MAY FLIES AND MIDGES OF NEW YORK 27 



ee Head widest toward the rear ; mandible with its outer canine 

 shaped like a shoemaker's last, the heel pointing laterally 

 and the long, slender, acute toe obliquely forward ; labrum 

 pendent from the flaring but notched edge of the frons ; 

 maxilla without strong teeth at tip of its lacinia and without 



long hairs on its palpus Rhithrogena 



1) Eyes lateral ; form of body various ; claws smooth or toothed below 



Baetinae 

 c Gills completely concealed under an enormously enlarged, four- 



spined dorsal thoracic shield Baetisca 



cc Gills exposed; thoracic dorsum normal 

 d Outer caudal setae fringed on both sides 

 e Gills on abdominal segments 1-7, double 

 f Gills filamentous 



g Each a pair of simple filaments Leptophlebia 



gg Each a pair of clusters of slenderer filaments 



Habrophlebia 

 ff Gills lamelliform, at least on the middle segments 



g Lamellae of each gill similar Blasturus 



gg Lamellae of each gill markedly differing in form at tip 



(see pl.8, fig.S) Choroterpes 



ee Gills absent from one or more of segments 1-7; one pair more 



or less elytroid, covering those behind it 



/ Gills present on the seventh abdominal segment, elytroid on* 

 the third or fourth segment; a pair of tubercles on the 

 apical margin of each segment beside the middorsal line 



g Head smooth above Ephemerella 



gg Head armed above with a pair of erect occipital tubercles 



D r u n e 1 1 a, gen. nov. 



ff Gills absent from the seventh abdominal segment, elytroid on 

 the second segment; no dorsal abdominal tubercles 



C a e n i s 

 dd Outer caudal setae fringed only on the inner side 



e Posterolateral angles of the hinder abdominal segments pro- 

 longed into thin, flat, sharp lateral spines 



f Fore legs conspicuously fringed with long hairs; gill tufts 

 present upon the bases of maxillae and front coxae and at 



bases of lamellae on abdomen Chirotenetes 



. ff Fore legs without conspicuous fringes ; no maxillary or coxal 



gills ; no gill tufts at base of lamellae on abdomen 

 g Gills double on the basal abdominal segments ; end of max- 

 illa fringed with simple hairs Siphlurus 



gg Gill lamellae all single; end of maxilla fringed with pec- 

 tinated hooks Ameletus 



ee Posterolateral angles of the hinder abdominal segments hardly 



more than acute not prolonged in thin flat lateral spines 

 f Gill lamellae simple 

 g Lamellae obtuse at apex ; maxillary palpus rounded at the 



apex Baetis 



gg Lamellae acute at apex; end of maxillary palpus trun- 

 cated Centroptilum 



