I find there is a lack of breeding records of spider x spider. Supposedly people do it "all the time", the typical response for many things, but where are the records of this "very common" pairing because the few I do find, does not have a lack of viable offspring. With the oddity of this, I would like to even see someone ultrasound and see if all the follicles actually turn into eggs, there is a possibility it may not even make it to slug and get reabsorbed sometimes. Obviously not everyone has access to one though.
And since we are going to claim it is a dominant trait, not only should be be looking at the spider x spider pairing, we should be looking at breeding records for those spider offspring, something I have not see one record of. About all we know is it is not recessive lol.