10 Fresh-water Biology 



{Continued from page 8) 



16 — Head emarginate in front : basal gill filaments almost wanting Cynigmula 



— Head hardly emarginate: gill filaments well developed 17 



17 — Gills of segment 1 enlarged, inturned, and meeting beneath the thorax. .Rhithrogena 

 — Gills of segment 1 not enlarged, directed laterally 18 



18 — Inner canine of mandible about half as long as the outer Cynigma 



— Inner canine at least three fourths as long as the outer one (see fig. 13) .Heptagenia 



19 — Gills completely concealed under an enormously enlarged thoracic shield . . . Baetlsca 

 — Gills exposed : thoracic dorsum normal 20 



20 — Outer tails fringed alike on both sides 21 



— Outer tails heavily fringed on the inner side only 34 



21 — Gills present on abdominal segments 1 to 7 27 



— Gills wanting from one or more of these segments 22 



22 — One pair of gills thickened (operculate), covering those behind it 23 



— Gills all thin and freely exposed 27 



23 — Operculate gill on segment 4 Ephemerella 



— Operculate gill on segment 2 24 



24 — Hind wing sheath present Orelanthus 



— Hind wing sheath absent: small hairy species 25 



25 — Gills on segments 2 to 6 double: operculate gill triangular Tricorythodes 



— Gills on segments 2 to 6 single: operculate gill squarish 26 



26 — With three prominent tubercles on top of the head Brachycercus 



— With no tubercles on top of the head Caenis 



27 — Gills of the first pair unlike those that follow 28 



— Gills of the first pair similar to the others 32 



28 — Gills on segments 2 to 7 single clusters of slender filaments Habrophlebia 



— Gills on segments 2 to 7 flattened and more or less plate like 29 



29 — Margins of each gill on middle segments fringed Thraulus 



— Margins of these gills broadly lobed or entire 30 



30 — Gill on segment 1 simple; the others lobed at the apex Choroterpes 



— Gill on segment 1 double; the others slender tipped 31 



31 — Middle gills broad and lobed before the tip Blasturus 



— Middle gills narrow and uniformly tapering . . . Leptophlebia 



32 — With lateral spines on segments 2 to 9 Thraulodes 



— With lateral spines on segments 8 to 9 or on 9 only 33 



33 — Hind dorsal margin of segments 1 to 10 finely spinulose Paraleptophlebia 



— Hind dorsal margin of segments 7 to 10 only are so Habrophleboides 



34 — Claws of the fore legs unlike the other claws 35 



— Claws of the fore legs like those on the other legs 37 



35 — Claws of the fore legs deeply cleft, appearing double 36 



— Claws of the fore legs simple, long, slender, bearing a few bristles beneath: front 

 coxae bearing a thumb-like appendage on the inner side Ametropus 



(Continued on page 12) 



PLATE 2. MAYFLY NYMPHS 



Genera Figs. Length Distr. Waters Genera Figs. Length Distr. Waters 



Campsurus 5 17 S static Hexagenia 6 27 E, C static* 



Ephemera 2(6) 18 E both Isonychia 8 15 G lotic 



Ephoron* 7 15 E, C static Pentagenia 3(6) 24 E, C static 



Euthyplocia 1 29 S Potamanthus. . . 4 13 E, C lotic 



• In settling basins in streams conditions are static. Drawings mainly from Kennedy. 



