PHOTOPERIODIC CONTROL OF DIAPAUSE 615 



DISCUSSION 



The life cycle of Metriocnemus knabi, like that of most organisms, 

 is attuned to a seasonally changing environment; such adjustment is 

 required for survival. During the spring, summer, and early autumn, 

 the population appears to pass through several generations, but with 

 the approach of winter the full-grown larvae enter diapause, a period 

 of developmental arrest. This physiological state is maintained 

 throughout the winter, until, in the spring, development is resumed 

 and pupation and emergence of adults occur. 



The most obvious environmental factors that might control the 

 timing of such a seasonal cycle are temperature, quality or quantity 

 of food, and day length. The present study indicates that temperature 

 and food are probably of secondary consequence as effective agents in 

 the seasonal adjustment of this species. Day length, on the other hand, 

 offers the perfect indicator of season, repeating the same cycle year 

 after year with a minimum of variation. For those species capable of 

 responding to it, unerring attunement to season is guaranteed. The 

 present investigation has established that it is day length which is of 

 primary importance in controlling seasonal diapause in Metriocnemus 

 knabi. 



This analysis of the response of M. knabi to day length shows that 

 photoperiods of 8 to 12 hr are diapause-maintaining, while those 

 longer than this are diapause-terminating. The location of the critical 

 day length range is, of course, important in determining when diapause 

 will be terminated in the spring and when it will be initiated in the 

 fall. As will be shown below, the 12- to 13-hr critical range established 

 experimentally for M. knabi correlates closely with the length of daily 

 illumination at that time in the spring and fall when the transition is 

 made by the organisms from the diapause to the nondiapause condi- 

 tion, and vice versa. 



In nature, the daily photoperiod to which M. knabi will respond 

 depends upon the minimum light intensity to which the larvae are 

 sensitive, which for this species was found to be below 0.0025 ft-c. 

 This appears to be the lowest threshold sensitivity yet recorded for a 

 photoperiodic animal (Lees. 1955). This extreme sensitivity brings 

 up the question as to why moonlight is not effective in inducing develop- 



