210 



COLEOPTERA 



CHAP. 



it is very widely distributed. We have 13 species in Britain, 

 all the genera being represented. 



Fam. 9. Dytiscidae (Water-beetles). Antennae bare; hind 

 legs formed for sunmming, not capable of ordinary walking : nieta- 

 sternum without a- transverse line across it ; behind closely united 

 with the extremely large coxae. Outer lobe of maxilla forming a 

 two-jointed palpus. ' The Dytiscidae, or true water-beetles, are of 

 interest because unlike the aquatic Neuroptera they exist in 



water in both the 

 larval and imaginal 

 instars ; nevertheless 

 there is reason for sup- 

 posing that they are 

 modified terrestrial 

 Insects : these reasons 

 are (1) that in their 

 general organisation 

 they are similar to 

 the Carabidae, and 

 they drown more 

 quickly than the 

 majority of land 

 beetles do ; (2) though 

 the larvae are very 

 different from the 

 larvae of terrestrial 

 beetles, yet the ima- 

 ginal instars are much 

 less profoundly 

 changed, and are capable of existing perfectly w r ell on land, and 

 of taking prolonged nights through the air ; (3) the pupa is, so 

 far as known, always terrestrial. The larvae and images are 

 perfectly at home in the water, except that they must come to 

 the surface to get air. Some of them are capable, however, when 

 quiescent, of living for hours together beneath the water, but 

 there appears to be great diversity in this respect. 1 The hind 

 pair of legs is the chief means of locomotion. These swiinming- 

 legs (Fig. 97) are deserving of admiration on account of their 

 mechanical perfection ; this, however, is exhibited in various 



1 Src J. Linn. Soc. Zool. xiii. 1876, p. 161. 



A 



FIG. 96. Cybister roesdi ( = laterima,rginalis De G.) 

 Europe. A, Larva (after Sclriudte) ; B, <J imago. 



