INSECTS IN RELATION TO PLANTS 



213 



to milkweed (Asclepias). In many instances an insect feeds indiffer- 

 ently upon several species of plants provided these have certain attri- 

 butes 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 Rutacese. 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 cabbage butterfly (Pieris rapci) though con- 

 fined for the most part to Cruciferae, such as cabbage, mustard, turnip, 

 radish, horse-radish, etc., often devel- A 



ops upon Trop&olum, which belongs to 

 Geraniaceas; all its food plants, how- 

 ever, have a pungent odor, which is 

 probably the stimulus to oviposition. 



Most phytophagous insects 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 indifferently on all sorts of herb- 

 age, as does also the caterpillar of Dia- 

 crisia 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 gipsy moth " holds 

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

 setts, according to Forbush and Fernald, it fed in the field upon 78 

 species of plants, in captivity upon 458 species (30 under stress of 

 hunger, the rest freely), and refused only 19 species, most of which 

 (such as larkspur and red pepper) had poisonous or pungent juices, 

 or were otherwise unsuitable as food. The migratory locust is noto- 

 riously omnivorous, and perhaps eats even more kinds of plants than 

 the gipsy moth. 



Galls. Most of the conspicuous plant outgrowths known as "galls" 

 are made by insects, though many of the smaller plant galls are made 



FIG. 251. Holcaspis globulus. A, 

 galls on oak, natural size; B, the gall- 

 maker, twice natural length. 



