688 



THE AMERICAN NATURALIST [Vol. XLIII 



In 1908 the first appearance occurred the evening of 

 March 23, when the maximum and minimum air tempera- 

 ture, were 47° and 42° F. The water temperatures of the 

 two ponds under observation on the following morning 

 were 42° and 48° F., respectively. On the above dates 

 of first appearance from 1903 to 1908, the U. S. Weather 

 Bureau Station at Ithaca obtained the following maxi- 

 mum and minimum air temperatures : 



1905 64 



From this it is 

 ting 50° F. or n 

 to emerge. 



To verify thi: 

 gration across t 

 mary follows in 



obvious that a temperature approxima- 

 ore almost invariably caused the species 



conclusion, a careful record of the mi- 

 le railway in 1908 was kept and a sum- 

 tabular form : 















49° 22° 

 37 ! 19 







! i 



M;,nh :~n 





15 



i 13 

 24 



15 

 22 

 55 







) i7 coid ' cioud - v 





r flurries." 1 



Aprj! 1: 











In this table as in the preceding 50° F. appears the 

 normal effective temperature of migration. The crest 



