活雷(lei)達(da)(da)在(zai)非洲的尼羅河(he)中,有(you)(you)一種生活在(zai)淤泥(ni)里的河(he)象(xiang)魚(yu)。它們的尾巴能(neng)發射無線(xian)電(dian)(dian)(dian)波,而背鰭(qi)底(di)部則是(shi)無線(xian)電(dian)(dian)(dian)波的“接受器(qi)”。河(he)象(xiang)魚(yu)的尾巴把電(dian)(dian)(dian)波發出去(qu),要(yao)是(shi)碰到小魚(yu),電(dian)(dian)(dian)波立即(ji)反射回來,“接受器(qi)”接受了電(dian)(dian)(dian)波,就知道(dao)小魚(yu)在(zai)什么地方,河(he)象(xiang)魚(yu)立即(ji)竄出去(qu)把小魚(yu)咬住。在(zai)軍事(shi)上有(you)(you)重(zhong)要(yao)用途的雷(lei)達(da)(da),其原理(li)與(yu)此(ci)相(xiang)同。河(he)象(xiang)魚(yu)憑(ping)此(ci)偵(zhen)察利(li)器(qi),還能(neng)及時(shi)地發現(xian)敵(di)人,在(zai)危險沒有(you)(you)迫近身邊(bian)時(shi)就倏然逃走,因此(ci)即(ji)使是(shi)最兇猛、狡(jiao)詐的敵(di)人對它也(ye)無可奈何(he)。