1«68. 



53 



above it, in front of it, and laterally ; while the under ones make 

 it possible for it to see at the same time directly downwards as 

 it swims on the surface of the water. 



3rd. The structure of the antenna) is remarkable, and differs greatly 

 from that of the Dytiscidcd, though it is very like what we find in 

 Parnus. Each is inserted in a cavity at the side of the head ; 

 the first joint is very small, the second is large and dilated, and 

 the third, also large, is inserted at the side of the second, while the 

 remaining joints are so compressed and soldered together that it is 

 not decided whether the antenna? consist altogether of ten or 

 eleven joints. 



4th. While in the Dytiscidce the mesosternum is small and feeble, and 

 the metasternum is lai'gely developed, in the Gyrinidcd the meso- 

 sternum is large, while the metasternum is correspondingly reduced 

 and small. 



5th. The structure of the legs is most remarkable in the Gyrinidce, 

 and afi*ords in several respects one of the most interesting examples 

 of the modification of organs to serve special functions that could 

 well be instanced ; w^iile the four posterior legs are formed into 

 powerful swimming organs, the anterior are quite different — they 

 are elongate, and are so placed that they can be packed under the 

 body so as to off'er not the least impediment to the most rapid 

 motion, while by one or the other being thrust out the course of 

 the insect is instantly changed, or when both are thrust out 

 retarded, and thus the Gyrini are enabled to perform those rapid 

 and eccentric motions which have attracted the attention of all 

 who have eyes and can use them. This rudder-like function of 

 the front legs is also perfected by the peculiar position in which 

 they are placed, a position so strange that what should be the 

 under surfaces of the anterior tarsi look towards one another, 

 instead of downwards : dependent on this is also a peculiar modifi- 

 cation of the tarsi, which are compressed laterally, so that, not- 

 withstanding the peculiar position of the legs, the broad aspects 

 of the tarsi are presented upwards and downwards as in other 

 beetles ; still stranger is the fact that what is in reality the side of 

 the tarsus is thickly furnished in the male with peculiar hairs such 

 as are placed in other beetles on the real under surface of the tarsi. 

 Had these hairs been placed in a position anatomically the same as 

 they are in other beetles, they could have been of no use for the 

 purpose for which they are intended ; thus they are in a position 



