314 PRINCIPLES OF ECONOMIC ENTOMOLOGY 



animal is doing its best to perpetuate itself and multiply 

 its kind, and every interference is from its stand-point 

 injurious. 



FOOD HABITS. 



It is very evident that one of the first and most important 

 relations concerning an insect is to be found in its food 

 habits. 



Insects, like other animals, may be considered primarily 

 herbivorous. That is, the most general plan of nutrition 

 for them would be to feed u])on fresh, growing leaves of 

 plants, with feeding ujjon fruits, seeds, bark, wood, roots, 

 decaying wood as adaptations, while the assumption of car- 

 nivorous habits either as predaceous or parasitic species, or 

 to go further, sucking blood of higher animals may be 

 considered adaptations in another direction. 



It is on this broad, general basis that we may consider 

 herbivorous insects in the main injurious, and carnivorous 

 insects, especially if predaceous and parasitic on other insects, 

 as beneficial. 



In this connection we may refer to the principle which 

 has been termed ''unity of habit" and treated by Dr. B. D. 

 Walsh. This is in effect that in any given group we may 

 expect to find similar habits among all the species, and if for 

 any species the habits are unknown, the^^ may be expected 

 to follow those of the known species. To this law there may 

 be striking exception, however, and along certain lines 

 peculiarities should be considered the rule and not the 

 exception. To find the most certain action of the law we 

 may, I think, say that in all particularly specialized groups 

 "unity of habit" is practically general. In less specialized 

 groups variation is more frequent. For example, in the bark 

 beetles, ScolytidcE, we would be astonished to find a leaf- 

 eating larva, or in Aradidce a species that does not live under 

 bark is exceptional. In Lepidoptera, the larval diet is gener- 

 ally herbivorous and the two or three carnivorous species 

 notably the exception. We safely assume that all plant 

 lice are plant feeders and treat them accordingly. The 



