11 



A ground beetle 

 ordinarily useful, 

 but injurious if in 

 excessive num- 

 bers, eaten by the 

 robin. 



part of the food of many ground-frequenting birds. But 



ground beetles feed to a greater or less extent on other insects. 



The question then arises, is not the robin 



doing harm by eating ground beetles, and 



does it not merit the destruction of its eggs 



and young by the crow? If the robin's habit 



of eating these beetles is harmful, is not the 



crow rendering a service by destroying a bird 



apparently so injurious as the robin? If there 



were too many robins might they not eat too 



many ground beetles and thus become the in- 

 direct cause of the destruction of much vegeta- 

 tion by saving the lives of the caterpillars 



and other harmful insects that the ground 



beetles, had they been spared, might have 



destroyed? 1 



Many ground beetles that are eaten by the robin feed much 



on vegetable matter. 2 This makes these beetles doubly useful 



in one respect, for they can main- 

 tain their numbers when insect food 

 is not plentiful, and so be ready to 

 check any increase of insects which 

 may occur. On the other hand, if 

 they become too numerous they 

 may do serious damage by destroy- 

 ing grass, grain or fruit. I have 

 witnessed attacks made by certain 

 of these beetles on grain and straw- 

 berries, and were they not held in 

 check by birds they might become 

 serious pests. Their destruction 

 by robins and other birds tends 



to keep these beetles within those normal bounds where they 



1 These questions can be answered only by one having a thorough knowledge of the food of 

 our ground beetles, a knowledge which no living man yet possesses; but enough has been 

 learned to throw some light on their food habits. Insects that feed promiscuously on other insects 

 are regarded as beneficial in so far as they take insect food, even though they may destroy many 

 so-called useful insects; for, as the injurious insects far outnumber the useful species, any check 

 upon the general increase of insect life must result in a balance of good. 



1 The predaceous beetles of one genus (Calosoma) and those of some closely allied genera are 

 exceptions to this rule, and are believed to feed entirely on animal food, as their structureffits 

 them for that alone. They feed ravenously upon both beneficial and injurious insects, and when 

 abnormally numerous they devour one another. These beetles, however, are eaten by crows 

 and probably not by robins. 



Calosoma scrutator, useful ground 

 beetle, eaten by crows. 



