36 



the larval skin will be found still enveloping most of the abdominal 

 segments, the head having slipped round to the ventral surface. But 

 when the pupa finally leaves the case the skin is left behind. 



THE PUPAL LIFE OF C. PUSIO. 



The pupal stage in warm weather lasts four days, but towards 

 winter it may become much longer. The activities of the pupa chiefly 

 consist of continuous movements of the abdomen which no doubt aid 

 in respiration. In larva No. i the movement causes a strong current 

 to pass in at the posterior end of the case and out through the central 

 opening of the operculum ; but in C. pnsio very little current can be 

 detected and what there is seems to be in the opposite direction ; the 

 weakness of the current is due to the very small size of the central 

 hole of the operculum. During the four days of the pupal stage 

 considerable change takes place in the abdominal spines and hairs ; at 

 first quite transparent and very difficult to see, they gradually become 

 stiffer and more chitinous. This fact may help to explain how it is 

 that when development is complete the pupa is able to leave its case. 

 If a one-day old pupa be partially extracted from its case by gently 

 pressing the tube, it will when left alone withdraw again into its 

 original concealed position by means of the undulatory movements of 

 its abdomen. It seems probable that at that time the backwardly 

 projecting spines are not sufficently strong to grip the walls of the case. 

 If the spines harden when the imago is ready to emerge, then during 

 the movement of the abdomen the spines will grip the walls, and 

 prevent any backward movement, while forward movements will be 

 facilitated. This will cause the head and thorax to press against the 

 operculum until it finally gives way, allowing the exit of the pupa into 

 the stream. Thus the hardening of the spines may with some truth be 

 said to compel the pupa to leave its case. 



Emergence of the And now the pupa, lashing the water vigorously by means of its 



powerful tail-fin, swims to the surface, where the back of the thorax 

 immediately bursts the surface-film, and the thorax itself splitting along 

 the back allows the fly, when the right moment has arrived, to liberate 



