20O 



ELEMENTARY ZOOLOGY 



which sing so shrilly from the trees, the seventeen-year 

 cicada (Cicada septendecini) (oftentimes called locust) 



being the best known of 

 this family. Its eggs 

 are laid in slits cut by 

 the female in live twigs. 

 The young, which hatch 

 in about six weeks, do 

 not feed on the green 

 foliage, but fall to the 

 ground, burrow down to 

 the roots of the tree and 

 there live, sucking the 

 juices from the roots, for 



FIG. 66. The seventeen-year cicada, Ci- sixteen years and ten or 

 cada septendecim ; the specimen at left , ,, iiru 



showing sound-making organ, ,./., ven- eleven months. When 



tral plate; /, tympanum. (From speci- about to become adult, 

 men.) 



the young cicada crawls 



up out of the ground and clinging to the tree-trunk molts 

 for the last time, and flies to the tree-tops. 



The plant-lice (Aphididce) are small soft-bodied 

 Hemiptera which have both winged and wingless indi- 

 viduals. In the early spring a wingless female hatches 

 from an egg which, laid in the preceding fall, has passed 

 the winter in slow development. This wingless female, 

 called the stem-mother, lays unfertilized eggs or more 

 often perhaps gives birth to live young, all of which are 

 similarly wingless females which reproduce partheno- 

 genetically. This reproduction goes on so rapidly that 

 the plant-lice become overcrowded on the food-plant and 

 then a generation of winged * individuals is produced from 



* It has been shown by experiment that the winged individuals, which are 

 able to leave the old food-plant and scatter over new plants, do not appear 

 until the food-supply begins to run short. At the insectary of Cornell Uni- 

 versity ninety -four successive generations of wingless individuals were bred, 



