THE YELLOW MAY DUN 37 



Dove towards the end of the season. Other 

 species belonging to this group are Ecdyurus 

 volitans, which is to be found on the Thames, 

 and Ecdyurus lateralis, which differs consider- 

 ably in appearance from the other species, 

 and superficially resembles Baetis rhodani, 

 one of the olive duns. 



Family . . EPHEMERID^ T 



~ TT L may dan. 



Genus . . Heptagema 

 Species . . sulphurea 



PLATE VI 



Heptagenia sulphurea is known as the yellow 

 may dun in Great Britain, and the yellow hawk 

 in Ireland. It is allied to the march brown 

 in that the larvae of both belong to the group 

 which Pictet, the eminent Swiss entomologist, 

 described as " Flat larvce," which term expresses 

 the broad, squat appearance of the immature 

 insect when clinging to the underside of stones 

 and rocks. 



The yellow may dun occurs abundantly on 

 the Hampshire chalk-streams " hatching out " 

 as evening advances, though the fly may also 

 be seen during the daytime. Of a brilliant 

 sulphur hue, with metallic-looking dark eyes, 

 H. sulphurea is one of the handsomest of the 

 EpHFMERiDvE. Nevertheless, it is possibly of 



specially directed to the sub-imago and the nymph, may prove 

 the necessity of creating a new species to receive this fly. The 

 male genitalia are practically indistinguishable from those of 

 the march brown. 



