EPHEMERID/E OF THE CHALK STREAMS. 9 



problem presents any serious difficulty. Turning first to the 

 <pit'stion of condition, we find that the wings of duns are dull 

 and covered \vith short hairs, especially at the lower edges : 

 whilst the wings of spinners are transparent, shiny and hair- 

 less. No very close scrutiny is necessary to distinguish between 

 them, especially if assistance is obtained from a small magni- 

 fying glass, while the matter will be settled at a glance as soon 

 as a little experience has been gained. On the wing the dun 

 is slow and clumsy, but the spinner is active and alert and 

 sometimes takes a lot of catching. 



The difference between the sexes is well and clearly marked. 

 Males, whether duns or spinners, bear a token at each 

 extremity. At the head their large and highly coloured eyes 

 give an immediate clue to their sex, and if t.his is not enough 

 an inspection of the lower end of the abdomen, beneath the 

 roots of the setae, will reveal what are known as the " abdo- 

 minal forceps." These are not so well marked in duns as in 

 spinners, but they are nearly always discernible with the naked 

 eye, and glaringly obvious when sought with a glass. 



In the case of a male spinner the setae are as a rule half 

 as long again as the rest of the fly, sometimes twice as long, 

 whilst the front legs are at least twice as long as any of the 

 remaining four. The front legs of female spinners and duns 

 of both sexes are not elongated, their setae are roughly the 

 same length as the rest of the fly. The eyes of females of 

 both conditions are small and dark, placed well to the sides 

 of the head. 



Apart from the eyes, duns as a rule are very much alike 

 in' both sexes, but spinners often show great variety. Some- 

 times, however, they also bear a fairly close resemblance, as in 

 the case of the well known Blue- winged Olive ; here the sexes 

 are inclined to be nearly matched as far as colouring is con- 

 cerned, but of course the males show the usual differences in 

 eyes, forelegs and setae. The spinners of the Blue-winged Olive 



