The May-flies and Stone-flies 
69 
condition, being divided, each into two parts, by a narrow impressed line 
or by a broader space (Fig. 105). The two parts differ in the size of the 
facets of the ommatidia, i.e., eye-elements, and it has been ascertained (Zim¬ 
merman, 1897) that this difference in size of facets 
is accompanied by other and more important 
structural differences, which make it certain that 
the two parts of the eye have different powers of 
seeing. One part is especially adapted for seeing 
in the dark, or for detecting slight differences in 
intensity of light, but is ill-fitted for exact sight, 
while the other part is adapted for seeing in 
daylight, and for making a more exact picture of 
outline. As the mating flights occur usually at 
twilight or in the evening, Zimmerman believes 
that this modification of the eyes of the males 
is to enable them to discover the females in the 
whirling shadow-dances. Chun has recorded a 
similar division and difference in the eye of 
certain ocean crustaceans and believes that the 
“dark eyes” are used for seeing in the dimly Fl( fi i° 4 ' May " fly ’. C(Bms 
,. , l T . . . ... . .... . dirmdiata, possessing only- 
lighted water below the surface, while the light one pair of wings. (Much 
eyes” are for special use at the brilliantly lighted enlarged.) 
surface. I have noted similar conditions in the eyes of both male and 
female net-winged midges (Blepharoceridae), small, two-winged flies of 
particularly interesting life (see p. 319). It is unusual to find such parallel 
adaptations in forms so unrelated. 
The May-flies show an anatomical condition of much interest to ento¬ 
mologists in the paired openings for 
the issuance of the eggs. Insects have 
their organs arranged in pairs, one on 
each side of the middle line of the 
body, as the legs, wings, mouth-parts, 
antennae, eyes, spiracles, etc., or exact¬ 
ly on the middle line, as the heart, 
alimentary canal, and ventral nerve- 
cord. That is, the typical insect body 
is bilaterally symmetrical, and the 
more apparent this symmetry is the sim¬ 
pler and more generalized the insect 
is believed to be. All other insects but the May-flies have the two egg- 
ducts, one from each egg-gland, fused inside the body, so as to form a short, 
single, common duct on the median line. But the May-flies have the ducts 
Fig. 105. —Section through head of 
male May-fly, Potamanthus brunneus, 
showing composition of compound 
eye and two sizes of eye-elements 
(ommatidia). (After Zimmer; greatly 
magnified.) 
