764 The Life- History of a Water- Beetle [May 9, 



the latter, and the max. palpi, which are short in the former and 

 lonu; in the latter, in which they are used under water as feelers, 

 just as are the antennte of D//tiscus. 



Passing over other less remarkable differences in the heads of the 

 two types and coming to the body, one at once notices the different 

 disposition of the legs : in Dyiiscua the first two pairs are close 

 together, in Hydrocharis the three pairs are about equidistant. In 

 Dytiscus the basal segment of each hind leg — the coxa marked ;^)* — is 

 large, and the two coxse are fused into a single piece which is firmly 

 fixed into the body. In Hydrocharis the coxa is long and narrow ; 

 the two coxge are separate, and each is hinged on to the body. The 

 firm fixing in Dytiscus gives it a much more powerful leg-drive than 

 the hinging gives to Hydrocharis, and hence Dytiscus is a more 

 efficient swimmer. 



These differences between the two types are therefore connected 

 with differences in function. The antennae of Dytiscus are feelers, 

 while those of Hydrocharis are connected with breathing, and the 

 disposition of the legs and their methods of attachment to the body 

 are connected with differences in mode of progression, Dytiscus being 

 a " swimmer," and Hydrocharis chiefly a " creeper " on the submerged 

 vegetation. 



In these two groups of water-beetles, the Hydradephaya repre- 

 sented by Dytiscus, and the Paljiicornia represented by Hydrocharis, 

 we have two types of adaptation to an aquatic existence. Each type 

 has originated independently of the other, that is, they are not 

 descended from a common aquatic ancestor. Each represents a part 

 of a large terrestrial family, and each has probably developed an 

 aquatic habit as a result of competition, stronger land forms having 

 driven the weaker off the land and into the water. 



Just as each group has originated under the stimulus of competi- 

 tion, so, within each group, competition has moulded the different 

 forms, and the peculiar details in the life-history of any one form 

 are just those whicli enable it to retain its place in the community 

 to which it belongs, and to hold its own in the great struggle for 

 existence. 



[F. B. B.] 



