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 hypophlieas feeds only 
on sorrel (Rumex acetosella), so far as is known. ‘The chry- 
somelid Chrysochus auratus appears to be limited to Indian 
hemp (Apocynum androsemifoliumy 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- 
cee. Anosia plexippus eats the various species of Asclepias 
and also Apocynum androsemifolium; 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 rape) though confined for the most part 
to Cruciferze, such as cabbage, mustard, turnip, radish, horse- 
radish, etc., often develops upon Trop@olum, which belongs to 
Geraniacee ; 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 Rosaceze 
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 
. 
