66 
It will be noticed that not in a single case do any measurements of 
the ditferent stages overlap. In abnormal individuals, or in rare cases 
where the larva undergoes onh^ 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 
larv83 reared from eggs of a single moth and on the same food, is 
shown graphicalh^ 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 64:. The length of the larval 
stage is thus seen to decrease from 37i da3^s, recorded in April, to 14 
da3'S 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. ^^^^^^^^^L_ gave onh^ negative results. The 
■ — effect of different diet on growth 
IV. I has been watched, but there 
Fig. 7.— Diagram illustrating relative width and vari- SCCmS tO bc but little relation 
ation ill width of the head casts of larvae in third between the twO, CXCept pOSsi- 
and fourth instars (original). i i • i • 
bly m one case, where it was 
found that larvee grew much faster on g-reen cowpea pods than on 
either corn or cotton. 
The effect of temperature on growth was also investigated by rear- 
ing larvae simultaneously at three different temperatures. This was 
done by means of a commercial chicken-egg incubator, which could be 
kept at a temperature of from 80° to 90° F. ; an ordinar}^ ice box, 
maintaining a temperature of from 50° to 60° F., and the laboratory 
breeding room, which w^as subject to the usual daily fluctuations in 
temperature. - 
During the latter half of April a lot of larv^ 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 be^^ond the third molt. The following- 
table gives a comparison of the three lots: 
