FRESH WATER 



195 



the pond-skaters (e.g. Gerris), which dart about on the surface film, 

 or drift, or even leap. On their body and limbs there is a " velvety 

 pile/' which keeps them from being wetted. They search busily 

 for insects which get trapped on the water. Some, such as Velia 

 cur r ens y which " loves the eddies and currents of backwaters on 

 burns and streams, and is very abundant in Scotland/' 1 are adept 

 swimmers under water as well as skaters on the surface ; but 

 there are others, like the slender Hydrometra stagnorum, that 

 moves rather slowly with long slender stilt-like legs on the surface, 

 which cannot be sub- 

 merged without great 

 risk of drowning. No 

 Nature Study exer- 

 cise is more delightful 

 than watching these 

 pond-skaters or water- 

 measurers (Hydrome- 

 tridae), and trying 

 to understand the 

 wonder of their move- 

 ments; and after some 

 observation of the 

 freshwater forms it 



might be interesting FIG. 113. The development of the tadpole, i, Portion of 

 tO tell about Some frog spawn; 2, the same after ten days; 3, a newly hatched 



tadpole. The remaining figures show the stages reached 



that Walk On the Sea at successive dates 4, at one week ; 5, three weeks ; 

 the Species Of Halo- 6, seven weeks ; 7, eleven weeks ; 8, thirteen weeks ; 



bates in particular, and 9 ' fourteen weeks after hatchin *- 



that occur in open ocean hundreds of miles away from land. 



In Professor Miall's indispensable book an account is given of 

 many of the common aquatic insects, not merely of water-beetles 

 (Coleoptera) and water-bugs (Rhynchota), but of dragon-flies 

 (Odonata), may-flies (Ephemeridae), stone-flies (Perlidae), alder- 

 flies (Sialidae), caddis-flies (Trichoptera), two-winged flies (Diptera), 

 and so on. We cannot, however, do more than refer to this 

 interesting book, which lays special emphasis on the study of 

 life-histories. Nothing is easier than to collect in spring the 



1 See (Sharp) p. 222. 



