May, 1953 
sutures and at wing bases, and usually on 
mesosternum; wings hyaline, veins Sc and 
R, of fore wing faintly tinged with tan near 
wing base; hind wing, fig. 270, narrow, 
costal projection absent, only two longitu- 
dinal veins present; all coxae yellow-brown, 
legs otherwise usually white, femora some- 
times faintly shaded with gray-brown. Ab- 
dominal segments 2-6 white or very faintly 
stained with tan, black spiracular marks 
present; apical tergites yellow-brown, ster- 
nites white; genitalia and caudal filaments 
white or faintly stained with yellow. 
FeMALE.—Length of body 4.5—-5.5 mm., 
of fore wing 5.5-6.5 mm. Head, thorax, 
and abdominal tergites chestnut brown, legs 
light yellow, wings hyaline, longitudinal 
veins yellow; hind wing as in male; ab- 
dominal sternites white, often with a faint 
pinkish tinge; caudal filaments light yellow. 
Known from Illinois, Manitoba, and On- 
tario. 
Illinois Records.—AroMA Park: July 8, 
1948, Ross & Burks, 1¢; Aug. 6, 1947, 
Burks & Sanderson, 2¢. East DUBUQUE: 
at light, July 21, 1927, Frison & Glasgow, 
18g, 692. ELizaBETHTOWN: at light, July 
14, 1948, Mills & Ross, 1¢. Erie: Rock 
River, June 26, 1947, B. D. Burks, 1¢. 
Miran: Rock River, June 4, 1940, Mohr 
& Burks, 7¢. Momence: Aug. 5, 1938, 
Burks & Boesel, 2¢ ; Aug. 16, 1938, Ross 
& Burks, 1¢. Muncie: June 8, 1927, 
Frison & Glasgow, 12. Oakwoop: May 
29, 1948, B. D. Burks, 1¢. Rock IsLANp: 
73, 162 (Walsh 1862:380). Rockton: 
Rock River, Aug. 4, 1948, Burks & Stan- 
nard,1¢,192. SHAWNEETOWN: 1 ¢ (Trav- 
er 1935a:699). Urpana: July 11, 1898, 
ey. Hart; 19. 
3. Baetis elachistus new species 
This species agrees with amplus (Trav- 
er), described from North Carolina, in 
having the wing veins yellow-brown, the 
hind wing without a costal angulation, and 
abdominal tergites 2-6 brown. The two 
differ in that the fourth genital forceps seg- 
ment in amplus is three times as long as 
wide, while this structure in elachistus is 
only as long as wide; the genitalia of 
elachistus likewise are distinct in that the 
basal forceps segment is more prominently 
produced at the mesoapical angle than it is 
in amplus. 
Burks: THE MAyFLiss oF ILLINOIS 
127 
Mate.—Length of body and of fore wing 
each 5.0-5.5 mm. Head dark brown, with a 
white spot at each lateral margin of frontal 
shelf; each antennal scape and pedicel white, 
shaded with tan at apexes, flagellum tan. 
Thorax very dark brown, almost black; legs 
white, coxae shaded with brown, fore femur 
stained with tan, which becomes slightly 
darker toward apex, middle and hind femora 
shaded with tan at apexes, all tibiae brown 
shaded at bases; wings hyaline, veins yellow- 
brown, fore wing with 10 to 12 highly anas- 
tomosed, stigmatic crossveins, no marginal 
intercalaries in subcostal interspace, usually 
none also in first R, interspace, although 
one short intercalary sometimes present 
here; hind wing, fig. 271, narrow, only two 
longitudinal veins present, costal angulation 
absent. First abdominal segment dark 
brown, segments 2-6 uniformly dark yellow- 
brown, tergites 7-10 brown, sternite 7 yel- 
low-brown, 8 and 9 brown, lateral margins 
of 9 dark brown; first genital forceps seg- 
ment brown, mesoapical angle slightly pro- 
duced, fig. 290, second segment tan, third 
segment tan, slightly bowed, one and ‘one- 
half times as long as second segment, fourth 
segment tan, globose, as long as wide; each 
caudal filament tan in basal fifth, gradually 
merging into white distad. 
Holotype, male—Duncans Mills, Illi- 
nois, Spoon River, October 20, 1941, B. D. 
Burks. Specimen in alcohol. 
Paratypes.—Same data as for holotype, 
54. Specimens in alcohol, wings and geni- 
talia on microscope slides. 
4. Baetis frivolus McDunnough 
Baetis frivolus McDunnough (19256: 174). 
Matve.—Length of body 4-5 mm., of fore 
wing 5-6 mm. Head very dark brown, 
lighter at lateral angles of frontal shelf; an- 
tennae brown; eyes brown, each stalk shorter 
than in most species of genus. Thorax very 
dark brown dorsally, slightly lighter later- 
ally and ventrally; anterolateral margins of 
mesoscutum, area along outer parapsides, 
and margins of mesosternum yellow-brown; 
wings hyaline, anterior longitudinal veins of 
fore wing shaded with tan, base of fore 
wing brown, stigmatic crossveins anasto- 
mosed, hind wing long, narrow, costal pro- 
jection and third longitudinal vein lacking; 
legs with all coxae brown, fore leg faintly 
shaded with gray-brown, middle and hind 
