340 THE RIVER-SIDE NATURALIST. 



The genus Baetis, in which will be found many of the 

 smaller Ephemeridce, which are at times so prevalent on 

 most of our chalk-streams, is defined as follows : 



Stephens : Head large and transverse ; eyes large, 

 united on the crown in males, remote in females ; thorax 

 ovate, stout ; wings, four, the anterior long, narrow, obtuse, 

 considerably reticulated ; posterior small, somewhat ovate ; 

 abdomen moderately long and tapering, two filaments at 

 base ; anterior legs long. 



He divides his eighteen species into A, wings dis- 

 tinctly and rather thickly reticulated ; B, wings faintly 

 reticulated. 



Pictet : Eyes simple in both sexes, but much larger in 

 the male, in which they are separated by a very short 

 space ; wings, four, with numerous transverse nervures ; 



I 2 



OUTLINE EYES OF BAETIS, MAGNIFIED (Pictet). 

 i, Male (eyes nearly united). 2, Female (eyes remote). 



the anterior long and straight, having complete nervation, 

 and the costal edge angular ; forceps large and well arched ; 

 two setae, without any rudiment of a third. 



Eaton : Oculi of male divided into two unequal parts ; 

 the upper segment, cylindrical or somewhat turbinate, is 

 faceted solely on the terminal surface ; the lower and 

 much smaller segment, oval in form, is annexed to the 

 under orbit of the former, and is faceted all over with 

 facets of less diameter of the turbinate part ; hinder oculi 

 large, foremost much smaller ; wings anterior or mesotho- 

 racic, large, ovate-oblong, gradually rounded off from the 

 terminal to the inner margin ; hind-wings small or absent ; 

 neuration incomplete ; sub-costa somewhat curved ; median 

 caudal seta aborted. 



The genus Cheon was separated from the Ephemera of 

 Linnaeus by Leach as having only two wings instead of four. 



