They are extremely short-sighted.

At a hand's-breadth's distance, the lifeless prey, unable to shake the web, remains unperceived.

Besides, in many cases, the hunting takes place in the dense darkness of the night, when sight, even if it were good, would not avail.
If the eyes are insufficient guides, even close at hand, how will it be when the prey has to be spied from afar?
In that case, an intelligence apparatus for long-distance work becomes indispensable.

We have no difficulty in detecting the apparatus.
Let us look attentively behind the web of any Epeira with a daytime hiding-place: we shall see a thread that starts from the centre of the network, ascends in a slanting line outside the plane of the web and ends at the ambush where the Spider lurks all day.

Except at the central point, there is no connection between this thread and the rest of the work, no interweaving with the scaffolding-threads.