On Aquatic Carnivorous Coleojitera or Dytiscidce. 253 



placed punctures armed with short acuminate spines (Hyderodes) ; in Dytiscus the 

 surface is smooth with an irregular double row of large punctures arranged alono- 

 its middle and each bearing a rather elongate spine, with a peculiar flattened and 

 bifid extremity ; in Hydaticus there is a single series of such spines along the middle, 

 and in Thermonectes this series becomes oblique in direction and the punctures 

 approximated ; this is also the case in Acilius where moreover the peculiar spines 

 become very elongate, and in Eretes this peculiar oblique series is much more 

 developed and placed near the apex of the tibia ; in Cybisterthese peculiar long spines 

 with bifid extremities form a dense tuft placed at the hinder outer angle of the upper 

 face of the tibia, the rest of the surface towards the base bearing also scattered short 

 spines ; the series of developments exhibited by these spines, and their culmination 

 in the highest forms indicate that they have some functional importance, though it is 

 difficult to imagine what this may be. The inner edge of the upper face beai's a 

 series of sj)ines, and the outer edge is fringed with dense elongate swimming hairs. 

 The inner face of the hind tibia is without sculpture, and its inferior margin bears 

 elongate swimming hairs, which however are sometimes very scanty and are never 

 so largely developed as the swimming hairs on the outer side of the tibia ; as before 

 remarked, in Cybister this inner aspect of the tibia is only indefinitely distinguished 

 from the upper face, while in the Noterides it is nearly absent, owing to the com- 

 pression or flattening of the limb. The apex of the tibia is fringed densely with 

 spines of variable lengths, and degrees of regularity, and bears two spurs which ai-e 

 always largely developed, and are placed on the inner side one at the upper, one at 

 the lower angle : in proportion as the swimming legs become largely developed so 

 do thesq spurs become remarkable, so that in Cybister they are about so long as the 

 tibia itself : they become bifid at the extremity in Laccophilus, and the Thermonectini; 

 in the Cybistrini the lower of these spurs becomes broad and flattened, while 

 the upper one is canaliculate along its lower aspect, and in Megadytes is frequently 

 bifid or even trifid at the apex; it is probable that the dilated lower sj)ur in 

 Cybistrini gives support and strength to the tarsus, and it is rendered additionally 

 useful for this purpose, by being itself supported by a prolongation backwards of 

 the lower face of the tibia immediately below this spur. 



The tarsus, like the other parts of the swimming leg, is the seat of great variation 

 in its development ; in the higher species it has undergone a great change by 

 which it is well adapted to serve its purpose as a means of locomotion in water 

 but in the lower forms, Hydrovatus, Celina and Methles, and more especially in 

 the last two, little or nothing can be pointed out to distinguish it from the tarsus 

 of the Carabidse. It is invariably five jointed and does not, as do the other feet, 

 ever show any diminution of the number of its joints. In the higher forms, it 

 has undergone a process of lateral compression so that it is flattened and presents 

 only two faces, the joints are accurately coadapted in form and rigidly fixed 

 together, even the claws become straight and rigid ; and at the same time it is 



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