CH. X 



POND-SKATERS 



347 



smell when crushed or 

 seized. Many Heteropoda 

 are dimorphic or even poly- 

 morphic, that is, within 

 one species are found two 

 or even three kinds, irre- 

 spective of sex. The adult 

 fern ale, especially, may have 

 , short win^s, or 



' '*^* 



no wings at all. Dimor- 

 phic species are common 

 among the aquatic Heter- 

 optera. It is noticeable 

 that none occur among the 

 tree-haunting species of the 

 sub-order. Wingless forms 

 may have their advantage 

 in species which live and 

 breed in water, or lead a 

 comparatively stationary 

 life on land. They are not 

 for one thing tempted to 

 incur the greater risks of 

 the air. But at least an 

 occasional winged form is 

 necessary for migration to 

 fresh sites. 



Just as in the carnivorous 

 Coleoptera we find some 

 (Gyrinus) which skim over 

 the surface of the water, 

 and others (Dytiscus, etc.) 

 which seek their prey in 



FIG. 102. Hydrometra stagnorum, 

 and rostrum of do. 



