620 PHOTOPERIODISM IN INVERTEBRATES 



the experiment began January 12. Thus the larvae used in Experiment 

 3 had been in diapause over three months before the start of the 

 experiment, as contrasted with Experiment 2 where the larvae were 

 just recently in diapause. Perhaps during the period of diapause there 

 occurs a physiological development which enables the larvae collected 

 late in diapause to respond more readily to inductive daylengths. 



Additional evidence for the latter hypothesis comes from the fact 

 that a few larvae eventually pupate after being maintained for a very 

 long time on an 11 -hr photoperiod at 23 °C. In Experiment 7, for 

 example, an individual larva which was reared and maintained on an 

 11 -hr day length finally pupated after 152 days. Similar examples 

 came from mass cultures kept in the laboratory on an 11 -hr day 

 length for several months. In a cuhure collected July 16, 1953, several 

 pupations occurred on and following February 26, 1954, more than 

 7 months later, and in another culture collected October 10, 1954, 3 

 larvae pupated April 4, 1955, about five months later. These facts 

 suggest a gradual development toward a readiness to pupate, so that 

 eventually a few pupations occurred even on an 11 -hr photoperiod. 



It will be recalled that observations in nature indicated that pupa- 

 tions began in the spring on day lengths that were almost 1 hour 

 shorter than those in the fall when pupations ceased. Perhaps this was 

 the result of such a physiological development by the larvae during 

 the winter, enabling them to pupate on shorter photoperiods in the 

 spring than in the fall. It is planned that this will be a matter for 

 future research. 



When larvae received 12 hr of light per day plus an additional IVi 

 hr given either continuously with the 12-hr light interval or at different 

 times during the dark period ( Experiment 11), diapause was ter- 

 minated. Night interruption also effectively induced pupation even 

 when the total amount of light given daily was only 12 hr (lOVi hr 

 plus IVi hr night interruption), an ineffective amount when given as 

 a single continuous photoperiod. The greatest proportion of pupations 

 in both night-interruption experiments occurred when the Wi-hv 

 interruption was given either soon after the long light period ended or 

 shortly before it began. This could be indicative of a carry-over effect 

 of the light phase during the short intervening period of darkness, but 

 the evidence is adequate for only a suggestion. 



