67 



which is part of the hibernating generation to continue into the follow- 

 ing year ; and October, the second, third, and fourth. The hibernating 

 assemblage contains representatives of at least three generations — the 

 second, third, and fourth of the season — and these are of all ages, from 

 young just fixed to those adult and quite ready to reproduce, or per- 

 haps with a part or all of their brood already brought forth. 



A comparison of the intervals between birth and maturity shows, 

 as might have been expected, shorter growth periods with the ad- 

 vancement of the season, those of three successive generations of the 

 descendants of the first overwintering female being 40, 37, and 34 

 days respectively, and those of the second overwintering female be- 

 ing 43, 37, and 33 days. The reproductive periods of the successive 

 generations differ little in length, but widely in productivity, the 

 June and October generations containing fewer young than those 

 brought forth in July, August, and September. The October young, 

 irideed, were all produced during intervals of warm weather between 

 periods of heavy frost. 



Evidently there can be no computation worth making of the 

 actual or possible rate of multiplication of the San Jose scale which 

 does not take account of the facts here given concerning the maximum 

 and minimum numljcrs of the generations of the annual cycle, as 

 shown by the first-born and the last-born series respectively, together 

 with the proportion of each generation which are males and of the 

 various rates of multiplication in different parts of the season. On 

 the basis of the above data of West's experiment, together with Per- 

 gande's percentages of males and females for the different genera- 

 tions, P. A. Glenn, of my office staff, has worked out a day by day 

 computation of births and deaths of both males and females for the 

 period of 152 days during which reproduction was in progress at 

 Urbana, with the result that there would have been produced by 

 October 28, under ideal, optimum conditions, 32,791,472 descendants 

 of a single female of the hibernating generation, of which 32,440,025 

 would have been still alive, 49.4 percent would have been a week 

 old or less, 27.6 percent between one and two weeks old, 13.6 percent 

 between two and three weeks, 5.5 percent between three and four 

 weeks, and 1.11 percent more between four and five weeks old, and 

 only 2 percent would have been mature. 



I am assuming that, as the average growth period at Urbana was 37 

 days, each week after birth would add about one fifth of the growth to 

 adult size. The import of these figures may be better realized if they are 

 expressed in the area which the total number of scales present in 

 their various sizes October 28 would cover if placed in a single layer 

 edge to edge. The average diameter of an equal number of male and 

 female scales is 1.1 mm. Taking this at 1 mm. and making propor- 

 tionate allowance for the lesser size of the scales at the various stages 

 of growth, we find that the entire product of reproduction alive Octo- 



