al-ʿAnkabūt 29:41

"The likeness of those who take protectors apart from God is the likeness of the spider that takes for itself a house. And the frailest of houses is surely the house of the spider."

This is the verse that gives the sūra its name, and the simile carries several layers.

First — the verb is shared: ittakhadhū (they took) and ittakhadhat (it took). The same verb on both sides.

Second — the object is shared: bayt (house). A house is the place where a person takes shelter, where he is protected.

Third — the verdict: awhanu'l-buyūt — the frailest of houses.

The root و-ه-ن: the loosening of strength, the ebbing of capacity. The root was treated at Luqmān 31/14 (wahnan ʿalā wahn — the mother's hardship in carrying) and at Muḥammad 47/35.

The same root: one the weakness of what carries, the other the weakness of what is sheltered in. This is an overlap that can be verified from the text.

I record this as my own reading, and its ground is the choice of words: a spider's web is not without function — it serves the spider very well; it holds its prey there. What the verse says is that the web is frail as a house. That is, against wind, against rain, against a movement of the hand.

So the simile says not that the protectors taken serve no purpose at all, but that they cannot be a shelter. The word in the sentence is bayt, not ṣayd (prey).

وَتِلْكَ ٱلْأَمْثَٰلُ نَضْرِبُهَا لِلنَّاسِ وَمَا يَعْقِلُهَآ إِلَّا ٱلْعَٰلِمُونَ (43) — "these likenesses We set forth for people; but none understand them except those who know."

A similar sentence was treated at Fāṭir 35/28 (innamā yakhsha'llāha min ʿibādihi'l-ʿulamāʾ) and it was recorded there: the sentence is placed as the conclusion of a list of observations. The same structure is here too: the simile is given, then the condition for understanding it is stated.