DYTICUS MARGINALISA WATER BEETLE 31 



firmly and immovably welded to the ventro-lateral portions of 

 the metathorax. 



Just as the body itself is well shaped and fashioned for easy 

 passage through the water, so, too, are the hind-legs modified 

 to form efficient oars. Their femora are flattened so that they 

 work easily to and fro upon the fused ventral plates of the anterior 

 abdominal segments and upon their own coxae ; their tibiae and 

 the five joints of the tarsus are likewise flattened to present a 

 broad face to the water, and are further furnished with fringes 

 of long hairs along each edge. These hairs are so arranged 

 that they spread out and meet the water, thus broadening the 

 face of the oar during the backward stroke, but yield and are 

 pressed together during the forward " recovery." The forward 

 movement is also facilitated by the articulation of the tarsus 

 with the tibiae being such that the former can be slightly rotated 

 upon its own axis so as to present its narrow edge to the water 

 in the same way as a " feathered " oar presents the edge of its 

 blade to the air. The claws upon the last tarsal joint of the 

 hind-legs are smaller and weaker than those of the other legs. 

 The middle-legs are also employed a little in swimming, but 

 they are shorter and stouter than the hind, and far less modified 

 in form ; for the tibia is nearly cylindrical ; and while the three 

 first tarsal joints are somewhat flattened and fringed with short 

 hairs, the fourth and fifth are cylindrical and devoid of hairs, 

 and the claws are well developed, as, too, are those of the front 

 legs. These claws are of service to the animal in enabling it to 

 lay hold of weeds and other objects beneath the surface of the 

 water, and so to keep submerged ; for since the body is relatively 

 lighter than water, immediately the hold is released the animal 

 floats obliquely, tail uppermost, to the surface. 



The tarsus of the front leg of the male has its first three joints 

 peculiarly modified. All three joints are very broad and flat, 

 and their lower surface is thickly studded with minute cups 

 raised on short stalks ; but on the first joint there are two 

 much larger, though unequal, cups of similar structure. Similar 

 minute cups are also present in large numbers upon the under 

 surface of the first three tarsal joints of the middle-legs. These 

 remarkable organs are used by the male as adhesive suckers 



