ioi 



tlieir attacks are made successively. 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 i)lant and its constant insect visi- 

 tants. With respect to these two corn insects, however, it must be ad- 

 mitted that no proof is apparent that such adaptation of life histories and 

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

 tion of species whose significant peculiarities were already established 

 when they came together. 



A similar but more striking example of a serial succession of injur- 

 ies to the same plant is to be found among the strawberry insects, as I 

 showed several years ago." Three coleopterous larvae belonging to the 

 same family (Chrysomelidse) but to different genera ( Colaspis, Graph- 

 ops and Typophorus), and to species native in the United States, are all 

 so closely adapted to underground life and to the root-feeding habit that 

 they are distinguishable from one another only by rather slight and in- 

 conspicuous characters. They are often associated in large numbers in 

 the same fields, living wholly on the roots of strawberry plants, which 

 they affect in an identical manner, so that from the appearance of the 

 injury itself one could not possibly tell which of the three species was 

 present in the field. One of these root-worms, the Colaspis larva, feeds 

 also on the roots of other plants, especially on those of timothy and corn, 

 but the other two larvae have been found only among strawberry roots! 

 They seem thus to be strict competitors for food from the same part of 

 the same plant, and as their locomotive capacity is poor, they are unable 

 to avoid one another's company by migration under ground. 



The strawberry plant, however, grows continuously throughout th; 

 season, and each of these three insects, having a short larval period, feeds 

 on strawberry roots for only a part of this growing season. It is an inter- 

 esting and striking fact that the life histories of the three competing in- 

 sects are so related that the larva: do not infest the plant at the same time, 

 but follow one another in close succession, beginning early in May and 

 ending late in fall. The first of the species, the Colaspis larva, feeds 

 from about May to the end of June, the Typophorus larva follows in 

 July and August, and the Graphops larva begins in August and continues 

 until fall. 



Consistently with this difference, the species concerned hibernate in 

 different stages of development — Colaspis apparently as an egg, Typoph- 

 orus undoubtedly as an adult, and Graphops as a larva in its subter- 

 ranean cell, from which adults emerge the following June to lay their 

 eggs in July. With such a distribution of their attack, each of these 

 three species is able to maintain itself on the strawberry in numbers as 



""On the Life Hi.stories and Immature Stages of Three Eumolplni." PsycUe, Vol. 

 4. Nos. 117-llS. January-February, 1884; and No. 121, May, 1SS4. 



