E：代词指代包括名词前面所有的修饰语，这里they指代single locusts，placed伴随状语，逻辑主语还是这个single locusts。
A is NOT "less concise"; it's actually necessary, to specify that we're now discussing the behavior of locusts in general, not single locusts.

E，表示单个的蝗虫很兴奋。应该是单个的蝗虫和其他的蝗虫一起都很兴奋。所以第二句引入新的主语“locusts“比沿用第一句的主语”single locusts“要好。

when placing them with others of their species, then，主语是locust，locust不能主动placing locust

E then多余，过去分词已经表示了when引导状语从句一样的时间概念；they和their完全指代single locusts，不优

这里是要强调蝗虫群一起嗡嗡嗡，一起很兴奋，而不是单个的蝗虫放进群里以后，他自己变得很兴奋，开始嗡嗡嗡。所以they指代locusts会更好。

locust n. 蝗虫。 locusts是复数形式。愣是没明白为什么 single + plural noun, 而且谓语动词还是复数. single做形容词解释为“单一的”，修饰的名词一定以单数形式出现。e.g.: Every single house in town had been damaged

The original subject is "single locusts".

The second part of the sentence doesn't describe single locusts. The subject of choice E is still "single locusts", so E is nonsense.

The appearance of "locusts" in choice A is NOT "less concise"; it's actually necessary, to specify that we're now discussing the behavior of locusts in general, not single locusts.

D: 划线部分后面的they就没有合适的指代对象了

e: they 完全指代single locusts，这不合逻辑

then 是副词，不是连词，不能连接两个句子。

老师绝壁想说的是只有 IF 和 WHEN 可以连接。笔误了吧

but 是连词，连接两个分句。
选项E：伴随状语placed with others of their species和其主句placed with others of their species之前不能出现副词then，只有if可以和then连接。

A选项两个locusts没有赘余的问题吗？

只有if可以和then连接。