THE TEEEAPIlSr SCALE. 



33 



The orchard larvae considered in Table XXIV, Part I, had very 

 favorable weather during their twigward migration. The relation 

 of this migration to the prevailing temperature is shown graphically 

 in figure 9. The migration curve shows the shortness of the interval 

 between the start of migration and its maximum. 



The migration started on August 13 and was 50 per cent complete 

 upon August 17. Thus, one-half of the migration was made during 





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Fig. 9.— Curve of the twigward migration of the terrapin scale for the orchard larvae of 1912. (Original.) 



the first four days of the period. This curve is typical for the 

 migration in favorable seasons. 



The larvae considered in Table XXIV, Part II, encountered very 

 unfavorable weather during the period of migration. Cold inter- 

 fered at the beginning, and rain continued throughout most of the 

 period. The effects of these conditions are shown graphically in 

 figure 10. The solid curve represents a normal migration, the dotted 

 curve the migration under prevaihng conditions. This graph 

 20782°— Bull. 351—16 — -3 



