萤火虫在山谷里清河上飞舞

七月一日，我看到了一些萤火虫，它们在山谷里清河上飞舞呢。它们的数量很少，因为今年的最盛期大半完了。
我捉到了一只。它是雌的，因为它的发光器稍小。

On July 1st, I saw some Japanese fireflies flying on the clear stream in the mountain. They were few because the peak of their active season in this year have almost gone.
I caught one of them. It was female because it has a somewhat small illuminant.

I haven't see them for many years，miss them~~~
I like your words " 山谷里清河上”，like a poem，haha，good job！

萤火虫在山谷里的请→清河上飞舞

七月一日，我看到了一些萤火虫，它们在山谷里的请→清河上飞舞呢。它们的数量 很少 / 不多，因为今年的最盛期大半完了→萤火虫最活跃的高峰期快接近尾声了。
我捉刀→到了一只。它是雌的，因为它的发光器稍→较小。

* ~ 请=please                                    刀=knife
      清(澈)的=clear                          捉到=(have) caugh
  ~ both “很少” and "稍小" is ok, but in my language sense, "不多" and "较小" may be better. But they just for your reference only.

On July 1st, I saw some Japanese fireflies flying on the clear stream in the mountain. They were few because the peak of their active season in this year have→has almost gone.
I caught one of them→one among them. It was a female because it has a somewhat small illuminant.

* The revised version of the last sentence is just for your reference only.

中文就不改了，改英文。

On July 1st, I saw some Japanese fireflies flying above the clear stream in the mountain. The peak of their active season in this year has almost passed.
I caught one of them. It was a female because it has a somewhat smaller light-emitting organ.

萤火虫在山谷里请河清澈的溪流上上飞舞

七月一日，我看到了一些萤火虫，它们在山谷里请河清澈的溪流上飞舞呢。它们的数量很少，因为今年的最盛期大半完了。
我捉刀到了一只。它是雌的，因为它的发光器稍小。

On July 1st, I saw some Japanese fireflies flying on the clear stream in the mountain. They were few because the peak of their active season in this year have almost gone.
I caught one of them. It was female because it has a somewhat small illuminant.

萤火虫在山谷里请河上飞舞

    七月一日，我看到了一些萤火虫，它们（正）在山谷里请（清清的）河上飞舞呢，它们的数（量）很少，因为今年的最盛期大半完了他们活动的鼎盛期大半已过完了。
     我捉刀（到）了一只。它（应该）是雌的，因为它的发光器稍小。

On July 1st, I saw some Japanese fireflies flying on the clear stream in the mountain. They were few because the peak of their active season in this year have almost gone.
I caught one of them. It was female because it has a somewhat small illuminant.