BIOLOGY OF MAYFLIES OF THE GENUS BAETIS. 



breeding cages of wire cloth, six inches long and three inches in diameter. These 

 cages were covered top and bottom with canvas. A small stone with algae was 

 placed in the bottom, and the cage, half submerged, was anchored in the stream. 

 As the nymphs neared maturity, they were separated into groups of four to a 

 cage. 



For a more detailed study, an egg-mass was carefully loosened from a 

 stone and transferred to a Syracuse watch glass in the laboratory. The water 

 was changed twice daily. Nymphs appeared in eleven days as contrasted with 

 twenty-eight days in the cooler water of the creek. As soon as hatched, the 

 nymphs were transferred to Syracuse watch glasses, ten nymphs to each glass. 

 The water was changed twice daily, and fresh bits of green algae added every 

 other day. When the fifth instar was reached, the nymphs were separated into 

 groups of two per dish. The cast skins of each specimen were kept separate. 

 Table A is a summary of the number and length of the stadia. 



A nymph of the first instar (Fig. 8) is .74 mm. in length including antennae 

 and tails. It has no gills and no middle tail. The nymphs are very active, and 

 start eating almost immediately. The food consists of diatoms at first ; then green 

 algae and decaying higher plant tissue are added in gradually increasing quantities. 



A nymph of the fifth instar is 1.5 mm. long including antennae and tails. 

 Gills show as tiny outpockets of the body wall, and the middle tail is evident for 

 the first time. The color pattern is faintly visible. As the nymph grows older, 

 this color pattern is more clearly defined. There is however, much individual 

 variation in the differentiation of the pattern. At the seventh instar the gills 

 reach their full relative size, and are freely moved. 



Nymphs of the same instar are not uniform in length. This becomes 

 apparent at the fifth stadium. The critical stage is at molting. This takes place 

 generally during the warmer part of the day. 



Specimens kept in cages and nymphs free in the stream began emerging 

 October 22, 1918. The main pulse started October 25, and continued for four 

 days ; stragglers appeared even ten days later. 



A nymph ready to transform (Fig. 20) swims to the surface of the water. 

 Strong pulsations in the dorsum of the prothorax cause a rent in the nymphal 

 skin. The subimago emerges and flutters through the air to some support. There 

 it sits with wings uplifted and forelegs thrust stiffly forward, waiting for the 

 subimaginal molt. This takes place from eighteen to twenty-four hours later. 

 Then the imago or true adult emerges and the mating flight follows. 



A mass of eggs laid October 28, 1918, was brought into the laboratory for 

 the purpose of rearing the nymphs. These specimens emerged during the second 

 week in May. There were again twenty-seven stadia (Table A). 



Nymphs reared in the creek from eggs laid October 28, 1918, did not 

 emerge until August, 1918. A new cycle started in August was completed the 

 following May. 



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