4r6 



GENERAL BIOLOGY 



Their chief problem has been locomotion; how to get 

 through the water speedily, in order to capture their prey 

 and to escape from their enemies. Becoming adapted, 



Fig. 245. Diagram of the ventral 

 aspect of a divingheetle (Coptotomus 

 interrogatus) a, antenna; b, mouth; 

 c, c, coxal cavities for the fore and 

 middle legs; d, labial palpi; e, eye; 

 /, maxillary palpi; g, lateral margin 

 of the prothorax; h, epipleura of 

 the wing cover (elytron); ^', pro=ter- 

 nal process; /, met asternal fork; k, 

 hind coxa with /, the inner, and o, 

 the outer lammae ; p, the coxal 

 process and q, the coxal notch: r, 

 trochanter of the hind leg; s. femur; 

 t, tibia; u, tarsus of five joints; v, 

 spurs of the middle tibia. /, 2, j, 

 4, 5, 6, ventral abdominal segments 

 5<i, st^, sf^, sterna pro-,meso-. and 

 meta-thorax, respectively. 



therefore, to progression 

 through the water — a medium 

 fo sufficient density to offer 

 considerable resistance — they 

 have acquired a remarkable 

 uniformity of appearance. It 

 would be hard to find another 

 large family of organisms all 

 the members of which are of 

 so nearly the same shape. 

 Whether large or small, they 

 are all of one form — that, the 

 form of a submarine boat — 

 compact and evenly contoured 

 and pointed at both ends. 



It is generally agreed that 



the ancestors of the Dytiscidas 



were ground beetles, more or less like the existing members 



of the family Carabidae,of which Calosoma (fig. 246) is a 



