II — Notes on the Biology of Mayflies of the 



Genus Baetis 



While gathering material for a study of May-fly embryology, some ob- 

 servations were made on the habits of a species of Baetis. This led to rearing 

 experiments that were carried through successive generations for two and one- 

 half years. A comparative study was made of two other species of the same 

 genus. 



The determination of these species as B. posticatus, B. pygmaea, and 

 B. propinquus was made by detailed comparison with specimens in the Cornell 

 University collection. 



BAETIS POSTICATUS. 



The mating flight of B. posticatus was observed late in the afternoon of 

 May 6, 1918. Males and females flying together rythmically rose and fell in 

 vertical lines. In numbers varying from hundreds to two or three, they danced 

 fitfully up and down; sometimes rising short distances above the surface of the 

 water ; sometimes soaring exultantly out of sight. Then they dropped with wings 

 out spread. A female left the throng followed by a male. He flew up from 

 below and grasped her prothorax with his forelegs. The posterior of his abdomen 

 was flexed up and forward. His tails were extended on either side of the head 

 of the female. Then he seized the seventh segment of her abdomen with his 

 forceps. The egg-valve was pushed open and the penes inserted into the sperm 

 receptacle in the seventh segment. The female continued to fly, carrying the male 

 along with her. In this manner they copulated for about a minute; then the male 

 rejoined the dance, and the female flew away to deposit her eggs. She alighted 

 on a partially submerged stone, wrapped her wings about her abdomen, and 

 crawled into the water. After walking about feeling the stone with the tip of 

 her abdomen, she suddenly stopped and braced her legs. As the abdomen swayed 

 from side to side, the eggs came from the oviducts. They were pressed to the 

 surface of the stones. The masses are irregularly ovoid (Fig. 21), and contain 

 from eighty to three-hundred eggs. These eggs are covered with a sticky sub- 

 stance which causes them to adhere to each other and to the stones. Some of the 

 females were washed away while ovipositing ; others managed to crawl weakly 

 out of the water. The males continued their flight until exhaused. Shortly 

 after sun-down the dead bodies of all floated away in the current. 



A stone coated with freshly laid egg-masses was covered loosely with a 

 piece of china silk, and submerged in the running water of the creek. Nymphs 

 hatched in twenty-eight days. They were left in the silk covering until too large 

 to escape through fine wire mesh. Then they were transferred to cylindrical 



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