石蝇
n. primitive winged insect with a flattened body; used as bait by fishermen; aquatic gilled larvae are carnivorous and pve beneath stones
1.stonefly and mayfly larvae have gills, with which they extract oxygen from the water.
石蝇和蜉蝣的幼虫有鳃 能够使它们从水中提取氧气