AQUATIC INSECTS IN THE ADIRONDACKS 427 



them well. They cling closely to twigs, bark, etc., and will allow them- 

 selves to be lifted from the water without stirring. 



Nymph. Length 5.5 mm; setae 3.4 mm additional; abdomen 

 2.5 mm; antenna 2.3 mm. 



Body stout, with thick thorax, and short and rapidly tapering ab- 

 domen. 



Color pale brownish, darker above, obscured by adherent silt, by 

 diatoms, vorticellae, hydras, and other adherent organisms. 



Abdomen with large and sharp, flat teeth, made by the projecting 

 posterolateral angles of the third to the ninth segments, largest on 

 segment 7, smallest on segment 3. 



Gills present on segments 1-6; on segment i minute rudiments; on 

 segment 2 thick, elytroid, covering the functional gills, squarish, the distal 

 and external angles rounded, the basal-internal angle square; there is on 

 the anterior face a piliferous carina, regularly arcuate, near the external 

 margin, extending from the outer basal to the inner distal angle. The 

 gills on segments ^-6 diminish in size posteriorly ; they have the shape 

 of the hind wing of a pierid butterfly, and bear a long dense peripheral 

 fringe of respiratory filaments. These filaments are unilaterally several 

 times branched on the anterior side, and are so closely crowded that they 

 overlap in regular series around the margin of the gill leaf. They are 

 longest at the distal end, where they exceed the length of the gill leaf 

 itself. 



Setae slender, thinly fringed along both margins, the middle one appar- 

 ently a little shorter in the males, a httle longer in the females than the 

 other two. 



Hexagenia variabilis Eaton 



1843-45 Palingenia limbata Pictet, Hist. nat. Neur. v. 2 (Epheiii.) p. 146, 

 pi. 12 (the original description: this was Serville's name for another 

 species) 



1853 Palingenia limbata Walker, List neur. ins. Brit. mus. 3 : 548 



1861 Pa 1 ing en i a bili n eata Hagen, Synopsis Neur. N. Am. p. 41 (a full 



description) 



1862 Palingenia limbata Walsh, Acad. nat. sci. Phil. Proc. p. 373 



1863 Palingenia limbata Hagen, Ent. soc. Phil . Proc. 2 : 176 



1863 Palingenia limbata Walsh, Ent. soc. Phil. Proc. 2:197-99 (makes 

 the species the type of his new genus Hexagenia) 



1868 Hexagenia limbata Eaton, Ent. mo. mag. 5 : 85 



1871 Hex ag e n i a limbata Eaton, Ent. soc. Lond. Trans, p. 65, pi. 1, 

 fio-. 7, and pi. 4, fig. 3 and 3a (description m Latin) 



1885 Hexageni a vari a b il i s Eaton, Linn. soc. Lond. Trans. (2) 3 : 55, 

 pi. 7, fig. lie 



1890 Hexagenia variabilis Hagen, Stett. ent. zeit. 51:11-13 (dis- 

 tinguishes this species from b i 1 i n e a t a Say by the form of the forceps 

 of the male) 



1892 Hexagenia limbata Banks, Am. ent. soc. Trans. 19 : 345 (listed) 



