May, 1953 
orthopteroid archedictyon. The cubital 
intercalaries of the fore wing consist of 
three or four long, straight veins, the pos- 
terior one of which is attached to the anal 
wing margin by a series of confused, short, 
and irregular veinlets. The hind wing has 
a blunt costal angulation. In the fore leg 
of the males, fig. 23, the tarsus is normally 
developed but the femur is quite short. In 
the females, the median caudal filament is 
well developed, while, in the males, it is 
reduced to a minute rudiment. 
The nymphs, fig. 59, have prominently 
toothed, downcurved mandibular tusks; the 
gills have short, relatively inconspicuous 
marginal ciliae, figs. 52B, 52C. 
Reliable characters for separating the 
females to species have not yet been found. 
KEY TO SPECIES 
Aputt Mates 
Mesonotum dark brown; apicolateral angle of 
each penis lobe rounded, fig. 66.......... 
Me PF Pe Cire. yer be ws 1. leukon 
Mesonotum light yellow, shaded with tan; 
apicolateral angle of each penis lobe acute, 
em Ces MPa. hea 2 DYSiee cn 5 2. album 
Mature Nympus 
Gills on abdominal segments 2-6 with lateral 
tracheal branches pigmented, figs. 52B, C, 
Dec be ode ie On eee ee 1. leukon 
Gills on abdominal segments 2-6 with lateral 
tracheal branches hyaline........ 2. album 
1. Ephoron leukon Williamson 
Ephoron leukon Williamson (1802:71). 
Polymitarcys albus of authors, 
misidentification. 
Rearing work and field observations 
carried on here in Illinois show that the 
mature nymphs of this species, when ready 
to transform, migrate to the shores of the 
large rivers in which they develop. At 
dusk, they congregate in the shallow water 
or even in the wet mud at the edge of the 
water. The subimagoes emerge there, leav- 
ing their cast nymphal skins floating on the 
shallow water or partly submerged in the 
mud. These subimagoes take flight at once, 
and the males molt to the adult stage almost 
immediately. Molting occurs in the air, 
during flight, as the legs are nonfunctional. 
The adults then disperse to mate and lay 
their eggs. All the adults that emerge 
during one evening are, apparently, dead by 
the following morning. The length of adult 
Burks: THE Mayr ies oF ILLINOIS 33 
life I observed for this species in Illinois 
was about one hour. My observations do 
not agree with those of Howard (Needham 
1905:60; 1920:285), also made in Illinois. 
However, Howard’s observations were made 
on a mixture of individuals belonging to the 
genera Ephoron and Potamanthus. My ob- 
servations on leukon agree closely with those 
made by Ide (1937a:25) on this species in 
Ontario. 
Mave.—Length of body 12-14 mm., of 
fore wing 11-13 mm. Vertex of head light 
yellow, shaded with very dark gray between 
ocelli; anterodorsal area of mesonotum 
dark brown, metanotum a lighter brown; 
legs light yellowish to snow-white (in freshly 
killed specimens), with each fore femur and 
tibia stained with purplish gray; abdomen 
white, with variable areas of gray shading 
Fig. 59.—Ephoron leukon, mature nymph, 
dorsal aspect. Small figure at left represents 
enlargement of fore leg to show detail. 
