CHAPTER 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 insects 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 

 under contribution, the latter have not only vigorously sustained the 

 attack but have even pressed the enemy into their own service, as it were. 



Numerical Relations. The number of insect species supported by 

 one kind of plant is seldom small and often surprisingly large. The 

 poison ivy (Rhus toxicodendron) is almost exempt from attack, though 

 even this plant is eaten by a leaf-mining caterpillar, two pyralid larvae 

 and the larva of a scolytid beetle (Schwarz, Dyar). Horse-chestnut 

 and buckeye have perhaps 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. Turn- 

 ing 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 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 num- 

 ber some 400 species. 



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

 Thus the caterpillar of Heodes hypophlaas feeds only on sorrel (Rumex ace- 

 tosella), so far as is known. The chrysomelid Chrysochus auratus appears 

 to be limited to Indian hemp (Apocynum androsamifolium) and to milk- 



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