BURROWING MAYFLIES 279 
color differences that he gives on the following page (1863, p. 200); for I fear Walsh 
did not take into account the color differences due to age, and, though he examined 
thousands of specimens at a time, these thousands may well have been all of practically 
the same age—all of one day’s brood. 
Several forms have been named upon the basis of slight and inconstant color differ- 
ences. Dr. Hagen (1890) thought to reduce the species to two in number because of differ- 
ences he found in the form of the penes of the male, whether gently curved and finger- 
form (bilineata) or hooked and pointed (/imbata Pictet = variabilis Eaton). I thought for 
a long time I could recognize males of these two species; and in my earlier papers I 
have treated variabilis as a distinct species, but a careful study of more material has 
shown intergradients and additional forms. Four of the more typical forms of male 
appendages are shown in Plate LXXXI, figures 61, 62, 63, and 64. In a general way 
it may be stated that the long, straightish penis goes with the lighter coloration of wings 
and body and with northward distribution; but there are exceptions to this also. A 
separation of species on such characters as these should not be made without a careful 
study of at least two things: 
1. Changes of form due to age.—A casual examination of the subimago, when the 
penis of the adult is clearly outlined within that of the subimago, shows that there will 
be a considerable change of form at the final molting. Figure 65 of Plate LXXXI 
shows this condition. The inclosed penis, it may be seen, will be of the form shown in 
figure 62 of the same plate. 
2. Changes due to functional activity.—There is a relatively immense sperm mass 
gathered at the outlet of the vasa deferentia of the newly issued adult male, whose 
presence there may have something to do with the prominences of the penes and whose 
discharge may allow for much retraction. 
A good many names have been applied to the different forms of this genus, but 
after a careful study of a good bit of material from many localities I am unable to ree- 
ognize more than two good and distinct species in the eastern United States—a lowland 
species from lakes and rivers, Hexagenia bilineata Say, and an upland bog-stream species, 
H. recurvata Morgan.* 
The former is, of all our mayflies, the most important, the most abundant, the 
most observed, the most characteristic of the river fauna. The adult male is figured on 
Plate LX XI. The female would be similar, larger in size, with shorter fore legs and much 
shorter tails. The nymph is figured on Plate LXXII. I have examined nymphs and 
exuvie from many lake and river localities and have been unable to find any constant 
differences between them to indicate more than a single species. 
The material for this species that I have studied is as follows: 
Mississippi River material, as stated in detail in preceding pages, including adults 
from Emerson Stringham collected about and above the Big Dam, on dates ranging 
from July 12 to September 15, showing several great swarms in July and several reappear- 
ances in dwindling numbers to the end of the season; adults collected by H. E. Schra- 
dieck on the outer walls of the biological laboratory at Fairport, lowa, July 12 (when 
@ With this species we are not here concerned. It is still insufficiently described, but its determination is made possible 
by the figures of the genitalia that were published by Miss Morgan in the Annals of the Entomological Society of America, volume 
6, page 395, 1913. Figures 8 and 12 of Plate LX XII herewith show that its nymph is readily distinguished from the lowland 
form by the relatively greater length of the mandibular tusks and by the unbranched condition of the rudimentary first gill. 
This is an early season species that swarms in May and then disappears, none being seen after early summer. 
