May, 1953 
External Morphology 
The external features of both nymph and 
adult mayfly are described in detail in The 
Biology of Mayflies (Needham et al. 1935). 
Only those characters used in identification 
of Illinois species are mentioned here. 
Many of the morphological structures 
which have proved useful in classification 
are shown in fig. 7 of a generalized mayfly 
adult and in fig. 8 of a generalized nymph. 
The terminology for the adult thorax is 
essentially the same as that used by Velma 
Knox (1935). The location of the various 
margins and areas of the wings is illustrated 
in figs. 7 and 9. The disc of the wing is 
the middle part, bounded roughly anteriorly 
by R, and posteriorly by Cu, but not in- 
cluding the marginal or extreme basal areas. 
The spines on various parts of the penis 
valves which have proved of diagnostic 
value, and their locations, are shown in fig. 
10, of the generalized male genitalia. 
All mayfly nymphs are strictly aquatic 
and respire by means of gills, which vary 
greatly in size and form in different species. 
Typically, there is one pair of gills on each 
of the first seven abdominal segments. They 
may be lamelliform (platelike), filiform 
(threadlike), or a combination of these 
two forms, figs. 51-55, 9la—d, 96, 113, 172, 
199, and others. 
When mayflies first emerge as winged 
insects, in the subimago stage, the surfaces 
of the semiopaque wings are covered with 
microtrichia; the wing margins and the 
caudal filaments are clothed with numerous 
long, slender setae. Beneath the subimaginal 
pellicle the adult eyes, legs, genitalia, and 
caudal filaments can be seen, contracted and 
wrinkled. Most mayflies remain in this 
stage about a day before shedding the sub- 
imaginal pellicle. Immediately following, the 
legs in most species and the caudal filaments 
become greatly lengthened and the eyes of 
the males greatly expanded; the wings are 
clear and hyaline, and they lack microtrichia 
and marginal setae. In a few mayfly adults, 
the legs are aborted, while, in some others, 
they are so greatly reduced as to be useless. 
Although mayflies typically have two pairs 
of wings, the anterior pair being much the 
larger, there are some species in which the 
hind wings are lacking. 
In most adults, the compound eyes occupy 
the greater part of the head and, in the males 
Burks: THE MAyrtiks oF ILLINOIS 17 
of most species, the eyes are larger and 
closer together on top of the head than in 
the females. In the males of some genera, 
the eyes are greatly expanded and each is 
divided into an upper and lower portion, 
which may be further differentiated by size 
and color, figs. 255-257. In the nymphs, the 
compound eyes show a considerable range in 
size and position but none shows the high 
degree of development exhibited in adults. 
The colors of the compound eyes of the 
male as given in the species descriptions 
and keys are those of the eyes while the 
specimens were still alive or had been dead 
only a short time. In mayflies, such as 
Baetis, which have divided eyes, the color 
of the upper portion of each eye is the sig- 
nificant one; the lower portion, as is well 
known, is a day-eye, with pigmentation so 
extensive that this part of the eye appears 
black. In such forms as Stenonema and 
Heptagenia, occasional specimens are to be 
found in which the entire eye is completely 
black, before death as well as after. These 
specimens should be disregarded, as this 
blackening is apparently due to the corneal 
layer of the eye separating from the hypoder- 
mis. After specimens have been killed and 
are thoroughly dry, the colors the eyes had 
in life disappear completely. 
The other color characters as given in 
the descriptions in this report are, wherever 
possible, those of fresh, dry specimens. All 
colors fade or change somewhat within a 
few days after death of the specimens, re- 
gardless of the method of preservation; 
however, this loss of color is relatively slight 
in dry specimens stored away from the 
light. Specimens preserved in 70 per cent 
alcohol soon change color almost completely, 
becoming, after a few months, only a dull 
tan or yellow, even though the color in life 
may have been of various shades from dark 
brown to white. On dry specimens, the 
salient features of the color pattern are 
relatively permanent, but the delicate tints 
of red, green, gray, or yellow of fresh 
specimens eventually disappear or become 
scarcely discernible. 
The system of nomenclature for wing 
veins used by Traver (1935a:119) has been 
followed in this report. It may be noted 
that this differs from the system of Tillyard 
(1923) in several respects. For instance, 
the branched, convex vein in the center 
of the fore wing called MA by Tillyard 
