On Aquatic Carnivorous Coteoptcra or Dijliscidce. 243 



A-gabus however it usually extends to the apex. In Hyphydrus the elytral ridge 



shows a very peculiar structure ; a little in front of the extremity there is an 



elongate prominent tongue, which projects towards the edge of the wing-case and 



even a little beyond it, leaving a small space between it and the inner face of the 



wing-case, and into tlie deep groove so formed fits the upper portion of the ventral 



plate ; as the result of this the two wing-cases at the extremity are so firmly locked 



together, that they can only be separated by first lifting them up from the body 



till the extremity of the ligula is free : a similar but smaller ligula is seen in 



Ccelambus, and in some other genera of Hydroporini it is found in a still less 



perfected state. In Queda and Pachydrus the elytral ridge attains its greatest 



development ; it is elongate, and for nearly its whole length forms with the face of 



the elytron a kind of groove into which are received the edges of the vertical 



segments ; near its extremity it becomes more prominent and thickened, and waved 



in such a manner as almost to form a broad double ligula, the elytra are thus locked 



together in an extremely firm manner. In Pelobius there exists two structui'es of 



an unique character on the inner face of the wing-case ; one is a raised longitudinal 



band at the extremity near the suture, on each elytron ; these bands are marked 



with transverse strife, and are a part of an apparatus for producing sound. The 



second is a large protuberance near the outer margin, just in front of the middle ; 



this protuberance forms a cavity into which is received a horny process on the upper 



edge of the basal ventral segment, and like the ligula in Hyphydrus serves the 



purpose of very firmly locking together the two wing-cases. 



WixGS. — Wings are always present in the Dytiscidse, and are well developed 

 and large, except in a very few species (Colymbetes bifarius No. 75 7 e. g.) where 

 they are rudimentary. They are folded in a complex manner, and placed in close 

 adaptation to the inner surface of the wing-cases, not touching the body, and thus 

 when the elytra are more than usually diaphanous (as in many Laccophili) the 

 veins of the wings may be slightly visible through the wing-cases, giving the latter 

 the appearance of being marked or coloured in an unusual manner. There are 

 two different folds to adapt them to the size of the wing-cases ; one of these is 

 a simple lapping over of the basal portion, but the other is a very complicated 

 longitudinal and transverse fold and is situated on the front part of the wing just 

 beyond the middle, and is exactly similar to the corresponding fold of the wing 

 in the Carabidi« and Cicindelidoe. The costal and subcostal nervures are close 

 together, and form the front border of the wing, the former of them is continued 

 to the apex of the wing, and it is by traction on this costal nervure that the wing 

 is unfolded, as may be readily ascertained by cutting off the wing of a freshly 

 killed specimen, and then pulling the base of this nervure with a pair of forceps ; 

 the subcostal nervure on the other hand is continued only as far as the transverse 

 fold (or carpa) of the wing, when it is turned backwards, so as to leave an irregular 

 cell or stigma immediately behind the costa ; this subcostal nervure is, when 



TRANS. ROY. DUB. SOC, N.S., VOL. II. ^ ^ 



