290 



THE AMERICAN NATURALIST 



[Vol. XLIII 



the winged members of the early generations, the per- 

 centage of which increases as conditions become locally 

 less favorable, the aphis largely leaves the field in which 

 it originally started, and early breaks the force of its 

 attack by a general distribution of it. The depredations 

 of the root-worm, on the other hand, increase with the 

 growth of the insect until about September first, and in- 

 crease also, at a rapid rate, from year to year in a field 

 kept continuously in corn. It follows, as a consequence, 

 that the principal damage by A pit is maifUradicis is done 

 to the corn while it is young, and that by Diabrotica to 

 the well-grown plant. 



This serial order of injuries to the corn plant, due to 

 the relation of the life histories and rates of multiplica- 

 tion of these two competing insects, is an advantage to 

 both of them, and, indeed, to all three, corn included, since 

 the plant would be more seriously injured or more cer- 

 tainly destroyed if both its insect enemies attacked it 

 together than it is where their attacks are made succes- 

 sively. Competitors for food from a living plant find 

 it to their advantage, and to that of the plant they feed 

 upon, to avoid a simultaneous competition; and such a 

 plant-insect group would, of course, prevail, other things 

 being equal, over a competing group not so adjusted. 

 Natural selection tends, no doubt, to establish these mu- 

 tually advantageous relations between a plant and its 

 constant insect visitants. With respect to these two corn 

 insects, however, it must be admitted that no proof is 

 apparent that snch adaptation of life histories and habits 

 as we here see is due to anything more than an accidental 

 collocation of species whose significant peculiarities were 

 already established when they came together. 



A similar but more striking example of a serial succes- 

 sion of injuries to the same plant is to be found among 

 the strawberry insects, as I showed several years ago. 3 

 Three coleopterous larva? belonging to the same family 



3 "On the Life Histories and Immature Stages of Three Eumolpim," 



