35 



mon aiul the others seaivo. Theii- fccinidity may 1k' simihir and 

 the chief death factors acting upon them identical 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 stndy the species separately this would not 

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

 as forming one group tlien this small factor will make the diffei-- 

 ence. 



Tabk- 1 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. 



A B C Aggregate 



First generation 30 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% .^0 50 50 150 



Adults hatched 50 37.5 25 112.5 



Adults reaching maturity (2nd 



generation) 20 20 14 00 



Eggs 80%. killed - - 050 500 350 1500 



Larvae 50% killed - 130 100 70 300 



Pupae killed: A nil, B 25'/, 



C 50% 05 50 35 150 



Adults hatched 05 37.5 17.5 120 



Adults reachino- maturity (3rd 



generation) ' - - 32 19 9 00 



Eggs 80% killed 800 475 225 1500 



Larvae 50% killed 100 95 45 300 



Pupae killed: A nil, B 25%., 



C 50% 80 47.5 22.5 150 



Adults hatched 80 35.0 11.2 120.8 



Adults reaching maturity (4th 



generation) ^. 37 17 00 



