CELEAP IER. Vii 
INSECTS IN RELATION TO PLANTS 
Insects, in common with other animals, depend for food 
primarily upon the plant world. No other animals, however, 
sustain such intimate and complex relations to plants as in- 
sects do. The more luxuriant and varied the flora, the more 
abundant and diversified is its accompanying insect fauna. 
Not only have insects become profoundly modified for using 
all kinds and all parts of plants for food and shelter, but plants 
themselves have been modified to no small extent in relation to 
insects, as appears in their protective devices against unwelcome 
insects, in the curious formations known as * galls,” the various 
insectivorous plants, and especially the omnipresent and often 
intricate floral adaptations for cross-pollination through the 
agency of insect visitors. Though insects have laid plants un- 
der contribution, the latter have not only vigorously sustained 
the attack but have even pressed the enemy into their own ser- 
vice, as it were. 
Numerical Relations.—The number of insect species sup- 
ported by one kind of plant is seldom small and often surpris- 
ingly large. The poison ivy (Rhus toxicodendron) is almost 
exempt from attack, though even this plant is eaten by a leaf- 
mining caterpillar, two pyralid larve and the larva of a scolytid 
beetle (Schwarz, Dyar). Horse-chestnut and buckeye have per- 
haps a dozen species at most; elm has eighty; birches have over 
one hundred, and so have maples; pines are known to harbor 
170 species and may yield as many more; while our oaks sus- 
tain certainly 500 species of insects and probably twice as many. 
Turning to cultivated plants, the clover is affected, directly or 
indirectly, by about 200 species, including predaceous insects, 
parasites, and flower-visitors. Clover grows so vigorously that 
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