DIPTERA. 



177 



The skin of the larva is not cast off, but becomes the 

 cocoon of the pupa, simply growing stiff and angular. 

 In this condition they may be found floating on the sur- 

 face. When the perfect insect is complete, it escapes 

 through a fissure in the second segment, and after floating 

 about for a little time, supported by its old integument so 

 as to allow its wings to dry, it takes flight. 



The Wasp Flies (Eristalis\ everywhere to be seen on a 

 hot day, hovering before the flowers in our gardens, and 

 darting here and there with sudden jerking movements, 

 commence their lives under a very different aspect. Their 

 larva? are provided with a breathing apparatus, which 



FIG. 137. WASP FLIES. 



resembles the tail of a rat, and which they are able to 

 elongate or shorten, so as always to keep its tubular 

 extremity above the surface of the filthy stuff in which 

 these creatures live. They are often found in great 

 numbers in old tubs that contain stinking rain-water. 



The Gad Flies (Oestrus) have much the appearance of 

 humble-bees, and the hairs that densely clothe their bodies 



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