are mating flies. 其实是正在交配的幽蚊

145. Soon, millions upon millions of newly hatched lake flies are taking to the wing.
     不久 成百上千万只新羽化的幽蚊 全都扇动着翅膀飞到空中
146. Early explorers told tales of lakes that smoked, as if on fire.
     早期探险家看到这种湖面上“冒烟”的情景 还以为是湖底着了火
147. But these spiralling columns hundreds if metres high
     这些高达数百米的螺旋型“烟”柱
148. are mating flies.
     其实是正在交配的幽蚊
149. Once the flies have mated,
     幽蚊完成交配之后
150. they will all drop to the water surface,
     便会一起落到水面
151. release their eggs and die.
     产下卵 然后死去

来自《行星地球 第一季 第三集 Planet Earth S01E03 淡水》第 148 行