35 



mon and the others scarce. Their feciinditj may be similar and 

 the chief death factors acting upon them identicah Close ob- 

 servation will often show that a very small percentage of the 

 scarce species is killed by a factor that does not attack the more 

 common. If we study the species separately this would not 

 account for the difference of numbers, but if we consider them 

 as forming one group then this small factor will make the differ- 

 ence. 



Table I tries to illustrate this. What we have previously 

 said must be borne in mind, viz. : 



1 The number of an insect in a district is constant within 



certain periods. 



2 There is a ratio between host and parasite. 



3 That predators follow the line of least resistance when 



feeding, and, owing to their powers of locomotion, act 

 as a movable factor to keep the numbers constant. 



TABLE I. 



ABC Aggregate 



First generation 20 20 20 60 



Eggs 80% killed _ 500 500 500 1500 



Larvae 50% killed 100 100 100 300 



Pupae killed: A nil; B 25%; 



C 50%; 50 50 50 150 



Adults hatched 50 37.5 25 112.5 



Adults reaching maturity (2ud 



generation) 2(1 20 14 GO 



Eggs 80% killed 650 500 350 1500 



Larvae 50% killed ...130 100 70 300 



Pupae killed: A nil, B 25'/,, 



C 50% 65 50 35 150 



Adults hatched 65 37.5 17.5 120 



Adults reaching maturity (3rd 



generation) 32 19 9 60 



Eggs 80% killed 800 475 225 1500 



Larvae 50% killed 160 95 45 300 



Pupae killed: A nil, B 25%, 



C 50% 80 47.5 22.5 150 



Adults hatched 80 35.6 11.2 126.8 



Adults reaching maturity (4th 



generation) 37 17 6 60 



