MAYFLIES, DUNS AND SPINNERS 243 



male and female olive duns, and 9 and 10 the two 

 sexes of the dark olive dun. I am inclined to think 

 that the two flies are varieties of the same species, 

 but at the same time I shall not be astonished if 

 entomologists at some future date give different spe- 

 cific names to these two varieties of the olive. It is 

 not unusual to see one hatching out exclusively on 

 one day and the other on a second day, and possibly 

 on a third day the two mixed up indiscriminately. 

 The male olive dun is figured in Plate XVIII, and 



on the same page a drawing is 

 Male olive. given to a larger scale of the 



head in profile, and the ven- 

 tral view of the tail end of the abdomen. All the 

 plates of insects excepting the mayfly are shown with 

 their wings flat, because entomologists usually figure 

 them in this attitude as giving a better opportunity of 

 displaying their parts for identification. The male 

 olive, pale watery, iron-blue, and blue-winged olive 

 (duns as well as spinners) are distinguished from the 

 female by the presence of what appears to be a tur- 

 binate cap on the top of the head. Of this Eaton 

 says : — 



" The ocu/i, always much larger in the male than 

 in the other sex, are in him, in some genera, divided 

 each into two parts transversely ; the upper portion 

 has larger facets than the lower, and is sometimes 

 differently coloured. The division between these seg- 

 ments of the oculus may amount to nothing more 

 than a mere superficial furrow or impressed line 



