384 Annals Entomological Society of America [Vol. VI, 



The thickening of the upper lamella is greatest upon the front 

 gills. This thickening and the reduction in number of the 

 gills is most marked in the two mud dwellers, Caenis and 

 Tricorythus. In these, gills are present upon segments two 

 to six only. In all species of both genera the upper lamella 

 of the first gill is modified into a cover which conceals all of 

 those behind it. They are further protected by a shelf -like 

 extension similar to that just described in Ephemerella. In 

 the slight concavity of this shelf lie the delicate gills of seg- 

 ments 4, 5, 6, protected from the harsh gravel through which 

 the nymphs crawl. When breathing actively the stiff covers 

 are raised enough to allow water to circulate upon the gills 

 beneath, which vibrate freely and create a current. 



The gill covers of Tricorythus are scoop shaped, with the 

 concavity beneath, so that even when the cover is closed down 

 the gills are not under pressure, but are enclosed in a pro- 

 tecting box. The edges of the cover and those of the gills 

 beneath are margined with short hairs. This brush of inter- 

 mingled hairs makes an effective sieve which strains out 

 particles of mud from the incoming current of water. En- 

 trance of water at the base of the gill is prevented by a small 

 triangular extension of the second abdominal segment which 

 fits closely to the inner side of the elytroid cover. 



Gills of the EphemerincB. 



The most homogeneous series of gills is found in the Eph- 

 emerinse which in Fall Creek were represented by Potamanthus, 

 Polymitarcys, Ephemera and Hexagenia. They are single 

 and rudimentary upon segment one, (PI. XLIX, Fig. 27) and 

 double upon segments 2-7 (PI. XLIX, Fig. 26). They are long 

 and generally narrow, but this varies slightly with the genus. 

 The upper and lower lamella are both fringed with filaments 

 into which run branches of the trachea. The attachments 

 are not protected and the base of the gills appears to be an 

 unbroken continuation of the body wall, which is very flexible 

 and tough. The gihs of Potamanthus (PL XLIV, Fig. 7) 

 are the most generalized. They are nearly linear, lie limply 

 extended from the sides of the body and except for the scanty 

 fringe of filaments are almost identical with the gills of Lep- 

 tophlebia. 



