II 



FLIES WITH AQUATIC LARVAE 



133 



skin. The pupa is distinguished from that of most 

 other aquatic Diptera by the tufts of respiratory fila- 

 ments which project from the prothorax. It lies half 

 buried in the mud at the bottom of the water with the 

 thorax and respiratory filaments projecting. These 

 are swayed to and fro by the continual 

 bending of the body. At the tail-end 

 two lateral flaps, fringed with long 

 bristles, form a broad fin, which is 

 used to a limited extent for locomo- 

 tion. The pupa virtually consists of 

 the body of the fly inclosed within a 

 transparent skin. The organs are al- 

 ready complete externally, and even 

 in microscopic detail they very closely 

 resemble those of the perfect Insect. 

 The parts are, however, as yet very 

 imperfectly displayed. The wings and 

 legs are folded up along the sides of 

 the body, and are incapable of inde- 

 pendent movement. The red colour 

 of the blood gradually assumes a 

 darker shade. After two or three days FlG 39> _ Pupa of 

 the tracheal system, which was rudi- chironomus, side- 

 mentary in the larvse, but is now 

 greatly enlarged and extended, becomes filled with 

 air extracted from the water by means of the re- 

 spiratory tufts, and the pupa floats at the surface. 

 On the last day or so of the pupal stage, air passes 

 through the spiracles, and inflates the pupal skin. 

 At length the skin of the back splits, the fly extri- 

 cates its limbs and appendages, pauses for a moment 



