196 APPLIED ENTOMOLOGY 



and ground. This fluid which is sweet and sticky is eagerly fed upon by 

 ants. Falhng on twigs and leaves it dries there and a fungus grows in it 

 turning it black, and plants where aphids have been abundant often show 

 this by their black appearance. Some plant lice produce galls within 

 which they live for at least a part of their lives but most of them are not 

 thus enclosed, living on leaves, twigs, succulent plant stems or roots. 



Though there are great variations in the life histories of different 

 aphids, certain general facts hold for most of the group. In general, eggs 

 are laid in the fall, on a food plant of the species concerned, and these 

 hatch the following spring. The nymphs soon become full-grown and are 

 known as "stem-mothers" and without fertilization (there are no males) 

 produce eggs, or in most cases living young which like the stem-mother 

 are all females and on reaching maturity produce young in a similar way. 

 The production of young without fertilization of the parent is not uncom- 

 mon in insects and is called parthenogenesis or agamic reproduction. In 

 this case the production of these young alive rather than from deposited 

 eggs introduces the additional fact that these insects are also viviparous 

 except in (generally) one generation. The number of young produced by 

 each parent varies but will perhaps average about ten, a few being born 

 every few days, and the number of generations is variable but is also likely 

 to be about 10, though the first born young in each generation, being a 

 week or two older than the last born young, will gain enough time during 

 the season to produce more generations than the others. In fact, in 

 some species a range from 8 to 21 generations for late and early born 

 individuals has been observed, and an average number of 28 young pro- 

 duced per parent, so that the figures given above may be regarded as 

 conservative. But even with this moderate estimate, allowing only 10 

 young to a generation and 10 generations a season, the total product from 

 a single egg hatching in the spring and itself counted as the first generation, 

 would be 1,111,111,111, and this would be far below the actual number in 

 most cases, were it not for the enormous destruction of these insects by 

 their enemies and by unfavorable weather conditions. 



In many species instead of 10 young being produced per female as an 

 average, the number is likely to be nearer a hundred, and in those species 

 which also have more than 10 generations the total number of individuals 

 which would theoretically be produced in a season "would be sufficient to 

 completely cover the entire world with a continuous layer of plant 

 lice." 



With such a marvelous reproductive power as this it becomes evident 

 that despite natural checks to their increase, plants infested are liable 

 after a few weeks to be entirely unable to provide food for the hordes of 

 plant lice upon them. Accordingly we find that in most of the genera- 

 tions winged individuals may be produced so that they can migrate to 

 other plants. Winged and wingless forms may therefore be found at 



