PLANT-EATING INSECTS 



211 



may flourish indefinitely on this tree. But the latter fly 

 baek to the spruce, on which they lay their eggs. 



Fifth. The young that hatch from these eggs develop 

 into a winged generation comprising both males and 

 females. After pairing, each of the latter lays in mid- 

 summer a single egg on the spruce, from which emerges 

 a wingless, hibernating foundress of the original type. 



Thus this fivefold life-cycle requires two years, and 

 two different coniferous trees for its completion, while 

 parallel series of unisexual (female) generations may be 

 established, the possible continuance of which has not yet 

 been determined. If we arrange the facts in the form 

 of a genealogical tree, we get the following : — 



Wingless Female 

 (forms gall on spruce). 



Winged Females 

 (resident on spruce). 



Wingless Females 

 (form galls on spruce). 



Winged Females 

 (migrate to larch). 



Wingless Females 

 (resident on larch). 



Wingless Females 

 (continue on larch). 



Winged Females 

 (My back to spruce). 



Winged Males and Females 

 (resident on spruce). 



Wingless Females 

 (form galls on spruce). 



Closely allied to Chermes is the dreaded Phylloxera — 

 a small aphid which was introduced from North America 



