1911] May-Flies of Fall Creek, N. Y. 115 



shown on PI. VIII, fig. 6.) Thorax, mottled brownish; prothorax 

 quadrangular, its angles closely fitting to the mesothorax. Legs with 

 the margins of the second and third femora entire; the first femur fig- 

 ured on PI. VII, fig. 6. Abdomen, rounded above, flattened below; 

 without dorsal spines or tubercles. Gills on segments 3-7; postero- 

 lateral angles of segments 3-9 produced into flattened spines becoming 

 more acuminate posteriorly; posterior margins of segments 2-10 sparsely 

 edged with hairs; lateral margins of segments 3-9 spinose serrate. 

 Setae, pale, except for a single brown ring at the base of each. 



Tricorythus allectus Needham. 



This species was described by Prof. Needham in Bull. 86, 

 N. Y. State Mus. as Caenis allecta, but afterward referred by 

 him to the genus Tricorythus, in Bull. 124 N. Y. State Mus. 



Occurrence, habitat. These nymphs are closely associated 

 with Caenis, though they have not thus far been often found in 

 the ill smelling mud generally preferred by that nymph. They 

 clamber about in fine silt and sand, and the particles which 

 adhere to their hairy bodies make them practically invisible. 

 A handful of mud which appears to contain no sign of life, will 

 after a few minutes draining, reveal slowly moving bits of mud 

 which may prove to be either Caenis or Tricorythus according 

 to the quality of the mud. Tricorythus allectus is one of the 

 commonest species in Fall Creek. During June and July, the 

 imagoes may be found strewn upon the surface of little protected 

 inlets along its shores, or caught in the meshes of the spider- 

 webs on walls and bridges near it. 



Measurement. Length of body 6.5 m. m. Length of setae 4 m. m. 

 Mouth — parts and gill lamella (PL XL) The nymphs of 

 Tricorythus may be at once distinguished from those of Caenis 

 by the shape of the elytroid gill cover, rounded at the 

 end in Caenis, distinctly triangular in Tricorythus (PL XI, fig. 1). 

 Color yellowish, pale below; abdomen marked with transverse bands 

 of brown broken by a median longitudinal pale stripe. Elytroid 

 lamella prominent, purplish brown at the base. Body all over sparsely 

 beset with hairs. Antennae pale with basal segments brown. Legs 

 pale with a blackish spot at the proximal joint of each tibia. Gills not 

 wholly hidden by elytroid lamellae. Lateral spines on segments 2-9. 

 Setae 3. 



Caenis. 



This genus is plentifully represented by Caenis hilaris, Say, 

 and by nymphs of some other species not yet reared here. 



