There have been claims of double spiders and if this being true its seems to me it would be easier to breed the animal, preferably a male, to several female normals and you should see nothing but spiders.

The other thing to do is breed spider to spider and hold back several males and breed them to non spider females and see if one throws out all spiders.

I think so many factors play into slugs so to say it was spider related would be hard to do with any certainty.