212 NATURAL HISTORY OF AQUATIC INSECTS CH. 



wrinkled, but still retains a considerable length ; the 

 body shortens and becomes rather thicker than 

 before. After ten or twelve days the skin is hard 

 and opaque, and to a great extent detached from the 

 pupa within. 



After twenty-four or thirty-six hours from the time 

 of pupation two pairs of horns appear at the head 

 end. The fore pair, is about half the length of the 

 other, and the two pairs often curve towards one 

 another. The fore pair, when diligently observed, are 

 found to be present in the larva, but they become 

 much more conspicuous at the time of pupation by 

 the smoothing out of the adjacent parts. The large 

 posterior pair appear for the first time shortly after 

 pupation. These horns are really the respiratory 

 organs of the pupa, as is clearly shown by the large 

 tracheal trunks which enter them. 



The process of pupation is similar in Eristalis and 

 the Blow-fly, though more rapid in Eristalis. Twenty- 

 four hours after the posterior horns have appeared, 

 the proboscis, the wings and the legs may be seen 

 by opening the cocoon. The long tail has now no 

 connection with the body of the pupa. Near its base 

 are found a number of coiled tracheal tubes, collected 

 into a bundle. The main tracheal trunks, however, 

 persist. 



In a favourable season the fly is ready to emerge 

 eight or ten clays after pupation begins. The part 

 of the cocoon contiguous to the four horns is pushed 

 off in two unequal pieces. When this happens two air- 

 vesicles appear, one attached to each of the posterior 

 horns ; the smaller horns each receive a large trachea. 



