26 H. E. EWING. 



species was already reproducing parthenogenetically under 

 laboratory conditions, I decided to take up for the third time 

 heredity work with a parthenogenetically reproducing insect. 

 This time my efforts have been rewarded with some measure of 

 success. Most of the results obtained up to the present are here 

 submitted. 



LIFE HISTORY AND HABITS OF APHIS AVEN.E FAB. 



Before submitting the results of these experiments, and 

 entering upon a discussion of them, I shall review briefly the life 

 history and habits of the species under consideration. 



As is true of aphids in general, this species propagates itself 

 chiefly by means of parthenogenesis, but under certain conditions 

 (probably due chiefly to climatic changes) the true sexes appear, 

 mating occurs, eggs are deposited, and the parents die. In the 

 case of this species, according to Pergande, the true sexes do not 

 appear annually under out-of-doors conditions; but the asexual 

 females live over the first winter, the true sexes being produced 

 the next fall. Thus two years are required to complete the 

 cycle from egg stage to egg stage. 



Aphis avence lives on a large number of plants, but is especially 

 injurious to small grains and grasses. It will frequently winter 

 over on apple, pear, plum, or other deciduous trees. The eggs 

 that have passed the winter out of doors usually hatch about the 

 middle of March in most of the eastern states. It takes these 

 young aphids about a month to reach maturity; when they are 

 called the stem-mothers. The offspring of these stem-mothers 

 are all asexual females, and produce parthenogenetically, for 

 several or many generations, other asexual individuals. These 

 asexual females are of two classes; winged and wingless. Al- 

 though the winged or wingless condition of these two classes 

 constitute the chief ground for their differentiation there are 

 other differences also, but generally these are not very important. 



During the warmer part of the season the asexual females 

 reproduce very rapidly, and the time required for the complete 

 life cycle may not be more than six days. During the colder 

 parts of the year two weeks or even more may be required for 

 the completion of the life cycle. The asexual individuals vary 



