434 EFFECT OF TEMPERATURE UPON FACET NUMBER 



mediate between the 30 and 15° stocks. These intermediates were 

 to be expected. The parents were not removed from the bottles 

 until the end of the first 24 hours. Hence some of the larvas may 

 have been 24 hours older than others. Those 4 days old had already 

 passed the point X in development during which the facet number- 

 determining reaction is going on. They are also the first individuals 

 in the bottle to hatch. Those hatching later were not so far along 

 in development and hence were affected by the transfer to 15°. 



Obviously temperature is not capable of modifying the facet 

 nimiber throughout the immature stages, but is limited to a definite 

 stage in development. 



Subsequent experiments on Ultra-bar consider these points more in 

 detail. 



Effect of Temperature during the Pupal Period. 



Experiments of the same type as the preceding were carried out 

 on Ultra-bar at 27 and 15°. We will consider the effects of tem- 

 perature during the pupal period first. Subjecting the cultures to 

 27° for 5, 6, 7, 8, and 9 days before subsequent removal to 15° gave 

 counts which are characteristic of the 27° stocks both in mean and 

 in individual variation. The first four cultures had pupated before 

 the transfer to the colder temperature was made. In the last one 

 the imagos had begun to emerge from pupation as the transfer from 

 27 to 15° was made. Obviously in the last case a change of tem- 

 perature would have no effect, since there is no further change in 

 the number of facets after the adult is once formed. 



From the distribution of the counts in Table XV it is just as clear 

 that subjecting the immature stage to low temperature after pupa- 

 tion has no effect on the facet number. This is a particularly im- 

 portant point, since the facets themselves first become evident only 

 toward the end of the pupal period. The reaction which has deter- 

 mined the number of facets that are to form has greatly preceded 

 the actual formation of these facets (Fig. 11). 



