MAY-FLIES. 25 



complete transparency of many of these larvae the entire 

 circulation of the blood may be easily seen if the insect 

 be placed in a shallow vessel of water and viewed under 

 a magnifier of moderate power. 



The food of these larvae is apparently very varied. The 

 Rev. E. A. Eaton, who is probably one of the greatest 

 British authorities on the EpJiemeridce, says that though 

 sometimes the stronger larvae devour the weaker, yet 

 the diet is even in these cases partly vegetable. The 

 alimentary canal frequently contains much mud ; very 

 small organisms, such as diatoms and confervae, are 

 thought to form a large part of the bill of fare of 

 Ephemerid larvae. Although the mouth is atrophied 

 in the imago, it is highly developed in the larva. As 

 soon as the May-fly larva attains maturity the winged 

 insect escapes through an opening in the back, slowly 

 flies ashore, and afterwards rests for a variable period, 

 according to the species, amongst the vegetation on the 

 bank of the stream. In this condition the insect is 

 termed the sub-imago, as a further transformation after- 

 wards takes place, i.e., the shedding of another and 

 extremely delicate skin from the entire body and appen- 

 dages of the insect. After this second change the wings 

 are more transparent, the legs and tails considerably 

 longer than before, and the insect is then in its final, 

 or imago, condition. The sub-imago is an extra phase 

 of life peculiar to the May-flies, and, so far as is known, 

 does not occur in any other family of insects. In most 

 insects, however, an extremely delicate inner membrane 

 can be detected inside the ordinary skin of the nymph, 

 or pupa, this membrane being no doubt analogous to 

 that which invests the sub-imago of the May-fly. There 

 is, however, this remarkable difference — in the May-fly 

 the inner skin is retained for a considerable period after 

 the insect has left the nymph, that is during the sub- 

 imago state ; in other insects the two skins are shed 

 simultaneously, one within the other, and in these no- 

 such stage occurs. 



In localities where May-flies are abundant, numerous 

 specimens of the sub-imago may be seen flying away from 

 the water in almost a straight line, with a slow and steady 

 flight, generally about fourteen feet above the ground. 

 This flight occurs during the early twilight, and is so 

 characteristic that when once recognised it cannot be 

 mistaken. It is very distinct from the peculiar up-and- 

 down flight of the imago about to be described. 



