26 



BULLETIN 351, U. S. DEPARTMENT OF AGRICULTURE. 



The second column in each table gives the age in days. This is 

 calculated from the time the larva? left the brood chamber. The 

 sixth column gives the date upon which the specimens emerged. 

 There are added to Table XIX two columns of data to show the 

 percentages of larvae in the first instar and second instar at different 

 ages. An examination of these columns will show that 50 per cent 

 of the larva? had, passed from the first to the second instar upon the 





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Fig. 8.— Growth curves for the first instar of the terrapin scale. (Original.) 



eighteenth day, and that all had left the first instar by the twenty- 

 fourth day. Eighteen days is the normal time spent in the first 

 instar by larva? during favorable seasons. Figure 8 shows the deflec- 

 tion of the growth curve for larva? in the first instar which resulted 

 from the late emergence during the unfavorable season of 1912, as 

 compared with the curve for the favorable season of 1913. These 

 curves are derived from the data in Tables XVIII and XIX. The 

 curves are similar, but the broken curve shows clearly the effect of 

 unfavorable weather in 1912 at both the beginning and the end of 

 the instar. 



