50 Ituinoris NarurAL History Survey BULLETIN 
ll. CAENIS Stephens 
Caenis Stephens (1035: 61). 
Oxycypha Burmeister (1839:796). In part. 
Ordella Campion (1923:518). New name, 
unnecessarily proposed. 
The species of Caenis consist of small, 
predominantly white mayflies with shadings 
of purplish gray. These mayflies often 
emerge in enormous numbers, filling the 
air like snowflakes. 
Each antennal pedicel in the adults of 
both sexes is approximately twice as long 
as the scape. The vertex of the head lacks 
tubercles. The fore coxae are close to- 
gether on the venter; each fore leg in the 
males is as long as the body, with the tibia 
twice as long as the femur and slightly 
longer than the tarsus. In most species, 
there is a pair of submedian, dark brown 
spots on the pronotum. In each wing, there 
are very few crossveins, as in fig. 97 of 
Brachycercus lacustris; vein M, and the 
median intercalary vein extend to the wing 
base, and the cubital intercalary veins are 
long and relatively straight. The marginal 
ciliae are well developed in the adult wings; 
the subimaginal pellicle covering the wings 
is often only partly shed. The wings are 
whitish hyaline, with gray-purple shading in 
the first three interspaces and on veins Sc 
and R,. Each abdominal segment bears a 
pair of long, lateral ciliae in the subimago; 
these ciliae are reduced to small, lateral 
projections in the adult. The posterior 
margin of the male genital forceps base is 
slightly convex; each forceps has only one 
segment. The penis lobes are fused on the 
meson; each lobe is broad and flat, and 
slightly widened at the apex, fig. 108. In 
both sexes, the three caudal filaments are 
well developed, those of the males being 
the longer; these filaments are entirely white 
in all Illinois species. 
In the nymphs, the body is quite flat, with 
the pronotum narrower than the mesonotum. 
The nymph shown in fig. 114 to illustrate 
this genus has been drawn somewhat dis- 
tended in order to show the structure of the 
abdomen; when the nymph is alive, the 
abdomen is more compact than it is repre- 
sented in this figure. The head is smooth, 
lacking tubercles. As measured in dorsal 
aspect, each antenna is twice as long as the 
head and pronotum combined. The legs are 
relatively short and stout; the claws are 
small and slender, and have extremely mi- 
Vol. 26, Art. 1 
nute ventral denticles, fig. 94. The first ab- 
dominal segment bears a pair of prominent, 
single, filamentous gills, fig. 914; the gills 
borne by the second segment are single, 
quadrate, and operculate, fig. 91B. The gills 
borne by segments 3-6 are single and plate- 
like, each gill having the margins deeply 
fissured to produce a marginal fringe of long 
filaments; each filament is secondarily di- 
vided near the tip to produce two or three 
smaller filaments, figs. 91C, 91D. The three 
caudal filaments are relatively stout, with 
a whorl of three to five setae at each artic- 
ulation. 
I have observed the subimagoes of simu- 
lans to shed the subimaginal exuviae during 
Fig. 114.—Cacenis simulans, mature nymph, 
dorsal aspect. 
