No. G33] FOOD-PLANTS AND INSECTS 



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cases. As the smearing of the oils of one plant on an- 

 other does not occur in nature, the important point to 

 discover is whether there is really any similarity of odor 

 in the several plants of diverse groups that are some- 

 times utilized by a single species of insect. The facts 

 alluded to above in regard to the wide variety of selec- 

 tions made by different species would seem to answer 

 this question in the negative, as would our own human 

 sense of odor, which latter may, of course, not be reliable 

 when dealing with a group of animals so different from 

 man. We may, I think, rest assured that odor frequently 

 guides the insects to their food-plant, but we can not be- 

 lieve that oligophagous or polyphagous species have be- 

 come accustomed to a variety of plants due to a confusion 

 of similar odors. There does seem, however, to be one 

 very striking exception to this among the Pierid butter- 

 flies. As said before, these butterflies are confined to 

 Cruciferas almost exclusively, but one of our species not 

 infrequently occurs on the garden " nasturtium" (Tro- 

 pceolum). That the pungent taste of this plant is much 

 like that of a Crucifer is well known and further attested 

 by the common name, as the true nasturtium of the botan- 

 ists is a genus of Crucifera?, while the garden nasturtium 

 is a Geraniaceous plant. 



On account of the very close biological association be- 

 tween insects and plants in many ways it is true that the 

 two have been mutally specialized until they have become 

 highly modified in reference to one another, but this is 

 not the case with food-plants, as no benefit ordinarily 

 accrues to the plants and any idea of parallel evolution 

 must be restricted to a development of undesirable at- 

 tributes on the part of the plants and adaptations on the 

 part of the insects to overcome such barriers to feeding. 



To avoid these numerous difficulties, it seems clear 

 that the selection of food-plants by the Lepidopterous 



