II. A. SCHNKIDKRMAN 49 



of the wasp Mormoniella:'' At temperatures above i5°C, these larvae will remain 

 in permanent diapause until death ensues after a year or more. However, if at 

 any time the larvae are exp^osed to temperatures below i5°C for several months, 

 and then returned to higher temperatures, diapause ends and the larval-pupal 

 and pupal-adult transformations ensue. As has been observed in many other 

 species (27, 3), as the period of exposure to low temperature increases, upon 

 return to higher temperature diapause terminates more rapidly and a larger 

 percentage of the chilled larvae eventually develop. The most effective chilling 

 temperature is about 5°C"; lower and higher temperatures are less effective and, 

 as already noted, temperatures above i5°C' are totally ineffective. 



The lirst series of experiments were of the type familiar to an earlier gener- 

 ation of plant physiologists studying vernalization. Mormoniella larvae, 2-4 

 weeks after they had entered diapause, were exposed to alternating periods 

 at 5°(' and 25°C according to the following schedule: 



a) 5°C continuously 



b) 5°C for 7 (lays, 25° for i day, 5° for 7 days, etc. 



c) 5°C for 7 days, 25° for 2 days, 5° for 7 days, etc. 



d) 5°C for 7 days, 25° for 4 days, 5° for 7 days, etc. 



e) 5°C for 7 days, 25° for 7 days, 5° for 7 days, etc. 

 /) 5°C for 2 (la_\s, 25° for 2 days, 5° for 2 days, etc. 



After a cumulative exposure to 5° for ;/ weeks, 60 animals were removed from 

 each set of conditions, placed at 25° and their development observed. The 

 results, summarized in figure i, reveal that the effects of sub-threshold chilling 

 in terminating diapause can be undone by warming. Two days at 25°C undid 

 a great deal of the effects of 7 days at 5°, while 7 days at 25° completely undid 

 the effects of 7 days at 5° (31). 



These data are consistent with the hypothesis that at low temperature the 

 insect's brain synthesizes and accumulates a substance which, when it reaches 

 threshold concentration, enables the brain when returned to high temperature 

 to function as an endocrine organ and release the brain hormone. This sub- 

 stance is apparently broken down at high temperature, and it may be the 

 brain hormone itself. Chilling at temperatures above i5°C never terminates 

 diapause, and we may conclude that at temperatures above 15° the rate of the 

 breakdown reaction exceeds the rate of the synthetic reaction, so that the 

 substance never accumulates to threshold concentrations; below 15° the syn- 

 thetic reaction outstrips the breakdown reaction and the substance accumulates 

 to threshold levels. In other words, the temperature coefficient (Qio) of the 

 synthetic reaction is less than that of the breakdown reaction, and at about 

 i5°C the rates of the two reactions are equal (cf. 2, 3, 27, 48; 41, pp. 191-196). 



If this hypothesis is correct, and low temperature acts by effectively pro- 



Kindl>- supplied by Professor Phineas T. Whiting of the University of Pennsylvania. 



