organs are placed in repose on the under surface of the head. 
The gula (piéce basilaire of the French authors) is broad and distinct, each of the 
sutures separating its sides from the epicranium, terminates as in the Carabidee, in 
front by a deep impressed puncture; this puncture is placed at a considerable distance 
behind the mental suture, and from it there is directed outwards a fine but 
distinct suture, extending as far as the maxillary cleft; in this manner a large 
transverse piece issituated behind the mentum, but this piece in the middle is not 
separated from the gula by any suture, and it is doubtful whether it should be 
considered a lateral expansion of the front of the gula (which in such case is T 
shaped) or as a submentum (piéce prebasilaire of the French authors) : this same 
formation exists in the Carabide, but is there excessively obscure owing to its 
being of smaller size, and limited by more or less obsolete sutures.* 
Behind the eye there is a transverse suture or mark, which is clearly the 
remnant of the suture seen in the Carabide as separating the epicranium from the 
protocranium: in a few cases this suture is very obsolete, thus in Dytiscus 
duodecimpustulatus (No. 462) it can scarcely be traced : it occupies much the same 
position in relation to the hind margin of the head as it does in Carabidee, but is 
much nearer to the anterior part of the head than it is in that family, though it is 
more than probable that there exists much difference amongst the members of the 
Carabide in this respect ; indeed I find that in Carabus violacens, the suture in 
question is much nearer to the front that it is in Harpalus caliginosus. The 
relations of the parts near this suture may be summed up by saying that in the 
Dytiscidee as well as in the Carabidee the cheek is separated from the temple by a 
transverse suture, but that the cheek is very much shorter in the Dytiscide than 
* T believe myself that this transverse front bar of the gula is quite homologous with the submentum 
(piéce prebasilaire) of some other Coleoptera. 
