A-yup. If the spider gene is truly dominant, it will have a "super" (homozygous) form that will only throw spiders, but can't be visually distinguished from the regular heterozygous spider.

The other speculation is that it's homozygous lethal, and the homozygous ("super") spiders don't survive.

... And I bet if you do a forum search of the terms "spider homozygous lethal" you will get a few dozen threads on the topic but it's never been proven one way or the other.