45° 'Journal of Corjiparative Neurology and Psychology. 



different individuals in accordance with their degree of buoy- 

 ancy. 



Doubtless this behavior which occurs as a result of food scarcity 

 is in a certain degree abnormal. But there is always a variation 

 in specific gravity. In any lot of Corethras some may be rising 

 from buoyancy and others falling, though in normal conditions 

 the great majority appear nearly or quite motionless. The air sacs 

 exert a dynamic function in causing righting movements, since 

 they automatically bring the animal into horizontal position with- 

 out muscular effort. After every downward movement this is 

 noticed, in the automatic restoration of the horizontal position. 



THE PUPA. 



There is a marked change of behavior in the pupa. Certain 

 structural changes occur. They acquire a pair of broad flat cau- 

 dal plates, by which they propel themselves by powerful strokes. 

 They also acquire a pair of external floats at the forward end. 

 The body becomes gradually more opaque. The pupae stand 

 almost erect and have a ventral flexure. The locomotion becomes 

 rapid and more continuous, though a tendency to discontinuity 

 persists. The negative reaction to jarring, etc., becomes more 

 intense. Merely hfting the dish in which they are placed sets 

 them off into rapid flight, a feature unnoticed in the larvae. The 

 positive light reaction also becomes well marked. They will 

 move to the lighted end of the trough often without pause. If the 

 pupa is stimulated at the posterior end it commonly makes a 

 single movement. If stimulated elsewhere it usually makes a 

 number of movements in quick succession, indicating a tendency 

 to develop continuity of movement. If highly excited, as when 

 the dish in which it is placed is lifted, its movements become really 

 continuous. They have thus lost the two characteristic larval 

 features of asymmetry and discontinuity. In these changes of 

 behavior the pupae approach more nearly to the ordinary type 

 of behavior, showing the effects of the loss of transparency, which 

 necessitates the development of pupal organs, such as the caudal 

 plates and the floats, which are of use for only the few days of the 

 short pupal period, but which greatly favor its security. Towards 

 the close of the pupal period, it rises to the surface and when stim- 

 ulated darts downward. 



