A Spider Trap spawns two Hell Gazers, scaled to the stats of Flesh Arachnids so they're less threatening if encountered early, to the left and right of the player - about as far out as the two Meteor Sparrows from a Sparrow Trap would spawn out, but closer if there's a wall in that direction.

Nasty surprise if you activate one while dashing past a check, and probably less likely to result in weird issues than Throw Stun Traps seem to be.