No. 633] FOOD-PLAXTS AXD INSECTS 315 



resent probably nearly half of the known species, and a 

 considerable proportion of the several orders of insects 

 contain at least some species that are phytophagous in 

 the sense indicated above. Some of these, like the Or- 

 thoptera, are very primitive, while others of probably 

 equal or even greater antiquity are not phytophagous, so 

 that it is difficult to say whether the earliest true insects 

 were vegetarian, predatory or saprophagous. 2 This 

 question is perhaps not a very important one, for, as will 

 be pointed out later, a change from one type of food 

 habits to another has actually taken place independently 

 in several families of the highly specialized Lepidoptera. 



As we might naturally expect, it is possible to point 

 out in a very general way a progressive specialization 

 in the selection of food-plants which parallels to some 

 degree what appears to have been the path of evolution 

 among insects, as determined from the criteria fur- 

 nished by comparative anatomy, development and pale- 

 ontology. Thus, the primitive Orthoptera appear to se- 

 lect their food-plants with but little discrimination, while 

 the Lepidoptera and phytophagous Hymenoptera exhibit 

 almost unerring accuracy in their instincts to choose cer- 

 tain plants and consistently to ignore all others. Beyond 

 this, however, it is not easy to make any broad state- 

 ments, for among the most highly specialized groups we 

 find a great variability, at least in the number of food- 

 plants admitted to the menu, as well as in regard to the 

 botanical relationships of the plants regularly selected. 



It may be argued that selection of food-plants is a 

 somewhat dubious expression and that it may not accu- 

 rately represent the condition of affairs from the stand- 

 point of the larval insect. In most cases the larval food- 

 plant is really chosen by the adult female, who places her 

 eggs upon certain plants which then become of necessity 

 the food of the resulting larva?, which could not very 

 readily migrate to another kind of plant even should they 



microorganisms. 



