. Bulletin. Natural history; Natural history. Mav. 1953 Burks: The Mayflies of Illinois 33 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 n
. Bulletin. Natural history; Natural history. Mav. 1953 Burks: The Mayflies of Illinois 33 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. 525^ 52C. Reliable characters for separating the females to species have not yet been found. KEY TO SPECIES Adult Males Mesonotum dark brown; apicolateral angle of each penis lobe rounded, fig. 66 1. leukon Mesonotum light yellow, shaded with tan; apicolateral angle of each penis lobe acute, fig. 67 2. album Mature Nymphs Gills on abdominal segments 2-6 with lateral tracheal branches pigmented, figs. 525, C, 59 1. leukon Gills on abdominal segments 2-6 with lateral tracheal branches hyaline 2. album 1. Ephoron leukon Williamson Eplwron leukon Williamson (1802:71). Polymitarcys alius 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 eg
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