Spider is a dom*(sorry dom) there has yet to be proven a super form. there for a spider to a normal would give you 25% odds.

Unless there is a super spider I know not of? and my spiders have been playing tricks on me?

Edit:
Actually a Super would give 100% spiders. I dont see how you can get 50%? maybe a spider to a spider would drop those odds. But than I believe the super form of the spiders dont survive