78 Iuntinois NarurAL History SurvEY BULLETIN 
to almost white; genital forceps tan or red- 
brown; caudal filaments light yellow or tan, 
articulations dark brown. 
NympH.—Length of body 8-10 mm. 
Frontal tubercles of head virtually wanting, 
only faintly indicated; each gena slightly pro- 
duced above base of mandible as a small, 
subtriangular shelf, fig. 179; dorsal and 
lateral spines of mesonotal shield long and 
relatively slender; mesonotal shield relatively 
long and slender, with a maximum width, 
not including lateral spines, two-thirds as 
great as maximum length. 
Known from Illinois, Michigan, New 
Brunswick, Ontario, and Quebec. Develops 
in cool, fairly rapid streams. 
Illinois Record.—AromMaA Park: Kanka- 
kee River, June 4, 1947, B. D. Burks, 14. 
3. Baetisca obesa (Say) 
Baetis obesa Say (1839:43). 
Mave.—Length of fore wing 9-11 mm. 
Compound eyes tan, lower portion slightly 
darker; head and antennae yellow-brown. 
Mesonotum red-brown, darker at apex of 
scutellum; thoracic pleura yellow-brown; 
sternum yellow-brown, becoming lighter to- 
ward posterior margin. Each fore leg light 
yellow, apex of femur, of tibia, and of each 
tarsal segment darkened with yellow-brown; 
middle and hind legs almost white, brown 
shading at apex of each tarsal segment; 
wings hyaline, veins C, Sc, and R, of fore 
wing brown-shaded at bases, light yellow 
distad. Abdominal tergites dark brown, be- 
coming chestnut brown on posterior tergites; 
sternum light brown, slightly darker on 
apical three sternites; genital forceps, fig. 
174, and caudal filaments usually white or 
very faintly stained with tan; basal articu- 
lations of caudal filaments sometimes brown. 
NympH.—Length of body 8-10 mm. 
Frontal tubercles of head well developed, 
projecting as a pair of rounded protuber- 
ances; each gena produced above base of 
mandible as a triangular ledge, fig. 180; 
dorsal and lateral spines on mesonotal shield 
relatively short and stout; mesonotal shield 
relatively long and slender, with a maximum 
width, not including lateral spines, two- 
thirds as great as maximum length. 
Known from Illinois, Indiana, Manitoba, 
Michigan, New Hampshire, New York, and 
Wisconsin. Develops in cool, fairly rapid 
streams. 
Vol. 26, Art. 1 
Illinois Records.—Havana: Illinois 
River, April 18, 1894, C. A. Hart, 1 N. 
Momence: Kankakee River, June 1, 1937, 
B. D. Burks, 1¢. RicHMoNpb: at light, 
June 4, 1938, Ross & Burks, 1¢. Rock 
IsLAND: 2086, 149 (Walsh, 1862: 378). 
4. Baetisca bajkovi Neave 
Baetisca bajkovi Neave (1934: 166). 
Description of nymph. 
Baetisca bajkovi Neave. Daggy (1945:388). 
Description of adult 4. 
This species differs from dacustris only by 
minor and possibly intergrading characters 
both in the nymph and the adult. It may, 
thus, eventually be necessary to place 
bajkovi as a synonym of Jdacustris. Until 
actual intergrades have been found, it is 
best, however, to continue to separate the 
two. All specimens referred at present to 
bajkovi came from streams, while Jacustris 
has been reported as having come only from 
lakes. 
Mave.—Length of fore wing 10 mm. 
Compound eyes yellowish tan, with lower 
portion of each slightly darker; head and 
Fig. 181.—Baetisca bajkovi, mature nymph, 
dorsal aspect. 
