Bugs, Cicadas, Aphids, and Scale-insects 173 



ifir. 



mm. in width, an easy calculation shows that these conceivably possible de- 

 scendants of a single female would, if closely placed end to end, form a proces- 

 sion seven million eight hundred and fifty thousand 

 miles in length; or they would make a belt or strip 

 ten feet wide and two hundred and thirty miles long." 

 The remarkable plasticity of the aphids as re- 

 gards their possession or lack of wings and, on the 

 physiological side, their reproduction agamically or 

 sexually, introduces certain unusual conditions into 

 their life-history. Although each species is likely 

 to present idiosyncrasies of its own, a fair example 

 of the course of aphid life through a season may 

 be outlined as follows: In spring there hatch, from 

 eggs which have been laid the fall before, wingless 

 females, called stem-mothers, which produce young 

 agamically (i.e., from unfertilized eggs) either by 

 giving birth to them in active free condition or by 

 laying eggs. From these eggs hatch wingless females 

 which produce in turn other agamic broods of wing- 

 less females. But at any time in the course of these 

 successive agamic generations either all or a part of 

 the individuals of a brood may be winged, and these 

 winged females fly away to other plants and there 

 found new colonies which continue the series of 

 agamic generations. But toward the end of the 

 season, when the first cold weather announces the 

 approaching winter, broods, still parthenogenetically 

 produced, of sexed individuals, both males and fe- 

 males appear. "The males may be either winged 

 or wingless, but these true females are always wing- 

 less." These individuals mate, and each female 

 produces a single large egg which passes over 

 the winter to give birth in the following spring to a 

 wingless stem-mother that one which begins the 

 spring series of parthenogenetic generations. The unfertilized eggs, called 

 pseud-ova, produced in numbers by the spring and summer agamic mothers 

 (from which eggs the young frequently emerge while the eggs are still in the 

 body of the mother) should not be confused with the single fertilized egg 

 laid in the late fall by the mated females of the sexed generation. Although 

 these two sorts of eggs are alike in their earliest stages in the ovaries of the 

 females, differences very soon occur, the embryo in the pseud-ovum begin- 

 ning to develop before the formation of its own egg is properly completed. 



FIG. 246. Bodies of 

 aphids which have been 

 killed by Hymenopter- 

 ous parasites, the adult 

 parasitic flies having 

 emerged from the small 

 circular holes. (En- 

 larged.) 



