372 WILSON. [Vol. VI. 



from June to September, the most favorable time, as far as I 

 have been able to determine, being in August and the earlier part 

 of September. The animals appear in abundance only on warm, 

 still nights, and even then are rarely found unless the water has 

 been quiet for some days. When the conditions are favorable, 

 they come forth soon after dark and swim rapidly about at the 

 surface, sometimes in almost incredible numbers. There is 

 something capricious about their occurrence, and I have never 

 been able to find a satisfactory explanation of their vagaries. 

 Sometimes both species occur together ; at other times, under 

 apparently similar conditions, only one species will be found, or 

 one may be very abundant and the other rare. The sexes are, 

 as a rule, readily distinguishable, since the males are bright red 

 and swim very rapidly, while the females are paler in color (often 

 nearly white) and swim slowly. The males are always more 

 abundant than the females, and I have sometimes captured 

 thousands of males without finding a single female. The 

 females are often actively pursued by the males, and have a 

 habit of swimming in a circle, closely pressed by numerous 

 males, while the eggs and spermatozoa are being discharged 

 into the water. 



The best method of procedure is to capture the adults singly 

 with a hand-net by the light of a lantern, which serves to 

 attract the animals, and by means of which they can be dis- 

 tinctly seen. The sexes should be carefully separated, since 

 otherwise the eggs are immediately fertilized and are so thickly 

 covered with spermatozoa that it is afterwards difficult to study 

 them. As soon as one or two ripe males are placed with the 

 females, the eggs and spermatozoa are discharged. The animals 

 should then be as quickly as possible removed (so as to allow 

 the eggs to sink), and the water thereupon repeatedly changed 

 until all superfluous spermatozoa are removed. If the sexes are 

 kept apart, the eggs are as a rule not discharged, and I have 

 often endeavored to keep the animals until morning, so as to 

 study the development by daylight. Unfortunately, however, 

 the animals usually die when thus treated, and even if the eggs 

 are deposited and fertilized (either naturally or artificially) on the 

 following day, they do not develop normally. Artificial fertili- 

 zation is easily effected at night, and my best material has been 

 obtained in this way, since a minimal quantity of spermatozoa is 



