26 NEW YORK STATE MUSEUM 



e Hind wings present 



f Fore wings with numerous costal crossveins before the bulla ; 

 hind wings with a moderate number of crossveins 



Callibaetis 

 ff Fore wings without costal crossveins before the bulla ; hind 



wings without crossveins or with but 1-3 of them 

 g Marginal intercalary veinlets in pairs; hind wings oblong, 



with a short costal angulation Baetis 



gg Marginal inteivalary veinlets of the fore wing single; hind 

 wings linear, with a spur-like costal augulation 



C e n t r o p t i 1 u in 

 ce Hind wings absent Chloeon 



Nymphs 



a Mandibles with an external tusk-like ramus, visible from above ; gills 

 on abdominal segments 1-7 (often rudimentary on 1), double, flat- 

 tened, linear, the margins fringed with respiratory filaments 



Ephemerinae 



1) Mandibular tusks longer than the head (burrowing species) 

 c Witb no frontal prominence 



d Legs increasing in length posteriorly; gills of the first abdominal 

 segment simple; labruui longer than wide; maxillary palpus 



t\vo-joiuted Polymitarcys 



dd Legs decreasing in length posteriorly ; labrum wider than long ; 



maxillary palpus three-jointed Euthyplocia 



cc With a conspicuous frontal prominence 



d Frontal prominence rounded Hexagenia 



</(/ Frontal prominence bifid at tip Ephemera 



1)1) Mandibular tusks shorter than the head, inconspicuous, only their 



tips visible from above Potamanthus 



6&& Unknown C a m p s u r u s and Pentageuia 



aa Mandibles without projecting tusk-like ramus ; gills not as in a 



& Eyes dorsal ; body strongly depressed ; tarsal claws with lateral teeth ; 



dwellers in rapid streams and on wave beaten shores ; adopted to 



clinging to flat surfaces of rocks, timbers, etc..H eptageninae 



c Gills represented on abdominal segment 7 by simple, lanceolate or 



linear filaments, differing markedly from the lamellae of the 



preceding segments II e p t a g e u i a 



cc Gills of the seventh abdominal segment lamelliform, like those be- 

 fore them, but smaller 



d Gills on all the segments divaricate in pairs E c d y u r u s 



dd Gills of segments 1 and 7 approximated at their tips, being 

 decurved beneath the abdomen, those of segment 1 much 

 enlarged 



e Head widest toward the front; mandible with its outer canine 

 linear, truncate and denticulate on the end ; labrum retracted 

 far back from the flaring margin of the frons ; maxilla with 

 its palpus hairy and the tip of its lacinia armed with three 

 large teeth Iron 



