36 BULLETIN 351, U. S. DEPARTMENT OE AGRICULTURE. 



relation of the temperature to the migration of this rearing is shown 

 in figure 11. 



This migration extended from September 8 to October 5, 1912, 

 under unfavorable temperature conditions. There was a drop in 

 the average daily temperature from 71° F. on the first day of migra- 

 tion to 56° F. on the sixth day (September 13). This was followed 

 by a favorable day, when the average temperature rose to 69° F., 

 after which it dropped gradually to 40° F. at the end of September. 

 There was a rainstorm of three days' duration, September 23, 24, 

 and 25, but it came too late to modify the rate of migration to any 

 extent. Under these conditions 95 per cent of the rearing had 

 migrated by the end of the tenth day of migration, or by the fortieth 

 day after emerging. The maximum daily migration was made by 

 this rearing upon the seventh day after the start of migration; that 

 is, upon the thirty-seventh day after emerging. A comparison of 

 the curves, figures 9 to 11, shows that the curve for. optimum con- 

 ditions (fig. 9) tends to have a perpendicular ascending slope and 

 a very steep receding slope, and that unfavorable conditions tend 

 to flatten the curve and to cause serrations in the slopes. 



TIME REQUIRED FOR MIGRATION. 



The migration is made during the hottest part of the day. Usually 

 very few migrating specimens can be taken until after 12.30 p. m., 

 because of the low morning temperatures. The larvae start upon this 

 migration when the temperature reaches 70° F. By 1.30 p. m. 

 larvae are usually moving in great numbers upon the twigs. By 3 

 p. m. nearly all migrants have selected their locations and perma- 

 nently established themselves. 



LONGEVITY OF MIGRANTS. 



In case the twigward migrants are prevented from attaching, they 

 can five for 2 days, and a small percentage even for 3 days before 

 dying. 



The following experiment was made to determine the longevity of 

 a twigward migrant when prevented from attaching. A quantity of 

 material was kept under observation from August 30, 1913, until one 

 of the larvae was observed to start upon its migration. This speci- 

 men was then placed upon wrapping paper for observation. The 

 details of these observations are shown in Table XXV. 



