A. GLENN RICHARDS 



147 



pletion" (1). This definition implies that there is a series of thresholds in 

 development of any species. Experimentally this is indeed found to be 



100 



10 20 30 



NO. OF DAYS AT 15° BEFORE TRANSFERRING 



Fig. 2. Development of Oncopeltus eggs at 30° following various lengths of in 

 cubation at 15°C. A, percentage hatching at 30°; B, time required to hatch after tr 

 feral to 30°. After Lin et ciL, 8. 



ans- 



10 20 30 



NO. OF DAYS AT 14° BEFORE TRANSFERRING 



40 



Fk;. 3. Development of Oncopeltus eggs at 30° following various lengths of incuba- 

 tion at the subthreshold temperature of 14°C. Hatching does not occur at any tem- 

 perature if the exposure to 14° is for more than 27-28 days. A, A', per cent hatching at 

 30° in two separate tests; B, time required to hatch after transferal to 30°. After Lin 

 et al, 8. 



correct. Johnson (7) defined three different thresholds for the egg stage of 

 the bedl)ug: a 'developmental threshold' below which no embryonic dif- 

 ferentiation occurred, a 'hatcliing threshold' below which no hatching 

 occurred, and a 'develojimental-hatching threshold' below which develop- 

 ment would not be followed by hatching. He found these three thresholds 



