CHAPTER VIII 



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 injects 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 toxicodendroii) is almost 

 exempt from attack, though even this plant is eaten by a leaf- 

 mining caterpillar, two pyralid larvse 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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