Sorry, have to throw in my two bits ...

If it turns out that spider is completely dominant AND BOTH of your spider's parents where spiders then there is a 33% chance that he is homozygous for the spider gene and all of his offspring will be spiders.

Most, if not all, spiders so far are heterozygous for the spider gene. Heterozygous doesn't mean "normal looking" it only means that they have unmatched genes - in this case one spider and one normal copy of the same gene. With recessive mutations the heterozygous animals are supposed to look normal but with co-dominant and dominant they are mutants. So it is true that you shouldn't see "possible hets" offered for co-dominant genes but it is perfectly ok to talk about a "het spider" to refer to a spider (not normal looking) with one copy of the gene. If homozygous spiders are proven and they end up looking like heterozygous spiders then you will probably hear "het spider" all the time to distinguish between the ones with one and the ones with two copies of the spider gene. Homozygous spiders would be valuable breeders as they would produce on average twice as many spiders bred to a normal, 25% more spiders (twice as many of them homozygous) than breeding het spider X het spider, and all homozygous spiders when bred to another homozygous spider.

Also, the odds for each baby from a heterozygous spider X non spider is 50/50 for getting the spider gene. Each babies odds are independent of the others so you are not guaranteed at least one spider in a clutch. You will probably get about half spiders but you could get all spiders or even no spiders. The bigger the clutch the less likely of getting either extreme. With an 8 egg clutch from heterozygous spider X normal your odds of getting no spiders are only 0.4% which means you have a 99.6% chance of getting at least one. Also, your odds of getting 8 spiders is only 0.4%. If there where only 4 eggs you would have a 6.25% chance of getting all or no spiders (93.75% chance of getting at least one spider). So, your odds are good of producing spiders but there are no guarantees.

In fact, these odds are the same when breeding any het to a non het. It's just that with spiders you can see the hets. If you where breeding a het albino to a normal girl and produced 4 daughters you would have a 93.75% chance that at least one was het albino. You just wouldn't know which one until you bred them. I actually did this with a 1998 albino het male from NERD and so far two of the four daughters produced in 2000 have produced and one proved het albino. The other one only had two eggs and since she was only bred to a het albino male (he actually started out 66% chance but proved with the first clutch) she could still well be a het and I missed exposing it with only two eggs from het X het. Hopefully I'll get the last two daughters to breed soon but I know there is only a 6.25% chance of them all being hets.