pupal development and sub-optimal diets

[R.D.J. Catchpole]

Dear All,
Does anyone know of a reason why the length of pupal 
development should change in response to a sub-optimal 
larval diet? I've been looking at two species, 
D.virilis and D.melanogaster which respond in totally 
different ways. When the same number of larvae are 
placed on the same amount of food, larval development 
increases in D.melanogaster while pupal and total 
development time decreases. D.virilis does the 
reverse. The cause of changes in larval development 
are obvious but I cannot find any references to 
changes in pupal development, the accepted opinion 
appears to be that this doesn't happen. Any 
suggestions?????

Thanks in advance.
Roger