INSECTS IN RELATION TO PLANTS 253 



it is able to withstand a great deal of injury from insects. Corn 

 is attacked by about 200 species, of which 50 do notable injury 

 and some 20 are pests. Apple insects number some 400 species. 



Not uncommonly, an insect is restricted to a single species of 

 plant. Thus the caterpillar of Heodes hypophl&as feeds only 

 on sorrel (Rumex acetosella), so far as is known. The chry- 

 somelid Chrysochus auratus appears to be limited to Indian 

 hemp (Apocynum andros&rni folium) and to milkweed (As- 

 clepias). In many instances, an insect feeds indifferently 

 upon several species of plants provided these have certain 

 attributes in common. Thus Argynnis cybele, aphrodite and 

 atlantis eat the leaves of various species of violets, and the 

 Colorado potato beetle eats different species of Solanum. 

 Papilio thoas feeds upon orange, prickly ash and other Ruta- 

 cese. Anosia plexippus eats the various species of Asclepias 

 and also Apocynum androscemifolium; while Chrysochus 

 also is limited to these two genera of plants, as was said. 

 These plants agree in having a milky juice; in fact the two 

 genera are rather nearly related botanically. The common cab- 

 bage butterfly (Pieris rapes) though confined for the most part 

 to Cruciferae, such as cabbage, mustard, turnip, radish, horse- 

 radish, etc., often develops upon Tropczoluni, which belongs to 

 Geraniaceae ; all its food plants, however, have a pungent odor, 

 which is probably the stimulus to oviposition. 



Most phytophagous insects, however, range over many food- 

 plants. The cecropia caterpillar has more than sixty of these, 

 representing thirty-one genera and eighteen orders of plants; 

 and the tarnished plant bug (Lygus pratensis) feeds indiffer- 

 ently on all sorts of herbage, as does also the caterpillar of 

 Diacrisia virginica. Many of the insects of apple, pear, 

 quince, plum, peach, and other plants of the family Rosaceae 

 occur also on wild plants of the same family ; and the worst of 

 our corn and wheat insects have come from wild grasses. As 

 regards number of food plants, the gypsy moth " holds the 

 record," for its caterpillar will eat almost any plant. In Mass- 

 achusetts, according to Forbush and Fernald, it fed in the field 



