278 BULLETIN OF THE BUREAU OF FISHERIES. 
PoLtymiTarcys alone has three or four long, straight, intercalary veins between the first and second 
anal veins, joined together basally before their attachment to these veins. 
Eutuypiocia alone lacks all the preceding characters. 
Nymphs. 
EPHEMERA alone has a frontal prominence divided by a deep, round notch into two smooth spines. 
HeExAGEnIA alone has a rounded, shelflike, frontal prominence on the head. 
PENTAGENIA alone has numerous brown denticles on margins of frontal prominence, antennal 
basal folds, tusks, and front tibie. 
These three are the true burrowers, having upturning tusks, front feet flattened for digging, more 
or less cylindric bodies, and erect gills. 
CaMPSURUS nymph is unknown. 
PoTAMANTHUS alone has the tusks shorter than the head. 
Potymirarcys alone has long, smooth tusks as long as the head and hairy only at their dilated bases. 
EvuTHYPLociA alone has the enormous tusks, hairy almost to the tip and beset also externally 
with brownish prickles. 
These last three are the sprawlers, having tusks horizontally extended, elongate fore legs, and 
laterally extended gills. 
HEXAGENIA, the Brown Drakes. 
This genus includes the largest of our mayflies, measuring often an inch and a half 
in expanse of wings and nearly an inch in length of body, to which the long tails may 
add 2 inches of length; all this without counting the very long fore legs which are usually 
extended forward. The fore wings are marked with a brownish band along the front 
border, and there is usually a narrower border of brown around the outer margin of 
the hind wings. This color varies from a faint, brownish tinge in newly emerged individ- 
uals and in pale varieties to dark brown, almost black, in older ones or in other varieties; 
and when these bands are darker, then the cross veins of the middle area of both wings 
become bordered with brown. The body is brown above, yellowish beneath; there is 
a paler longitudinal middorsal stripe upon the thorax, dividing the brown into two 
broad stripes, as in the typical H. bilineata Say; and there are interrupted yellowish rings 
upon the abdomen, all of which pale markings tend to become obscured in the darker 
specimens. 
Hexagenia bilineaia is the name I apply to all the variants of the species that occupies 
the beds of our larger lakes and streams. The color differences appear to be only differ- 
ences of degree. Even the differences of the male genitalia—usually our ultimate criteria 
of species—are intergradient. 
Walsh (1863) thought there were two good species in the Mississippi River at Rock 
Island. He said (p. 199): 
Nothing is easier than to distinguish the living specimens of these insects [H. bilineata and H. 
limbata=variabilis Eaton] by the color of the eyes. In the former the upper half of the eyes is cinnamon 
brown, in the latter bright, greenish yellow; in both the lower half of the eyes is black. The dried 
specimens, especially those of the male, are very difficult to distinguish. * * * In the middle of 
July, when on the shallow area of the Mississippi known as “the slough’’ at Rock Island, H. bilineata 
appears in prodigious swarms, so that the bushes absolutely bend down with their weight. * * * I 
am sure that in the thousands of individuals, both male and female, which blackened the bushes there 
was not one with the upper surface of the eyes yellow or yellowish; the only variation I noticed from 
the normal color was that one male had the eyes a shade or two paler than the rest on their upper surface. 
I have not had the privilege of studying the Hexagenias of the Mississippi River 
alive, but I am unconvinced by this emphatic opinion and by the long table of other 
