All these sentences use the word "so" correctly except "Flies breathe through holes in
<span> their abdomen or thorax, so do bees.</span>" So should either begin a new sentence of be preceded by "and".
The conventional rule is to take the plural form of the verb when two nouns are joined by the conjunction 'and'. However, there are exceptions. Only then can you break the rule.
When the two nouns refer to one thing only, then you can use the single form. For example: The president and the CEO joins the meeting today. In this example, the president and CEO apply to one person only. Hence, you use the singular form of the verb.
It's a metaphor.
To help you understand why, here's the definitions of both:
Simile: it's a <span>figure of speech involving the comparison of one thing with another thing of a different kind, it's used to make writing seem more vivid.
Metaphor: this is also a</span><span> figure of speech in which a word or phrase is applied to an object or action to which it is not literally applicable.
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