Diaphanosoma orachyurum. 



347 



.Zacharias ('96, p. 53), gives August and September as the 

 maximum, and also says nothing about males. The species 

 was only occasionally present in the winter. Apstein ('96, p. 

 170), gives August as the date of the maximum for all species 

 of Daphnia. He does not mention a sexual period, though he 

 gives no especial attention to the subject. Had there been 

 such a period as is shown by D. retrocurva it could not have 

 been missed. 



Diaphanosoma orachyurum Sars. 



Figure 19. — Table I, Appendix. 



Table XXL— Average number per square meter of surface, stated in 



thousands and tenths. 



July 1-15 



July 16-31 



August 1-15 



August 16-31.... 

 September 1-15 . , 

 September 16-30 

 October 1-15 .... 

 October 16-31 . . 



1894. 



0.8 

 6.3 



18.0 

 Noobs. 



19.6 

 5.2 

 3.0 



1895. 



s 



6.9 

 31.5 

 32.2 

 27.1 

 17.2 



3.4 



0.0 



1896. 



s 



s 



8.9 



147.4 



108.3 



32.9 



0.4 



00 



This species is the least numerous of the limnetic Crustacea 

 which appear in large numbers, and has the shortest season. 

 Scattering individuals may be seen as early as the middle of 

 May, but they do not become a regular constituent of the plank- 

 ton catch before the middle of July or the earlier part of Au- 

 gust. They disappear in October, and are greatly reduced in 

 number by the cold storms which usually come in late Septem- 

 ber. Males appear about the middle of September, and the win- 

 ter eggs are then produced. The species was far more abundant 

 in 1896 than in either of the two preceding years, which agree 

 with each other fairly well. For this difference I can assign no 

 reason. The numbers were constantly greater in 1896, so that 

 the increased number was not the result of a few large 



