Bridge spiders build webs that double as acoustic arrays

What a cool thought!

What if it takes so much energy to move in the fourth dimension that the entities have to be vulnerable? They might be even easy to catch, or destroy by accident.

"Spiders invert this relationship -- they are 2D specialists that slice a 3D space with a trap, capturing 3D specialists without expending energy to navigate their prey's space."

I think that's a great way to put it, although "spiders" really should say "orb weaving spiders".

Jumping spiders don't build a web, they are active hunters in the 3D space with the ability to plan several steps ahead. Although they can't fly, they definitely hunt in 3D and don't really conserve energy.

Huntsman spiders are also active hunters, but mostly in 2D and waiting from an ambush position. They don't build a web. Neither do crab spiders, but I'd agree that these two families of spiders are trap-based and largely passive, even if the trap is not a web.

Labyrinth spiders (funnel web spiders) are a fun anomaly due to the complexity of their trap, which is 3D, but arguably it's a still a passive way to hunt.

My favorite would be the net-casting spider, which builds a tiny web that it holds, and then casts on its prey. The timing and coordination of that is mind blowing: https://www.youtube.com/watch?v=QyRPrax9MJU&t=19s

Not all spiders are 2d specialists who hunt with webs. Jumping spiders display extraordinary spatial reasoning in conjunction with excellent vision, which they use to stalk prey. Jumping spiders have been known to sit motionless for up to half an hour, scanning the landscape to plan the best route to something they can see (the FOV of their hi-res front-facing eyes is quite narrow, so this scanning is necessary to gather the required information).

We are the 4-dimensional invaders of this planet. :)

If one enjoys reading SF that involves intelligent spiders, then A Deepness in the Sky [0] is worth a look. Although the arachnoids in 'Deepness' are spider-like aliens, rather than evolved Earth-spiders, as in Children of Time. The latter also has evolved ants, as well. Both books are highly recommended.

[0] https://en.wikipedia.org/wiki/A_Deepness_in_the_Sky

Since the web is much bigger than whatever serves as spider ears, it should pick up much lower frequencies, if my physics intuition is working this morning.

> You'd expect it to react when the web vibrates.

I imagine if I were to set up a parabolic reflector, my reaction to sounds I hear would be different from my reaction of something hitting the reflector itself.