A homozygous form of a mutation does not have to be different than the Het. Thats what we are talking about. If its a recessive mutation you need a pair of the genes, if its a codominant mutation then a single gene gets you something and a pair gets you the super form. If its Dominant then a single gene gets you the mutation and a pair of the genes gets you the same thing but when you breed an animal that's homozygous for a dominant trait you are guaranteed the offspring will show that trait. Whether they are homozygous or heterozygous then depends on the other parent and luck if the other parent is heterozygous for the same trait.
The question is whether the Spider mutation is codominant or dominant and that depends entirely on whether the phenotype of the homozygous is different than the heterozygous. I would still have to assert that since the homozygous spiders all die they are a super form.
The answer is unknown and can only be proven by a living homozygous spider (something that can only be proven by multiple breedings to normals throwing only spiders or a genetic test).
Or else it comes down to a strict definition of whether or not a lethal homozygous gene is considered a super form despite no visible difference. I am not sure on that one.