Clause starting with "so well"

Question

I am learning English from reading articles. I am reading "Sight Unseen" from The New Yorker (link: https://www.newyorker.com/magazine/2015/04/13/sight-unseen-critic-at-large-kathryn-schulz), and there is a sentence...

Perhaps you have seen a stick insect sitting on a stick, or a leaf-shaped katydid hanging from a branch—but probably you have not, so well do they blend in.

The part "so well do they blend in" is really confusing to me. From my understanding, the clause tries to tell us that these insects probably blend in the nature extremely well, so the readers cannot recognize. However, I'm confused about the structure of the clause. Why is it able to use "so well" in the beginning of the clause and put "do" before "they"? It overall seems like a structure for a question for me, and I'm not familiar with it. Thank you very much for your help in advance!

Answer 1

Perhaps you have seen a stick insect sitting on a stick, or a leaf-shaped katydid hanging from a branch—but probably you have not, so well do they blend in.

It's called subject-auxilary inversion. Here, the inversion is triggered by the preposing of the complement "so well".

The basic order would be "... they blend in so well", where there is no inversion.

Preposing of a complement like this is a literary device, often adding emphasis to it, as well as linking the theme to the previous discourse.

The expression could be paraphrased as "... because they blend in so well".

Answer 2

Let's look at a simplified version of your sentence:

You probably haven't seen one, so well do they blend in.

This is the same as:

They blend in so well that you probably haven't seen one.

The unusual word order is called subject-auxiliary inversion and you're already familiar with it for its much more common use in forming questions. But it's also used to form something called a condition clause. That's what we have here. The fragment "it blends in so well" changes from a causative statement followed by a statement of effect, to a condition clause following a statement. The emphasis shifts from the cause to the effect and is a stylistic choice. The semantic meaning of both sentences is the same.