18 THE LIFE-STORY OF INSECTS [OH. 



twigs ; these resistant eggs carry the species safely 

 over the winter. At springtide, when the leaves 

 begin to sprout from the opening buds the aphid 

 eggs are hatched, and the young insects after a series 

 of moults, through which hardly any change of form 

 is apparent, all grow into wingless 'stem-mothers ' 

 much larger than the egg-laying females of the 

 autumn. The stem-mothers have the power, unusual 

 among animals as a whole, but not very infrequent 

 in the insects and their allies, of reproducing their 

 kind without having paired 1 with a male. Eggs 

 capable of parthenogenetic development, produced 

 in large numbers in the ovaries of these females, give 

 rise to young which, developing within the body of 

 the mother, are born in an active state. Successive 

 broods of these wingless virgin females (fig. 6 a) 

 appear through the spring and summer months, and 

 as the rate of their development is rapid, often the 

 whole life-story is completed within a week. The 

 aphid population increases very fast. Later a gene- 

 ration appears in which the thoracic segments of the 

 nymphs are seen to bear wing-rudiments like those 

 of the young cockroach, and a host of winged 

 females (fig. Qb) are produced; these have the power of 

 migrating to other plants. We understand that wings 

 are not necessary to the earlier broods whose mem- 

 bers have plenty of room and food on their native 



1 Such virgin reproduction is termed 'parthenogenesis.' 



