1968] 
Kukalova — Mayfly Nymphs 
315 
half its length; anterior margin slightly concave; posterior markedly 
convex; a convex ridge extending transversely across the pronotum 
which also has a border along the posterior margin. Meso- and 
metathorax nearly equal ; longitudinal suture strongly developed ; each 
notum with a quadrate pigmented area with conspicuous swellings 
at attachment of leg muscles and with a pair of conspicuous spine- 
like projections in the latero-posterior regions; mesothorax slightly 
larger than metathorax. Legs subequal, with five distinct tarsal seg- 
ments; tarsal segments with a row of spines; pretarsus with two 
nearly straight claws (from dorsal view shown in right leg of third 
pair) ; tibia slightly shorter than tarsus; femur longer and broader and 
armed on the ventral surface with a row of spines. Wing pads at- 
tached to the thorax only along the articular area (of the adult 
wing) and independent of each other, i.e., not enclosed in a common 
wing case, as in existing mayfly nymphs; the wing pads projecting 
posteriorly at an oblique angle from the body; wings nearly homono- 
mous, the hind wing being slightly shorter; costal brace relatively 
much larger than in the adult wing; venation otherwise much as in 
the adult of Protereisma but MA independent from R and Rs basally; 
Rs clearly arising from Ri ; cross veins somewhat more irregular 
and reticulate than in the adult. Abdomen slender, the segments 
apparently subequal, although the distortion of some segments prevents 
certainty of this statement; the posterior part of each segment has a 
prominent median projection, comparable to that found in the pos- 
terior part of the meso- and metathoracic segments; markings on 
the terga as shown in figure 1 ; the cerci and the caudal filament 
arising from the posterior part of the tenth abdominal segment, the 
cerci with a dense row of long hairs projecting towards the caudal 
filament and the caudal filament with similar hairs on each side; 
gills of the first and second segments large but thin, showing a 
delicate series of lines as in the gills of some existing nymphs (e.g., 
Siphlonisca aerodromica Needham, as figured by Edmunds, Allen and 
Peters, 1963, plate 5) ; remaining gills much smaller and showing 
fewer lines. 
Dimensions: length of whole body, 21 mm; length of left fore- 
wing, 5.3 mm; length of right caudal filament, 8.3 mm; length of 
middle leg — femur, 4.0 mm, tibia, 2.2 mm; tarsus and pretarsus, 
2.6 mm. 
In addition to this specimen, six other nymphs of mayflies from 
the same deposit in Oklahoma have been studied; these are much 
younger stages and are without wing pads but the body structures 
