AQUATIC INSECTS IN THE ADIRONDACKS 4I9 



GENERA OF EPHEMERID NYMPHS IN EASTERN UNITED STATES 



a Body flat, widest across the rear of the head ; eyes dorsal ; legs depressed ; 

 adapted for clinging closely to flat surfaces. (Imagos have five freely 

 movable segments to the hind tarsi) 



(Heptageninae) Heptagenia, sens. lat. 



aa Head not so wide as succeeding parts of the body ; eyes lateral ; (imago 



' with but three or four freely movable segments to the hind tarsi, the basal 



segment at least coherent with the tibia) 



& Body widest across the mesothorax; legs of the first and second pairs 



about equidistant at the base. (Imagos have the cubitus and the first 



anal vein nearly parallel toward the base) (Baetinae) 



c Gills completely concealed under the enormously enlarged, conspicuously 



four spined mesonotum Baetisca 



CG Gills exposed ; mesonotum normal 

 d Outer caudal setae fringed only on the ianer sides ; gills on abdominal 

 segments 1-7 ; agile swimmers 

 e Gills simple 



/ Gills oval in outline, obtuse at the apex Baetis 



ff Gills lanceolate in outline, acute at the apex .. Centroptilum 

 ee Gills double, at least the anterior pairs 



/ Antennae hardly longer than the head ; the thin lateral margin of 

 the eighth abdominal segment produced posteriorly in a very 



large flat tooth; ocelli on the face Siphlurus 



ff Antennae longer than half the body; the posterolateral angles of 

 the eighth abdominal segment not forming a conspicuous tooth ; 

 ocelli generally on the top of the head 

 (J Antennae shorter than the body; gill tracheae pinnately branched 



Callibaetis 

 gg Antennae longer than the body ; gill tracheae palmately branched 



C loeon 

 del Outer setae fringed on both sides 



e Gills on abdominal segments 1-7, double, similar 



/ Divisions of the gills narrowly linear. Leptophlebia 



ff Divisions of the gills leaflike, each with a terminal filament 



B 1 a s t u r u s 



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



more or less elytroid, covering those behind it 



f Gills present on the seventh abdominal segment, elytroid on the 



third or fourth segment; a pair of minute tubercles at the apical 



margin of each abdominal segment beside the median line 



Ephe m erella 



ff Gills absent from the seventh abdominal segment, elytroid on the 



second segment : no dorsal abdominal tubercles.., C a e n i s 



hh Body somewhat compressed, widest across the base of the abdomen; legs 



of the first pair much more closely approximated at the base than those of 



the second pair ; all the legs appressed against the sides ot" the body and 



adapted for burrowing; mandibles usually produced anteriorly in a long, 



curved tusk. (Imagos have the cubitus and the first anal vein strongly 



divergent toward the base) Ephemerinae 



c Head without frontal prominence ; gill rudiment on the first abdominal 



segment simple I'olymitarcys 



