PHOTOPERIODIC CONTROL OF DIAPAUSE 



617 



JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC 



Fig. 3. Day length and pupation in nature. Day length curves were 

 drawn from data "taken from Tables of Sunrise, Sunset, and Twilight, 

 issued by the Nautical Almanac Office, U. S. Naval Observatory, 1946. 

 A, larvae collected 9 days earlier began pupating; B, larvae pupated day 

 after collection; C, larvae pupating in nature; D, larvae no longer 

 pupating in nature. 



and sunset, a second for this period plus civil twilight, and a third 

 for this period plus astronomical twilight. Light intensity at the end of 

 civil twilight (sun 6° below horizon) on a clear evening is about 0.4 

 ft-c; when the sun is 10° below the horizon, it is slightly less than 

 0.01 ft-c (at end of astronomical twilight the sun is 18° below the 

 horizon) (Kimball, 1916). Because of the very low threshold intensity 

 for the response of Metriocnemus knabi, it seems apparent that a part 

 of astronomical twilight must be effective. In Fig. 3 horizontal lines 

 have been drawn at 12- and 13-hr photoperiods to mark the critical 

 range for the midge's response, and vertical lines mark the approxi- 

 mate time pupation was found to begin in the spring and the period 

 within which it ceased in the fall. Although there appears to be a close 

 correlation between the timing of events in nature and the occurrence 

 of effective day lengths of critical duration, it is interesting that pupa- 

 tion began in the spring on photoperiods that were perhaps 1 hr 

 shorter than those on which it ceased in the fall, a point which will be 

 considered later in the discussion. 



Temperature plays only a modifying role in the control of diapause 



