451 



Classification of Adaptations to Food 



A ]ilnnt-insect group — a group, that is, composed of a plant and its 

 insect visitants — is not in fact usually marked, either as a whole or in 

 any of its several parts, by the presence of adaptive structures special to 

 that group. The structural adaptations of insects are as a rule much 

 too broadly shaped to fit ihem closely to any one plant, and where such a 

 fitting is found, it is clearly due to some other than the structural factor. 

 Such facts bring us to a consideration of the whole subject of the varia- 

 tions and classification of the adaptations of insects to their food re- 

 sources. 



These adaptations may be classed as structural, physiological, psycho- 

 logical, synethic.- local, biographical and numerical. All structural 

 ada]3tations are, of course, physiological in a sense, but I use the word 

 physiological, as a matter of convenience, for functional adaptations not 

 based on obvious structural peculiarities, as where an insect equally capa- 

 ble of feeding on the sap of two plants and readily availing itself of 

 either, nevertheless thrives and multiplies better on one than on the other, 

 the adaptation being evidently digestive or assimilative rather than obvi- 

 ously structural. The San Jose scale, for example, feeds readily on a 

 great variety of trees and shrubs, on some of which it thrives poorly and 

 spreads but little, while on others it multiplies enormously and spreads 

 with great rapidity. The word psychological may be applied to cases 

 of apparent choice or evident inclination, as between the various avail- 

 able food plants of the environment. Those fixed peculiarities of habit 

 or behavior which adapt an insect to one food plant or class of food plants 

 rather than to another we may call synethic adaptations, in the absence 

 of any existing word applicable in this sense; local adaptations are those 

 in which the usual haunts and places of resort of an insect species, how- 

 ever determined, bring it into common contact with an available food 

 ])lant, the frequency of this contact being quite independent of the de- 

 gree of the fitness of such plant for its food ; biographical adaptations 

 are those based on a correspondence between the life history of the in- 

 sect and its organic food supply, such that the latter shall always be ac- 

 cessible in sufficient quantity to meet the varying needs of the dependent 

 insect at the various stages of its growth ; and numerical adaptations are 

 the consequence of such an adjustment of the rate of insect multiplica- 

 tion to the plants or animals of its food that only the unessential surplus 

 of this food shall be appropriated, its maxinuini essential product being 

 left undiminished. 



These several classes of adaptations limit each other variously, the 

 most desirable food of an insect being that which is found within the 

 area common to all of them. That is, the most important food plants 

 of a vegetarian species will be those which are well within its structural 

 capacities of discovery, access and appropriation ; within its physiologi- 



- .\daptation.« of habit. 



