196 Annals Entomological Society of America [Vol. XIII, 



injury to their common host; and individual variations in the 

 length of the stages of the life cycle already referred to make this 

 easily possible. If there is any initial difference whatever 

 between the competing species as to the period of their attack, 

 natural selection may do the rest, and even if there is not, 

 mutation of habit may have the same effect. I happened, many 

 years ago, upon an apparent instance of this kind, when I was 

 studying the life histories of the so-called root-worms of the 

 strawberry — larvae of three species of chrysomelid beetles which 

 devour the roots of the plant. One of these larvae, that of 

 Colaspis brun?iea, begins its work in southern Illinois in early 

 spring and continues active through June; another {Typophorus 

 canellus aterrima) begins in June and continues into August; 

 and the third {Graphops nebulosus) begins in August and con- 

 tinues active through the fall, hibernating, in fact, in the larva 

 stage. 



It is a significant fact that another species {Graphops 

 pubescens) closely allied to the last and with a like distribution, 

 but living on another food-plant, refusing, indeed, the roots of 

 the strawberry and feeding only on those of the wild evening 

 primrose, has a very different life history from its near relative, 

 wintering as an adult instead of a larva, as does the strawberry 

 species. There is here a suggestion of a possible shifting of the 

 life history of the strawberry Graphops in a way to adjust its 

 demands for food to those of its competitors. Of course, this 

 seeming adjustment may have been a coincidence merely, and 

 I do not know of another instance of the kind; but, on the other 

 hand, I do not know that such instances have been sought. 

 Most of our best life history work has been done on insects infest- 

 ing the crop plants, where the natural reactions of plant and 

 insect are so generally disturbed or annulled by the over- 

 powering agency of man that we must look for clear cases of 

 interlocking life histories of competing insects among those 

 dependent on uncultivated plants; and here but little has been 

 done. 



The foregoing case may at any rate serve to illustrate the fact 

 that for a full understanding of the adaptations of insects to 

 their environment by way of their life histories, we must 

 not study merely the separate species in their natural habitats, 

 but must make local ecological groups our units for investiga- 

 tion, and inquire into the system of competitions, and adjust- 



