托福听力考试背景学问综合辅导(五十三)

托福听力考试背景学问综合辅导(五十三)CICADA Their life cycle takes 17 years in northern species (the so-called 17-yea

() 托福听力考试背景学问综合辅导五十三 CICADATheirlifecycletakes17yearsinnorthernspecies (theso-called17-yearlocusts)and13yearsinsouthern species;thetwotypesoverlapinpartsoftheUnited States.Thefemaledepositshereggsinslitsthatshecuts inyoungtwigs.Inaboutsixweeksthewingless,scaly larvae,ornymphs,dropfromthetreeandburrowintothe ground,wheretheyremainfor13or17years,feedingon juicessuckedfromroots. Thenymphsmoltperiodicallyastheygrow;finallythe full-grownnymphsemergeatnight,climbtreetrunksand fences,andshedtheirlastlarvalskin.Thewingedadults, whichgenerallyemergetogetherinlargenumbers,livefor aboutoneweek. Cicadalarvaedolittledamage,butwhenadultsappearin largenumberstheiregg-layingmaydamageyoungtrees. CicadasaresometimeskeptfortheirsonginAsia,asthey wereinancientGreece.LICHENusuallyslow-growing 13 第页共页