370 Aquatic Societies 



The head is so flat and flaring that the eyes appear 

 dorsal in position instead of lateral as in pond-dwelling 

 Mayfly nymphs. 



A more remarkable form is the torrent-inhabiting 

 nymph of Rithrogena whose gills are involved in 

 the flattening process. They also are flattened and 

 extended laterally and rest against the stone. But, 



' " Fig. 2ig. The nymph of an unknown mayfly from mountain 

 torrents, showing oval ventral attachment-disc formed be- 

 neath the body. 



most remarkable of all, the anterior pair is deflected 

 forward and the posterior pair, backward, to meet on 

 the median line beneath the body, and both are 

 enlarged and margined; By the close overlapping 

 of all the gills of the entire series there is formed a large 

 oval attachment-disc of singularly limpet-like form. 



A similar flat attachment-disc is formed on the 

 ventral side of the mayfly nymph shown in figure 219, 



