You already answered your own question of why a spider - poison-based.
There's also a big ol' boulder you can throw at people, plus sexy armor spells.
Regarding the spider, I think a giant serpent would fit the role better, since it feels earth and poisony.
Also, I repeat that I think a lvl one earthy damage spell needs to exist. I really don't want to have to use poison in my repetoir for damage.