INSECTS 



many species of aphids that infest our common field and 

 garden plants (Fig. ioi) and cultivated shrubs and trees, 

 to say nothing of those that inhabit the weeds, the wild 

 shrubbery, and the forest trees. Almost every natural 

 group of plants has its particular kind of aphid, and many 

 of them are migratory species like the rosy and grain 

 aphis of the apple. There are root-inhabiting species as 

 well as those that live on the leaves and stems. The 

 Phylloxera, that pest of vineyards in California and 

 Prance, is a root aphid. Those cottony masses that 

 often appear on the apple twigs in late summer mark 

 the presence of the woolly aphis, the individuals of which 

 exude a fleecy covering of white waxy threads from their 

 backs. The woolly aphis is more common on the roots 

 of apple trees, being especially a pest of nursery stock, but 

 it migrates to both the twigs and the roots of the apple 

 from the elm, which is the home of its winter eggs. 



An underground aphid of particular interest is one that 

 lives on the roots of corn. We have seen that all aphids 

 are much sought after by ants because of the honey dew 

 they excrete, a substance greatly relished and prized by 

 the ants. It is said that some ants protect groups of 

 aphids on twigs by building earthen sheds over them; but 

 the corn-root aphis owes its very existence to the ants. 

 A species of ant that makes its nests in cornfields runs 

 tunnels from the underground chambers of the nests to 

 the bases of nearby cornstalks. In the fall the ants gather 

 the winter eggs of the aphids from the corn roots and take 

 them into their nests where they are protected from 

 freezing during the winter. Then in the spring the ants 

 bring the eggs up from the storage cellars and place 

 them on the roots of various early weeds. Here the 

 stem mothers hatch and give rise to several spring gen- 

 erations; but, as the new corn begins to sprout, the ants 

 transfer many of the aphids to the corn roots, where 

 they live and multiply during the summer and, in the fall, 

 give birth to the sexual males and females, which produce 



[172] 



