A spider has a trigger-strand. This is the strand it is touching, probably with a forleg/foot, to read its web. In some situations, the spider detects a collision with the web and moves rapidly to its -center-, and from there, fairly directly to prey. In other situations, as, for example, when the prey is exceptionally small, and also still, the spider will, as before, occupy the center of the web, but in this case, it will ping webstrands (usually in pairs) with the forelegs/feet.

By this I mean it agitates them in pairs and reads both. Now this pairing is interesting in part because it is bisymmetrical; but it is also a way of creating a binary choice. A strand containing even a small insect or debris object will ‘echo’ in a way that a free strand will not. By feeling pairs that are proximate, the spider can detect many things at once, including prey size or strand involvement.

And this, for those who can metaphy it as it relates to thought and problem-solving… is a powerful, if not unbeatable heuristic, for it is actually a model of how to solve -any- kind of problem, including what kinds of problems to select. But this view is too well-seasoned for most. Simply understand: agitate the strands, and carefully observe both the unaffected and the affected replies.