THE LIFE CYCLE 341 



of parasites from one host to another; and there are some 

 remarkable changes of form accompanying the shifts. For 

 example, the witch hazel aphid(fig. ig2)that causes the con- 

 icalmantle galls (shown at fig. 326) upon the leaves, and that 



FIG. 193. The same aphid sh'own in figure 192, in the form assumed aft: 

 migration to the birch, a, dorsal, b, ventral, and c, lateral views of 1 1 

 adult (after Pergande). 



grows up inside them, is of the ordinary form of the common 

 aphids during its life within these galls (two generations). 

 But in midsummer, when its food supply begins to be cut 

 off by the drying up of the galls, it migrates to a new host 

 plant. It flies through the air in search of birch trees, and 

 finding them, settles upon the under side of the leaves to 

 dwell there the remainder of the season. There it gives 

 birth to numerous young, which will grow up for three suc- 

 cessive generations into the adult form shown in figure 193. 



