PHOTOPERIODIC CONTROL OF DIAPAUSE 619 



Lees, 1955) for the same species at Leningrad, U.S.S.R., 8° farther 

 north. It was pointed out that this should mean that mites at Leningrad 

 would enter diapause about 2 weeks earlier than those at Cambridge. 



Metriocnemiis knabi has a wide latitudinal distribution, and one 

 might expect to find evidence of similar geographic variation. The 

 present study involved experiments employing midges collected in 

 Maryland (ca. 38° 55' N. Lat.) and in North Carolina (ca. 35° 2' N. 

 Lat. ) ; there appeared to be little difference, if any, in the responses of 

 animals from these two latitudes, but the results of an earlier experi- 

 ment by Jenner (1951) suggest that geographical differences do oc- 

 cur. Midge larvae collected on May 8, 1950, at Petersham, Massa- 

 chusetts, were employed in an experiment started May 30. Five larvae 

 in separate culture dishes were placed on each of the following photo- 

 periods at room temperature: 6%, 11, 13V^, and 16Vi. After 46 days 

 only the 5 larvae on the 161/^-hr photoperiod had pupated; the failure 

 of the 13Vi-hr larvae to pupate would appear to demonstrate a dif- 

 ference in response from that obtained with Maryland or North Caro- 

 lina midges. Efforts to substantiate this difference have not yet resulted 

 in a satisfactory experimental test; thus, an intriguing question remains 

 for further research. 



The difference in rate of response on inductive photoperiods in 

 Experiments 2 and 3, both employing North Carolina larvae, offers 

 another interesting problem (Table I). In Experiment 2, 50% pupa- 

 tion on the 14-hr photoperiod occurred in 40 days and for the X'iVi-hv 

 group in 45 days, while in Experiment 3, this percent was realized 

 in 30 days and 36 days on the 13Vi-hr and 13-hr groups respectively. 

 Of the several possible alternative explanations to account for this 

 difference, two seem most worthy of consideration. First, the difference 

 might have been genetic, since the collections were made at different 

 places — Experiment 2 from Mulky Gap and Experiment 3 from 

 Duncan Valley, about 30 miles apart. Since Sarracenia purpurea 

 occurs only locally in the North Carolina mountains, this distance 

 might constitute an effective geographical barrier for these two midge 

 populations. A more probable explanation relates to the time when 

 the midges were collected and the experiments conducted. Experiment 

 2 employed larvae collected October 1 and was started October 12, 

 while the larvae for Experiment 3 were collected December 26 and 



