ADAPTATIONS 



341 



by the impulse of reproduction. Some pelagic fislies, especially 

 those of the mackerel and flying fish families, swim long dis- 

 tances to a region favorable for the deposition of spawn. Others 

 pursue for equal distances the schools of herring, menhaden, or 

 other fishes which serve as their prey. Some species are known 

 mainl}^ in tlie waters they make their breeding homes, as in 



^ 









■ > 



f ■♦; 



,-'^ :« N \ 



Fig. 202. — Galla on the rose caused by the gall fly, Rhoditea rosce. (After Kieffer.) 



Cuba, southern California, Hawaii, or Japan, the individuals 

 being scattered at other times through the wide seas. 



Many fresh- water fishes^ as trout and suckers, forsake the 

 large streams in the spring, ascending the small brooks where 

 their young can be reared in greater safety. Still others, 

 known as anadromous fishes, feed and mature in the sea, but 

 ascend the rivers as the im]nilse of reproduction grows strong. 

 Among such fishes are the salmon, shad, alewife, sturgeon, and 

 striped bass in American waters. The most remarkable case 

 23 



