AQUATIC INSECTS IN THE ADIRONDACKS 423 



Abdomen black and white ; segments 2-5 pure white (in old males) 

 with black spiracles; segment 6 yellowish; segments 7-10 black, paler 

 below. 



Male subimago (undescribed). Differs only in having the setae 

 about 5 mm long (as in the female) ; and in lacking strongly contrasting 

 black and white colors on the abdomen, all the colors being duller, the 

 wings merely translucent, with a fine fringe of hairs. 



Female imago. Length 3-5 mm, setae 5 mm. Color reddish 

 brown, darker on the thorax and paler beneath the body; discoloring 

 badly when pinned. Head and thorax brownish with ocelli paler and 

 eyes black. Thoracic dorsum brown, darker on the ridges, with a pair 

 of oblique pale stripes extending from the hind angles of the prothorax 

 to the wings, and a pair of narrow, submedian, longitudinal stripes on the 

 mesothorax. 



Abdomen reddish brown, apical segments paler. 



Female subimago. Differs in obscurer coloration; on the top of 

 the mesothorax there is a single wide middorsal longitudinal stripe. 



Known only from the St Lawrence river and Saranac Inn. Specimens 

 are deposited at Cambridge in the Museum of comparative zoology and in 

 the New York state museum. 



Nymph. Measures 4-5 mm; setae 2.3-2.5 mm additional; anten- 

 nae 2.2 mm. 



Body slender, graceful, smooth, clean ; brownish above, yellowish 

 beneath and on the sutures; a narrow middorsal yellowish line, dilated 

 on the middle of the mesothorax, and expanded again into a quadrate 

 spot at the front of the prothorax; paired yellowish markings beside this 

 line, and numerous small yellowish spots nearer the sides ; legs and 

 antennae yellowish. Setae with a brownish shade near the bare tip ; all 

 fringes short; the two outer setae fringed only on the inner side. 



Gills present on abdominal segments 1-7. small, separate, widely 

 extended and fully exposed; each leaf obovate, a little oblique, with a 

 chitinous thickened inferior border, this border, short on the foremost gill 

 leaf, reaches the apex on the sixth one, and comprises the greater part of 

 the narrow, reduced seventh one. 



The face is vertical, with the ocelH in front, somewhat as in Siphlurus. 



The nymph is an exceedingly agile, little fellow, darting hither and 

 thither with astonishing speed when one tries to pick it up. 



A few specimens of a larger, undetermined species of B a e t i s were 

 taken at the hatchery windows. 



Siphlurus alternatus Say 



Plate 11, figure 7 

 1824 Baetis alternatus Say, Godman's western quart, reporter, 2 : 304 

 1859 Baetis alternatus Leconte (ed.), Complete writings T. Say, 1 : 204 



1861 Baetis alternatus Hagen, Synopsis Neur. N. Am. p. 49 



1862 Baetis alternatus Walsh, Acad. nat. sci. Phil. Proc. p. 369 



1863 Baetis alternatus Hagen, Ent. soc. Phil. Proe. 2 : 169 



