622 PHOTOPERIODISM IN INVERTEBRATES 



SUMMARY 



1. Both the initiation and termination of larval diapause in Metri- 

 ocnemus knabi were shown to be under the control of day length. 

 Long days prevent and short days induce diapause; the entire critical 

 range separating long and short photoperiods was between 12 and 

 13 hr. 



2. Larvae were sensitive to light intensities below 0.0025 ft-c and 

 intensities from 0.0025 ft-c to 35 ft-c were equally effective. Such 

 extreme sensitivity indicates that larvae in nature respond to a portion 

 of the daily twilight period. 



3. The timing of events in nature correlated well with the occur- 

 rence of the experimentally determined effective photoperiods of 

 critical duration; however, in nature, diapause was terminated in the 

 spring when day lengths were shorter than those which prevailed 

 when diapause was initiated in the fall, indicating a physiological 

 conditioning. 



4. Temperature modified the photoperiodic response. Inductive 

 photoperiods were most effective at intermediate temperatures (23° 

 to 25°C). Constant low temperatures (12° to 8°C) reduced the rate 

 at which larvae responded; constant high temperature (ca. 30°C) 

 prevented pupation. 



5. Diapause occurred only in fully grown larvae. 



6. Ten to fourteen successive inductive cycles were required to 

 produce any pupations in diapausing larvae. Short photoperiods 

 introduced into a regime of inductive day lengths reduced the effective- 

 ness of the long photoperiods when given as often as every fourth 

 day; short photoperiods alternated with long were almost completely 

 ineffective in inducing pupation. 



7. When larvae on a diapause-inducing day length had the cor- 

 responding long night period interrupted by 1 Vi hr of light, pupations 

 occurred even though the total amount of light given per 24 hr was 

 less than the normal threshold duration when given as a single period 

 during each 24 hr. 



8. The effect of continuous darkness was similar to that produced 

 by long photoperiods, a feature shared with many other photoperiodic 

 arthropods. 



