EXCEPTION IN THE EPHEMERA, 325 



After the insect has once withdrawn itself from 

 the pupa-case, it generally retains the same appear- 

 ance and raiment as long as it lives, not casting its 

 skin like the larva, but having put on its permanent 

 clothing immediately upon its leaving the pupa- 

 case. But in the case of an aquatic insect, the 

 Ephemera, of which we have before spoken, a re- 

 markable exception to this rule has been noticed. 

 When these insects leave the pupa-case, any one, on 

 looking at them, would say that they had completed 

 their changes ; they appear to be furnished with 

 every part necessary to them, and not to have any 

 which is redundant ; yet they are destined to go 

 through a change equivalent to that which has just 

 taken place, if, indeed, it is not more apparently 

 difficult than it, and that is, they have to cast off 

 their skin. That they should be able to withdraw 

 from thence their head, their legs, their body, and 

 their long tails, would be no great difficulty for us 

 to comprehend, because numbers of insects at their 

 escape from the pupa-case do more than this ; but 

 in their case we are presented with a more per- 

 plexing enigma. In the transformation of other 

 insects, as we have already seen, and, indeed, in 

 that of the insects before us, the wings are at first 



