1913] 



Biology of May -flies. 



393 



must have been recurved in order to insert the penes inside 

 the egg valve. Copulation did not last more than half a minute. 

 When in copula, each pair was borne diagonally downward to 

 the ground, but always separated immediately upon touching 

 it. 



So far as known flight is a necessity for copulation and 

 egg-laying in May-flies. The alimentary canal and the legs 

 play a part in flight which is peculiar to this group. It is a 

 well known fact that adult May-flies take no food and that 

 the alimentary canal is inflated with air or gas. Sometime 

 before emergence the nymphs cease to eat and just before it, 

 they push their heads above the surface and appear to be 

 rapidly gulping in air. If dissected at this stage the alimentary 

 canal is found much inflated. It remains thus inflated through- 

 out life. The structure of the alimentary canal of adults 

 was studied by Fritze, '06 and by Sternfeld, '07. Sternfeld 

 found a complicated muscular dilator apparatus in the esoph- 

 agus. This he concluded to be a pump by which the mid- 

 gut was filled with air and by which its supply could be vol- 

 untarily controlled. He did not discover whether any change 

 occurred in the air taken in. No suggestion was made as to 

 when this pump was used, but it is probable that it functions 

 when the canal is first inflated by the nymph and afterward 

 in controlling the specific gravity during flight. This change 

 of the alimentary canal from its normal function to that of 

 a balloon is very important to flight. The lessening of the 

 specific gravity made possible by this modification makes the 

 work of the wings much easier. Since it is more important 

 that adults mate, than that they live a long time, this function 

 of the alimentary canal exceeds the former one in value. 



Fig. 1. Legs of male imago of Hexagenia bilineata. A, first leg; B, second 

 leg; C, third leg; 1, opposite side of legs showing tibial spur. 



