93 



continued for 152 days, closing October 28. The following table shows 

 the dates of birth and of maturity of the first-born and the last-born 

 of each generation . 



A new generation of young began to appear every thirty-seven 

 days, and the average period during which each female reproduced was 

 forty-three days. The average number of young produced by the over- 

 wintering females was 147.5, and the averages for the females of the 

 first, second, and third generations were 472.5, 509, and 247.5 respec- 

 tively. 



Two full and two partial generations were produced, and the 

 first representatives of a fifth generation were due to appear when 

 reproduction ceased. All of the first and nearly all of the second 

 generation reached maturity before the close of the season, but the 

 larger part of the partial third and nearly all of the partial fourth 

 generations were still immature at its close, and thus entered the win- 

 ter in this stage. 



This record may be considered as typical for the latitude of Ur- 

 bana. In the southern part of the state, where the season is some- 

 what longer, and in any season lengthened by an unusually early spring 

 or a late autumn, a partial fifth generation is no doubt produced. In 

 the northern part of the state, where the season is shorter, the fifth 

 generation probably never appears, and the fractional parts of the 

 third and fourth generations are much smaller than in the latitude of 

 Urbana. 



A small variation in the length of the season afifects very greatly 

 the abundance of the scale. This is true of all insects having a short 

 life-cycle, with several generations in a season, especially if each female 

 produces many young, unless, indeed, natural checks effectually re- 

 strain the species. The enormous fecundity of such insects is one of 



•Date computed. 



