PLANTS AS PASSIVE MEMBERS (COACTEES) 117 



organisms usually soon begin to take food, picking up all sorts of 

 material and rejecting those that are distasteful or painful (Holmes, 

 1911). After a few trials, rejection takes place at sight without test- 

 ing, the animal selecting only those objects that serve as nourishment, 

 and thus quickly learning to distinguish food from all other things. 

 However, many adult insects in particular render such learning un- 

 necessary by laying their eggs on the very material that is to supply 

 food for the young. Some adults, that in the larval stage were forced 

 to feed upon strange food, have been found to lay eggs upon this 

 same substance. Frequently, larvae that have started to feed upon 

 a plant selected by the female moth will not change to another host 

 plant (Picket, 1911; Brues, 1920, 1924). The general principle is 

 that organisms show a preference for a certain kind or kinds of food, 

 which may be selected from a considerable range of materials. Such 

 a choice may be exercised between groups of various kinds, e.g., 

 between species, or organs of food plants. 



PLANTS AS PASSIVE MEMBERS (COACTEES) 



The animal coactors of this group may well bear the collective term 

 of plantivores, since the more familiar word herbivore is neither defi- 

 nite nor inclusive. However, it is evident that such a designation is 

 often to be employed in a relative sense for animals whose food is 

 largely but not wholly vegetable, and hence differ only in degree from 

 omnivores. Plantivores sometimes select their principal food from the 

 dominants of a biotic community. Thus, insects on a fioodplain feed 

 in general on a variety of trees, though each kind usually prefers but 

 one or two species of the fioodplain dominants (Felt, 1906). By con- 

 trast, the salt-marsh caterpillar {Estigmene acraea Drury) feeds on 140 

 different species of herbs (Folsom, 1922). Metcalf (1924) has shown 

 that certain leaf hoppers feed on the plants present where the physical 

 conditions are suitable. A number of Phytophaga are confined to 

 plants belonging to a single family, a preference for willow and 

 Cottonwood being not uncommon (Folsom, loc cit.; Brues, 1924). A 

 striking instance of a similar predilection is furnished by the potato 

 beetle, Leptinotarsa, which passed from the wild Solarium rostratum 

 to the cultivated S. tuberosum to become one of the generally distrib- 

 uted North American pests. 



The general relations of a pure phytophagous group to its food 

 plants is well illustrated in a recent monograph of the aphids or plant 

 lice of Illinois (Hottes and Frison, 1931), which includes a discussion 

 of 251 species and varieties. Since the food plants and much of the 



