THE CORN ROOT-APHIS AND ATTENDANT ANT. 31 



only some minor details made out within the last two years. A highly 

 successful series of breedings made in my insectary by Mr. J. J. 

 Davis, a junior student in my department, working at the time as 

 my assistant, gives us some additional data concerning the rate of 

 multiplication of these insects under Avhat Avere evidently optimum 

 conditions. 



Most of these aphides were reared in small glass vials, each with 

 a layer of moist cotton in the bottom and containing a young food 

 plant. A single aphis just born was placed on each plant, the vial 

 was wrapped with paper to exclude the light, and its mouth was 

 (closed with dry cotton. As soon as the plant began to wilt a fresh 

 one was introduced, the aphides being carefully transferred to it by 

 means of a cameFs-hair brush.. Sometimes a glass tube was placed 

 around a food plant while growing in an earthen pot, the bottom of 

 the tube pushed into the earth and the upper end stopped with cot- 

 ton as before, the whole being darkened by a paper wrapping. That 

 these artificial conditions were highly favorable to the corn root- 

 aphis was shown by the fact that among several hundred specimens 

 bred and reared in this way not a single one developed wings, 

 although winged aphides were appearing abundantl}^ in the insec- 

 tary at the time. AYe have much evidence that the development of 

 winged aphides is gi-eatly stimulated, if not sometimes caused, by 

 some deficiency in conditions for the maintenance of an increasing 

 population — usually by a diminishing food supply. 



By the use of these data, together with others already in my pos- 

 session relating especially to the earliest generations of the year, I 

 am able now to present a virtually complete calendar for this spe- 

 cies of the annual succession of its generations, from the so-called 

 stem-mother — the generation which hatches from the egg — to the egg 

 again in fall. Beginning with the f?^'<f to hatch in the spring, if we 

 follow down the series of the frs-t horn of each generation we find 

 that 16 successive generations may appear, counting the eggs laid 

 in the fall as the last. If, on the other hand, Ave begin with the last 

 to hatch from the eggs in spring and follow down the series of the 

 last born of each generation, there are but 9 generations in all; from 

 which it folloAVsthat the number of mid-born generations is 12 — 

 the mean number for the year. The average interval between suc- 

 cessive generations of the first born is 11 days, that between the 

 successive generations of the mid born is 16 days, and that betAveen 

 generations of the last born is 18^ days. 



The first generation may be found in the field from April 8, our 

 earliest date for the hatching of the eggs, to June 20, our latest 

 date for the birth of the young of the second generation, a period 

 of 73 days. The eighth generation — the longest liA^ed of all, if we 

 except the eggs — continues from JuIa' 11 to the last of October, a 



