Spiders (EDH / Commander) [Revision 49]

This is not a good deck. Not to say it can't be good, but it's not good now, and I have no intention of making it as good. Many of the included cards are included for flavor; namely Hidden Spider, Witch's Web, and even the commander, Arasta of the Endless Web. There are many better mono-green tribes you can build, Spiders as a tribe do better in Golgari or Jund colors (not that Spiders are a particularly good tribe even in better colors), and overall better tribal support exists.
It was fun to build, and it's kinda fun to play, and that's all I intended with the deck, so it's kind of a success.
The issues with the deck are primarily inconsistent card draw, lack of tribal synergies, and no clear win-con.

If you want to improve it, you can:
A. Choose a new commander. Ishkanah, Grafwidow seems to be a very good commander and gives access to black, which gives you access to much better card draw and some more efficient creatures. Thantis, the Warweaver would give access to black and red, though doesn't provide much outside of that.
B. Remove the tribal focus. As I said, Spiders aren't a particularly good tribe, having no specific tribal support except for one Arena exclusive card (unless you want to count Arachnogenesis as tribal support).
C. Be willing to spend more money. If you want to keep it tribal, cards like Vanquisher's Banner, Herald's Horn, and Cavern of Souls would help the deck. Sylvan Library, Craterhoof Behemoth, and Doubling Season would also help a lot. I try to build budget (with a few cards as exceptions because I already had them) so none of those cards are included, despite how good they are.
D. Remove some flavor. A lot of cards are included because they featured a spider in some way. They don't help a lot with the synergy, but they look nice in the deck.

E. Thought of this much later, but you could also make it a Selesnya toughness matters deck. There are two selesnya spiders you can use in Ancient Spider and Pale Recluse to keep the spider theme, though it does require finding a new commander (Trostani, Selesnya's Voice would work well).