28 



J. PERCY BAUMBERGER 



TABLE 8 

 Effect of larval food on size of pupae 



the energy, wear and tear, growth and storage requirements. 

 On 3 per cent yeast the larval period approached normal at the 

 expense of the reserve stuffs in the pupa, for the latter is under- 

 sized. This is also true of 4 per cent yeast. On 24 per cent the 

 larvae usually reach a size of 6.5 mm. in length on the first day 

 and are therefore three days ahead of all larvae on media of 3 

 per cent yeast; still pupation occurs only 1.41 days earlier. Ap- 

 parently there is a certain periodicity in the larval life, since 

 there is a tendency for the larva to pupate after a certain length 

 of time whether it reaches the maximum size before this period 

 or is still undersized. The probable explanation of this phenom- 

 enon is that certain changes go on in the larva, since a meta- 

 morphosis of the nervous system and digestive glands is known to 

 take place during this period, at a definite rate if the minimum 

 necessary food substances are available. ^^ 



The pupal periods (table 5) of Drosophila, fed during larval 

 life on different concentrations of yeast, are also plotted in figure 

 6. The figures show that there is no consistent variation be- 

 tween the pupal period of larvae which lived for a long or a short 



1^ Mendel and Judson ('16) studied the proportional weights of skeletons of 

 retarded rats and found that the skeleton grows at a normal rate in retarded in- 

 dividuals. On normal food the growth of retarded individuals is accelerated, but 

 that of the skeleton is retarded till equilibrium between tissue and skeleton weight 

 is established. 



