THE PUPAL STAGE OF CULEX. 51 



The pupa does not eat. It breathes air through the apertures at 

 the ends of its siphons. It floats, thorax upward, by virtue of a 

 large air cavity lying under the hinder part of the thorax and the 

 anterior part of the abdomen. This cavity is bounded in front by 

 the legs, at the sides by the wings, and below by the mouth-parts. 

 It extends up at each side of the first segment of the abdomen, 

 where it is covered by the halteres, and into this part of the cavity 

 at each side opens a large stigma, held open by the fairly well- 

 developed cuticular lining (" intima "), and guarded near its entrance 

 by numerous spines. These two stigmata belong to the first 

 abdominal segment, and put the air-cavity just described into direct 

 communication with the tracheal system. As already mentioned, I 

 regard this cavity and these stigmata as being mainly, if not 

 exclusive^, hydrostatic in function, serving not only to make the 

 pupa float when at rest, but to make it float in a definite position, 

 with the thorax upward and the apertures of the siphons at the 

 surface of the water. 



The pupa is sensitive to light, and immediately darts backwards 

 when a shadow falls upon it suddenly. The movements, however, 

 though very rapid, are devoid of anything like steering. The larva 

 had to steer in its search for food, but the pupa has simply to get 

 out of the way of danger, and the direction of its flight is of little 

 importance, though, since the movement is always backward with 

 reference to the pupa, it is chiefly downward with reference to the 

 outer world. 



A sudden very loud noise, or a very gentle tap upon the vessel 

 containing the pupse, causes those at the surface to dart downwards, 

 but as slight sounds of various kinds produce no effect upon them, 

 I conclude that the tremor of the surface of the water, and not the 

 sound itself, was recognised by them. The setae on the first segment 

 of the abdomen are probably the organs by which this movement 

 is felt. 



As to the anatomy of the pupa, it is only necessary now to state 

 that at the beginning of pupal life the internal arrangements are 

 those of the larva ; at the end of that period they are those of the 

 imago. 



At the beginning of the fifth day of pupal life, the cuticle splits 

 along the mid-dorsal line of the thorax ; the thorax of the imago 



