64 THE REPORT OF THE No. 36 



Breeding Experiments, 



In this form, the duration of nymphal life varies very considerably due in 

 a large measure to the great differences in temperature to which the females born 

 early in spring and those bom during the summer are subjected. (See table 

 No. 1.) In our experiments the average nymphal life in the second generation 

 was 17.5 days, in the eighth it was 6.6 days. 



In the matter of reproductive capacity, this form is slightly less prolific 

 than the stem mother (see table No. 1). One hundred and eleven apterous forms 

 produced an average of 80.9 young per insect. 



Summer Migrant. 



Migrants are pi-oduced on cherry trees during a period extending from mid- 

 June to the middle or latter part of August.* The vast majority of them, 

 however, develop and migrate before mid-July. 



Description : The head, thorax, cornicles, and cauda of the migrant are 

 black and the abdomen varies in colour from dark or black green to dark brown. 

 It is about 2.16 mm. long. 



Factors Which Produce Alat^, 



A question which should be touched on here is : what agencies tend to produce 

 winged-forms? We are inclined to believe that three of them are, the influence 

 of over-population, the instinct to migrate, and, to a small extent at least, the 

 influence of generation. , , 



With a monophagous species such as Aphis pomi the appearance of alatse 

 is apparently due in a large measure to overcrowding. This hypothesis explains 

 why, by preventing crowding, it is possible to rear apterous lines of the green 

 apple aphis through from egg to egg. With a migratory species, however, such 

 as Aphis avence, another factor comes into play, viz. : the instinct to migrate. 

 In some of our experiments with the oat aphis one individual was reared on 

 each host plant (apple), but in spite of this superabundance of food and space, 

 all the lines gave rise to migrants — the aphis in order to complete its life cycle 

 had to migrate. 



And now to., return to the cherry aphis, this louse behaves more like a 

 monophagous than a migratory species. The migratory instinct appears to be 

 attenuated and seemingly is of little or no importance in the production of 

 winged forms. In our insectary work, it was observed that alata? did not develop 

 unless the plant lice were excessively • crowded. 



The influence of generation as a minor factor is suggested by the fact that 

 no winged forms occur in the 2nd generation of M. cerasi — at least we did not 

 obtain any. 



Breeding Experiments. 



In our experiments with a large number of migrants the following data 

 were obtained: — 



Duration of nymphal life. Migrants took from 7 to 13 days to reach maturity, 

 or in other words, one to four days longer than contemporary apterous viviparae. 



Reproduction: Migrants gave birth to young one or two days after they 

 were transferred to Lepidium. 



*A few migrants were taken on cherry on August 27th, 1917. 



