66 



It will be noticed that not in a single case do any measurements of 

 the different stages overlap. In abnormal individuals, or in rare cases 

 where the larva undergoes only four molts before it pupates, the meas- 

 urements are of course irregular and are here disregarded. 



The variation in head width, in Stages III and IV, in a lot of five 

 larvae reared from eggs of a single moth and on the same food, is 

 shown graphically in figure 7. 



INFLUENCE OF EXTERNAL CONDITIONS ON GROWTL. 



The effect of seasonal variation in temperature on the development 

 of the bollworm is easily noted by glancing down the right-hand 

 column of figures in Table XVIII, page Bi. The length of the larval 

 stage is thus seen to decrease from 37i days, recorded in April, to 14 

 days during the warmer parts of the summer, and to increase again in 

 the fall. 



Experiments were tried to ascertain if atmospheric conditions had 



any effect on growth, but they 



III. • gaveonly negative results. The 



I — effect of different diet on growth 



'V- I has been watched, but there 



Fig. 7.— Diagram illustrating relative width and vari- SCCmS tO be but little relation 

 ation in width of the head casts of larvae in third between the twO, CXCCpt possi- 

 and fourth instars (original). i i • i ■ 



bly m one case, where it was 

 found that larvae grew much faster on green cowpea pods than on 

 either corn or cotton. 



The effect of temperature on growth was also investigated by rear- 

 ing larvse simultaneously at three different temperatures. This was 

 done by means of a commercial chicken-egg incubator, which could ])e 

 kept at a temperature of from 80° to 90° F. ; an ordinary ice box, 

 maintaining a temperature of from 50° to 60° F., and the laboratory 

 breeding room, which was subject to the usual daily ffuctuations in 

 temperature. 



During the latter half of April a lot of larvae reared in the incubator 

 at an average temperature of about 85° attained full growth in four- 

 teen days, and the moth emerged twelve days later, making in all 

 twenty-eight days, as compared with forty-five days required in the 

 laboratory, where the average temperature was about 71°. In the ice. 

 box, where the temperature was below 60° most of the time, it proved 

 difficult to raise any larvae beyond the third molt. The following 

 table gives a comparison of the three lots: 



