6 EPHEMERID^ OF THE CHALK STREAMS. 



two hind-most segments carry the wings which are always 

 held rigidly erect when their owner is at rest. The hind wings 

 are always very small, and are sometimes altogether absent. 



(iii) The abdomen consists of ten segments tapering to a 

 graceful point, where we find an attachment known scientifi- 

 cally as the " setae," but more generally referred to by flytyers 

 as the " whisks." Some genera have three setae, others only 

 two, which should not be forgotten, as it affords much help 

 in identification. Of the forty species classified by the Pro- 

 fessors, fifteen are decorated with three setae, while twenty-five 

 possess only two, so that a casual glance at the tail-end of a 

 victim must immediately reduce the possibilities to terms 

 considerably more reasonable than the rather alarming propor- 

 tions which they originally assumed. 



The metamorphosis of the Ephemeridse is incomplete, that 

 is to say, the insect in its immature stages bears an appreciable 

 likeness to the adult form. At no period of its career is it 

 quiescent after it has once made its way out of the egg. 



The life of every member of this family is divided up into 

 four very distinct stages or conditions, (i) The egg. (ii) The 

 nymph, (iii) The subimago commonly called the Dun except 

 in the case of the Mayfly, (iv) The imago generally known 

 as the Spinner, again with the exception of the Mayfly. 



(i) and (ii) The egg lies on the bed of the river until in due 

 course possibly a month, sometimes much longer the nymph 

 emerges. This microscopic creature is invariably very active 

 and very hungry; it eats anything it can get, and as much of 

 it as it can hold. Decaying vegetation, or any other kind of 

 offal, serves for food, some species are carnivorous. This 

 large appetite produces rapid growth, with the result that 

 before long the nymph feels a certain tightness of the skin. 

 This troubles him not at all, for when he arrives at bursting 

 point he simply bursts and crawls out of a slit in his own 

 back, after which he carries on as before, forming for 

 himself a new 1 skin as he goes along. This swelling and 

 bursting process recurs at short intervals in the early stages 



