ii FLIES WITH AQUATIC LARVAE 119 



is caused by diminution of pressure. The sucker 

 implies such local diminution, and if a Simulium 

 larva, for instance, can employ negative pressure for 

 the purposes of attachment, it is conceivable that a 

 Corethra larva may be able to employ negative pres- 

 sure to extract a gas from its own blood for hydro- 

 static equilibrium. But until some mechanism able 

 to produce diminution of pressure has been found in 

 the air-passages of an Insect, the discussion is very 

 vague and unpractical. It will be remembered that 

 many other aquatic animals besides Insects can fill 

 some internal chamber of their bodies with a gas rich 

 in oxygen (see p. 37). 



The pigment upon the air-sacs of the Corethra-larva 

 may conceivably help in the respiratory process. 

 Pigment is not unusual in the respiratory organs of 

 Insects. The large tracheal trunks of some Dragon- 

 fly larvae are tinged with purple, and the tracheal gills 

 of some Caddis-worms show a pale violet colour. Gas 

 may be secreted by a chemical process in which the 

 pigment plays its part. 



The pupa floats in an upright position at the sur- 

 face of the water. At the head end is the great mass 

 composed of the head, wings and legs of the future 

 fly, all shrouded in the temporary larval skin. The 

 tail is broad, and like the tail-fin of a Lobster in shape. 

 It is the chief organ of locomotion possessed by the 

 pupa. When alarmed, the pupa descends a few inches 

 below the surface, by means of the powerful strokes 

 of its abdomen and tail. 



When pupation takes place the tracheal system of 

 the larva becomes disorganised. Most of the air 



