MAY-FLIES AND THEIR NEIGHBOURS 213 



place towards sunset in still weather, and the almost 

 simultaneous emergence of myriads from their 

 pups is one of the most remarkable phenomena 

 of insect life. The most attentive observers have 

 failed to detect an interval of more than a tew 

 seconds between the floating of a pupa or nymph 

 to the surface of the water, the cracking of its 

 skin and the flight of the winged insect into 

 the air. 



But a most curious second emergence has still 

 to be performed, for the insect is not yet freed of 

 its vestures, being enveloped in a superfluous, 

 glove-like skin which has to be got rid of. So it 

 alights on some plant-leaf or stem to complete its 

 development, and there gradually frees itself from 

 all restraining bonds. Sometimes in so great a 

 hurry is it to soar on the newly developed wings, 

 that it will ascend before this last skin is removed, 

 finishing the process of development in the air, 

 and looking like two flies, for this skin is a perfect 

 im ige of the insect, even reproducing the long 

 and slender tail filaments. Fig. 141 shows a 

 magnified representation of one of these cast 

 skins. 



If difficulties should arise in this last emer- 

 gence, occasionally the flies will mate without 

 completing their development, and quite a large 

 number die without casting this last skin. If we 



