332 THE AMERICAN NATURALIST [Vol. LI V 



1. The odor of the plant, and also its taste, which is 

 no doubt closely connected with odor. Associations rea- 

 sonably placed in this category would be the oligophagous 

 species occurring, for example, on various Cruciferse, 

 various Umbelliferse, and various Composite. An addi- 

 tional argument for the importance of this factor is seen 

 in the less common utilization by the same insect of sev- 

 eral plants in a family like the Solanacese 1 where a more 

 or less similar odor does not become a family charac- 

 teristic. 



2. Some attribute of the plant, perhaps an odor but 

 far less pronounced to our own senses than those men- 

 tioned above. Species restricted to plants like Legumi- 

 nosae or Violacese may be considered in this category. 

 Undoubtedly there is some attribute of such plants which 

 insects can recognize in a general way and not as a 

 specific characteristic of some single plant species or 

 genus. The ' 4 botanical instinct ' ' of some caterpillars that 

 has frequently been commented upon would appear to be 

 an exaggerated power of recognition of this sort. 



3. A similarity in the immediate environment or gen- 

 eral form of the food-plant. The effect of something of 

 this sort is seen particularly in oligophagous and also 

 polyphagous caterpillars feeding mainly on trees or 

 shrubs, such as the gipsy-moth, Cecropia moth, etc., and 

 those of certain species like some of the Arctiid moths 

 that feed upon a great variety of low plants. 



4. Apparently chance associations that have become 

 fixed, whereby diverse plants are utilized by oligophagous 

 species. Secondarily polyphagous species show these in 

 an exaggerated form. On account of their comparatively 

 rare occurrence these seem to be analogous to structural 

 mutations, although they appear to be strictly modifica- 

 tions of instinct. As has been pointed out on a previous 

 page, these are much more apt to occur in some groups 

 (families and genera) than in others. 



