On Aquatic Carnivorous Coleoptera or Dytiscide. 253 
placed punctures armed with short acuminate spines (Hyderodes) ; in Dytiscus the 
surface is smooth with an irregular double row of large punctures arranged along 
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 1s 
difficult to imagine what this may be. The inner edge of the upper face bears a 
series of spines, 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 are 
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 these spurs become remarkable, so that in Cybister they are about so long as the 
tibia itself ; they become bifid at the extremityin 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 spur 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 Carabidee, 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, 1t 
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 
2L 2 
