610 PHOTOPERIODISM IN INVERTEBRATES 



photoperiod; no pupations, fed or starved, occurred on the 11 -hr 

 photoperiod. Starvation apparently reduced slightly the response to 

 inductive photoperiods, but did not prevent pupations altogether. 



Photoperiod and Larval Growth (Experiment 7) 



Experiments described thus far employed fully grown larvae in 

 diapause. In order to determine whether diapause could occur at some 

 earlier stage in larval development, several of the smallest larvae 

 (about 2 mm in length) that could be found in a collection made in 

 North Carolina on October 1, 1955, were isolated individually. On 

 October 11, 1955, 7 of these were placed on a 131/2 -hr photoperiod 

 and 8 on an 11 -hr photoperiod (Experiment 7). Examinations made 

 at about 10-day intervals indicated that growth rate was essentially 

 equal under both conditions. No exuviae were seen, probably because 

 of their delicate nature. After about one month larvae under both 

 conditions were of a size of those normally used in experiments (about 

 6 to 7 mm in length). After 48 days one larva on a 13!/2-hr photo- 

 period pupated, as did the remaining 6 by the 72nd day. After 81 

 days, during which time no pupations had occurred on the 11 -hr 

 photoperiod, 4 short-day larvae were placed on a 13l^-hr photoperiod; 

 all of these pupated within 34 days after transfer. On the 11 -hr day 

 one pupated 152 days after the beginning of the experiment while 

 three still remained as larvae 190 days from the beginning of the 

 experiment. These results indicate that diapause induced by short 

 photoperiods must occur only in the last larval instar. Larvae grown 

 at favorable temperatures and photoperiods develop directly without 

 the intervention of diapause. 



Inductive Cycle Requirement (Experiments 8, 9, and 10) 



Two experiments were conducted to determine the number of 

 consecutive inductive cycles required to initiate pupation in diapaus- 

 ing larvae. In one experiment (Experiment 8) groups of 10 larvae 

 each were exposed to an inductive photoperiod of 13 hr from to 28 

 days, depending on the group, and then were placed on a noninductive 

 photoperiod of 11 1/2 hr. Results (Table IV) show that at least 10 

 inductive cycles were required to induce pupation, and maximum 

 response was approached only in groups receiving 19 or more indue- 



