HOMOPTERA 417 



Third type.— The parthenogenetically produced winged agamic females. 

 Fourth type. — The sexual forms, males and oviparous females. 



B. SEQUENCE OF GENERATIONS IN A MIGRATING SPECIES 



Only the first of a series of similar generations is counted. 



First generation. — The stem-mother. 



Second generation. — Wingless agamic females. There may be a series of 

 generations of this form here. 



Third generation. — Winged agamic females. These migrate to the secondary 

 host. 



Fourth generation. — -Wingless agamic females. There may be a series of 

 generations of this form here. 



Fifth generation. — Winged agamic females. These migrate to the primary 

 host and are the sexuparae. 



Sixth generation. — Males and oviparous females. The females produce the 

 fertilized eggs from which the stem-mothers are hatched, thus completing the life- 

 cycle. 



A remarkable fact that has been demonstrated by several ob- 

 servers is that the ntmiber of generations of the wingless agamic 

 form may be influenced by the conditions under which the aphids 

 live. In an experiment conducted under my direction by Mr. 

 Slingerland, in the insectary at Cornell University, we reared 98 

 generations of the wingless agamic form without the appearance of 

 any other form. The experiment was carried on for four years and 

 three months without any apparent change in the fecundity of the 

 aphids, and was discontinued owing to the press of other duties. As 

 the aphids were kept in a hothouse throughout the winters, seasonal 

 influences were practically eliminated ; and as members of each gen- 

 eration were placed singly on aphid-free plants and their young re- 

 moved as soon as born, there was no crowding. 



In order to determine the influence of crowding, members of the 

 sixtieth generation were placed on separate plants and their young 

 not removed. At the end of three weeks the winged agamic form ap- 

 peared, evidently in response to need of migration to less densely 

 populated plants ; while in other cages where the young were removed 

 promptly, no migrants appeared up to the end of the experiment. 



The family Aphididse includes a very large number of genera and 

 species. The genera are grouped into tribes and these into subfamilies 

 in various ways by different authors. Recent classifications by 

 American authors are those of Oestlund ('18) and Baker ('20). Four 

 subfamilies are recognized by Baker. The characters of these sub- 

 families given below are largely compiled from this author. 



Subfamily APHIDIN.E 



To this subfamily belong most of the species of aphids that are 

 commonly seen living free {i. e., not in galls) upon the foliage of 

 plants. But while most of the species feed on foliage, some of them 

 attack stems and roots. Their attacks on foliage in some cases merely 

 cause a weakening of it; in other cases, the leaves become curled or 

 otherwise distorted; such distortions are termed pseudo galls. True 

 galls formed by aphids are described in the accounts of the last two 

 subfamilies. 



