The May-flies and Stone-flies 



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 FlG - 104. May-fly, Cmis 

 .. , ' , , . .. . ditmdtata, 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.) 



