At level 13 it looks like this:
|Corpse Spiders (basic version)|
You get about 3 attacks each spider so the basic version does about 4 spiders * 3 attacks * 16% and the Leaping Spiders is 4 * 3 * 19%. That works out to 192% weapon damage and 228% weapon damage. Worth 3 points of anyone's mana!
At level 9 you unlock a rune for it. This changes the spell's visuals quite dramatically and improves it by increasing the damage the spiders do per bite and giving them a leap. This makes them better at attacking moving prey but it has been claimed that the basic spiders are better against stationary targets.
Runed it looks like this:
They're quite good at munching things as these unfortunate zombies discovered:
|What do spiders eat for dinner? Everything!|
Another subtle advantage of the Leaping Spiders is that although they don't attract aggro (ie no mob thinks I'm picking that spider as my target), they do disrupt pathfinding. Leaping Spiders are bigger than the basic spiders so they get in the way more.
In this shot the Skeleton King was briefly trapped. He hesitated as if he wanted to move towards someone but couldn't path find. (Although he escapes when he uses his Whirlwind or Vanish abilities).
If you cast it at monsters that are moving towards you they form behind it. You can cast it onto the ground by holding shift, so dropping them into the paths of monsters slows them a small amount (as they have to deviate around the spiders).
Like all summons it's dynamic. If you have spiders out and cast Soul Harvest or equip a better weapon you'll see their damage increase dramatically.
It's possible to set up traps. If you're waiting for a monster to spawn you can spam spiders to create a reception committee. The unfortunate Mira Eamon met such a committee in this shot - she died almost instantly:
|The ! denotes Haedric experiencing a what the fuck moment.|
And just in case anyone hasn't had enough spiders already here's a fabulous video from a fellow arachnid enthusiast. Hat tip to Plateau Gaming and FixtionFXR for this wonderful film.