HEMIPTERA 



67 



are remarkable for the numbers of the plants they attack, 

 few groups of plants being entirely immune, and for 

 their methods of reproduction and their fecundity. In 

 this group is illustrated the phenomenon of partheno- 

 genesis or reproduction by the 

 females without the intervention 

 of males. Accompanying this 

 phenomenon is another, alter- 

 nation of generations. In brief, 

 a typical life cycle for an aphid 

 is as follows: In the autumn 

 or at some other time during 

 the year, true males and females 

 appear. These mate and the 

 females produce true eggs, just 

 as do insects of other groups. 

 These eggs, when they hatch, 

 usually in the spring, produce 

 wingless females which in turn, 

 without the intervention of 

 males, give birth to living young. 

 These, upon becoming grown, 

 produce other living young, all 

 females, and either winged or 

 not. These forms are spoken of 

 as the agamic females. During 

 the summer the sexual individ- 

 uals appear as noted above. The males are either winged 

 or not, but the true females, which produce the winter 

 eggs, never have wings. There may be more than a dozen 

 generations in a year without any of the sexual individuals 

 and in some species the sexual forms have never been 

 found. The number of progeny which might, theoretically, 



FIG. 44. Woolly-aphis on 

 Apple Seedling. (Photo 

 by W. E. Rumsey.) 



