72 NATURAL HISTORY OF AQUATIC INSECTS CH. 



we should do better to say to ourselves : * Every 

 time that our weak perceptions have enabled us to 

 read the purpose of Nature, we have found the marks 

 of a wisdom superior to our own. It is therefore rash 

 .and arrogant to pronounce useless structures whose 

 purpose we are unable to assign. Although we cannot 

 imagine of what use these hooks can be to a pupa 

 buried in the earth, we may be well assured that they 

 have a definite purpose.' We shall presently see that 

 these parts are so necessary that the pupa would run 

 some risk of perishing if it were deprived of them. 



" This Insect, although aquatic, breathes air. In its 

 larval condition, as we saw, it brings its tail from time 

 to time to the surface of the water. In the pupal 

 state it can no longer move its limbs, and it is this 

 apparently which compels it to leave the water in 

 order to undergo its transformation. It creeps out 

 upon the shore of the pond or ditch in which it has 

 hitherto lived, and there, in some moist place, it digs 

 out a cell, whose walls it secures by pressure, and prob- 

 ably also by gluing them together with a tenacious 

 substance which is seen to exude from the hinder 

 part of the body when the Insect is annoyed. Within 

 its cell the Insect rests for a time. Its body swells up, 

 and at the same time shortens. The new parts form 

 beneath the larval skin, which at length splits, and the 

 pupa emerges. This it does with ease when the skin 

 is moist, but with great difficulty, as we have seen, 

 when it has become dry. It was the lack of moist 

 earth which explained why several of my larvae re- 

 fused to enter the ground ; why others, having entered 

 it, came out again, and why yet others perished 



