40 



BULLETIN 351, U. S. DEPARTMENT OF AGRICULTURE. 



A third larva. Table XXVI, taken September 1, 1912. at 11 a. m., 

 while in the first day of migration, gave a maximum rate per hour 

 of 101 cm., at 88° F., and an average rate for 2 hours of 92.05 cm. 

 (Fig. 14.) ' 



The rate of migration depends both upon the temperature above 

 70° F. and upon the length of time that has elapsed since the migra- 

 tion started. 



It is very exceptional for specimens to require more than one day 

 for the twigward migration. More than 90 per cent of the females 



Fig. 14.— Tracing of a female larva of the terrapin scale for the first two 

 hours of the twigward migration. K educed S times. Total distance, 

 1S4.1 cm. (Original.) 



complete this migration during the first 4 hours. Females may 

 occasionally shift to a more favorable position during the second 

 and third days' sojourn upon the twigs, but after this they remain 

 in one place. 



During the day preceding the twigward migration the female larvae 

 secrete a thin scale which covers the dorsal surface and gives rigidity 

 to the flat, leaf -shaped body. The larva 1 in migrating pass down the 

 petiole and move along the twigs toward the region of greatest illn- 



