1913] Biology of May-flies. 397 



Genitalia of the Female. 



In the simple condition each oviduct lies> well to the side 

 of the abdomen and opens between the seventh and eighth 

 sternites (Hexagenia). Each opening is perfectly distinct 

 (see dotted line Fig. 3, B) and there is no sign of an open 

 passage or vestibule between the two. 



In Siphlurus alternatus (PL LI, Figs. 53, 55) the lower 

 ends of the oviducts approach each other and open into a 

 common vestibule (C. V.) just inside the egg valve. Opening 

 into this vestibule is a soft membraneous sac (S. R). In fresh 

 specimens this sac shows prominently between the bases of 

 the oviducts (Figs. 53, 55). In figure 55 the sac and oviducts 

 are shown viewed from the inside; the nerve chain has been 

 severed so as to fully expose the sac. In the specimens thus 

 far examined, no spermatozoa have been found within this sac. 

 It is extremely probable, however, that this is a true seminal 

 receptacle, and that this is a specialization which nearly ap- 

 proaches the unpaired opening found in other insects. 



VI. THE EGGS. 



Under the ordinary conditions of their life a large pro- 

 portion of May-fly nymphs regularly perish before reaching 

 maturity. A great excess of young must be produced in order 

 to meet this loss and the success of different groups in main- 

 taining their existence becomes more than usually dependent 

 upon the number of eggs produced and the structures which 

 aid in their dispersal and safety during incubation. 



In insects whose lives are so brief as these, the eggs are 

 well developed even at emergence, and may then be readily 

 counted, the difference in size between the developed eggs 

 and the egg rudiments being very marked. It is easy, there- 

 fore, in mature nymphs, sub-imagos or imagos to determine 

 the actual fecundity. 



. The first count of May-fly eggs was made by Reaumur* to 

 determine the fecundity of some specimens which he captured 

 in his garden. He found egg masses protruding from the 

 abdominal openings, counted the eggs and found about 400 

 in each mass. His results have been several times quoted 

 by later workers, but no references has been found to any 



*Reaumur 1742. T. VI, Mem. XII, p. 495. 



