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THE AMERICAN NATURALIST [Vol. LIV 



temptations to avail themselves of a variety of foods and 

 we find that it is usually the most abundant species of 

 any group that are polyphagous. Conversely we may 

 say also that polyphagy, when present, greatly increases 

 the chances for the larvae to secure the necessary amount 

 of food for complete growth and tends to cause the spe- 

 cies to become excessively abundant. Once under head- 

 way, these two processes will act together and result in 

 the production of dominant species that tower above their 

 fellows. Examples of this are seen in the gipsy-moth, 

 the Cecropia-moth., the army- worm, Papilio glaucus men- 

 tioned above, the woolly bear (Isia Isabella), etc. We 

 must not lose sight of the fact, however, that this is only 

 one of many factors influencing dominance. The milk- 

 weed-butterfly, one of our most abundant native species 

 develops on a very common plant {Asclepias almost ex- 

 clusively) and is thought also to be a protected species. 7 

 Its dominance may be interpreted, like that of many de- 

 structive agricultural pests, as due to a plentiful and un- 

 failing food supply, coupled with other pre-requisites in- 

 herent in the insect itself. 



In spite of the many exceptions and variations which 

 have been enumerated, the fact stands out clearly that 

 the Lepidopterous insects show a very fixed instinct to 

 select definite plants for larval food; that many are ex- 

 tremely precise in this respect, some less so, and others 

 quite catholic in their tastes. Furthermore there is much 

 to show the existence of a so-called "botanical instinct' ' 

 in species, genera and even families, whereby evidently 

 related plants and these only serve as food. A few spe- 

 cies have departed from the general habit so far that 

 they have become carnivorous, and among the others we 

 find every gradation between the extremes of monophagy 

 and polyphagy. 



It has been claimed that the food habits may be modi- 

 fied experimentally, in that caterpillars reared on a 



