i9i: 



Biology of May-flies. 



387 



of nymphs collected there contained little else. Nymphs 

 taken in the same place a month later contained no fragment 

 of Meridion. May-fly food is most abundant in April and 

 May, especially for the running water forms. Later the thick 

 mats of Meridion, Cladophora arid Spirogyra begin to decay, 

 there is a diminished supply of water and consequently greater 

 competition for food. 



In the summer of 1911, a few experiments in feeding were 

 made upon Callibaetis fiuctuans, one of the most abundant 

 local May-flies in pools and open waters. Six pails about 

 one foot deep and seven inches in diameter were made from 

 strong muslin. A ring of wire was placed at top and bottom 

 to extend them. A string was tied into the upper one for a 

 bale and the pails were fastened to a frame and suspended 

 in a pool where the water was kept constantly fresh. They 

 were numbered 1, 2, 3, 4, 5, 6, and a different food placed in 

 each respective pail. 



On June 28, twelve nymphs of equal size were measured 

 and freed in pail. On July 1, nymphs were taken from each 

 pail and the stomach contents examined. For ten days more 

 the same food was given at intervals of two days. Occasionally 

 the pails were rinsed free of stale food. This was especially 

 necessary for the corn meal w^hich soured quickly. 



The mouth-parts of May-fly Nymphs. 

 Bcetince 

 The most generalized mouth-parts occur among the Baetinae 

 in the species which bite or tear fragments from roots and 

 stems. They consist of labrum, labium, mandibles, maxillae, 

 hypopharynx, and the epipharynx which is borne upon the 

 labrum. Those of Callibaetis fiuctuans (PI. XLVII, Fig. 14), 



