BURROWING MAYFLIES. 281 
any case) it flies forth over the waters with myriads of its kind, all together making a 
great company, filling the air and forming one of the great swarms that have been so 
often described and that are so well known on every stream side. 
Mating occurs in the air during flight, and almost at once thereafter the female 
seeks the surface of the water. She flies hither and yon, dipping the surface, and then 
falling flat upon it with wings outspread. Her eggs are liberated in the water, just when 
and how has not been actually observed; but I have seen many playing above the sur- 
face of the water, egg-laden, and I have picked many from the surface, all of which have 
been spent females with eggs all gone. 
When gravid females are injured, as by squeezing the thorax or snipping off the head, 
they at once extrude their eggs in two long, yellow packets of extraordinary size (similar 
to those of Polymitarcys, Pl. LXXVII, fig. 36). If these be placed in water, the 
masses crumble and the eggs tend apart in falling; thus they become disturbed on the 
bottom. The egg coat, which is slightly adhesive, quickly gathers a protective covering 
of silt and becomes well-nigh invisible. 
The eggs of a single female will usually number above 8,000. The body of the 
female mayfly has become hardly more than a scaffolding for carrying this mass of eggs. 
Her mouth parts are atrophied; her alimentary canal is an air reservoir; her muscles are 
nearly all muscles of flight; her chief appendages are outriggers for control of flight; 
and her body is filled with eggs from end to end, even up into the rear of the head. To 
produce this great mass of eggs and to get them fertilized and back safely into the water, 
is her great end in life. 
The habits of the young nymphs that hatch from these eggs have not been observed. 
Doubtless they have many enemies, such as the predacious burrowing gomphine drag- 
onfly nymphs. They have also parasites. I found a large nematode worm filling the 
body cavity of one Mississippi River specimen, and most of the nymphs have their 
gills thickly beset with the cysts of some parasite unknown to me. 
The grown nymph of Hexagenia bilineata may briefly be described as follows : 
Length, 28 mm.; tails, 12 mm. additional; antenne,6 mm. Body pale, becoming purplish brown 
on abdomen and on gills, bare and shining on top of thorax, hairy around all margins. Frontal promi- 
nence of head shelflike, elliptical in outline when viewed from above, marked with a median dark dot, its 
margin fringed with pale hairs. There isa densely hairy circular ridge or fold surrounding the bases of 
the antennz externally. The long, strong mandibular tusks are bare and shining brown in color at 
their extreme tips, but bear a marginal line of hairs externally, the fringe becoming longer and denser 
toward the base. 
Prothorax with its side margins widened by a fringe of long, horizontally extended hairs. Fore 
legs stout and twisted. Femur ovoid, with a small lobe beside the apical articulation, hairy in longi- 
tudinal patches, the brushes short on the sides and very long on the edges. Tibix greatly dilated 
apically and further widened by marginal fringes of hairs, with a single, large apical tooth and that close 
beside the base of the short cylindric tarsal joint. Claw very short and stout, not more than twice as long 
as wide. Middle and hind legs slender, similar, each with a pincherlike prolongation of the apex of the 
tibia inferiorly, beside the base of the tarsus, the prolongation bearing an obliquely placed comb of short, 
stiff hairs. All expanded margins of all the legs bear dense brushes of yellow hairs. 
The hind wings of Keokuk nymphs show a distinct outer border of brown. 
Abdomen purplish brown above, darker on the middle segments, on each of which is included a 
pair of oblique, pale marks that are divergent at the rear. Gill on abdominal segment 1 rudimentary, 
tuning-fork shaped; on 2 to 7 large, composed of nearly equal lanceolate, long-tapering divisions that are 
broadly margined with whitish respiratory filaments. Tails stout, tapering broadly, fringed each side 
with tawny hairs, the very slender tips bare. 
