I don't believe that a fear of anything beyond tormenting pain and death can really be genetically encoded. A fear of snakes is, more likely than not, attributed to learned behavior and individual personality than genetics. I think people react more to certain attributes of snakes than the actual snakes themselves; I.E. hissing.
Present a small child with a baby corn or hognose snake and they'll probably be enamored by it. Present them with an adult King Cobra as it flairs its hood, hisses, and lunges at them and they'll probably be filled with an overwhelming sense of terror.
From my casual observations most animals capable of hearing react very strongly to anything that sounds like hissing. It seems to be one of the best ways for different species to communicate their antagonism with one another. Imagine a Pomeranian aggressively approaching an adult monitor/iguana/tegu/whatever, barking and doing all that other annoying crap that I hate Pomeranians for. Lizard responds by facing the little dog, flaunting its teeth (which are barely visible), and hissing. Little dog runs away in terror.
Why should this be? The dog's wolf ancestors [probably] never encountered big lizards in its natural habitat. Unless the lizard in question was a perentie or a crocodile monitor none of them would have posed a threat to the wolf either (both species are Australian, and thus would probably not have been encountered).
Now, the little dog clearly did not have a pre-disposed fear of the giant lizard itself even though it was clearly large and strong enough to kill and eat it. If it did it wouldn't have approached in the first place. It did have an immediate reaction to it hissing though. I imagine this originates from it's (and our) ancestors' encounters with venomous and constricting snakes. Many people don't initially fear snakes until they've learned to do so; either by being taught so by their parents or experiencing one hissing and lunging at them.
As for spiders... I have no idea. Perhaps our ancestors would commonly seek food hiding in crevasses and would get a less-than-pleasant surprise when it stuck its hand in. Of course, that would probably have also lead to a pre-disposed fear of land crabs which we don't have. Land crabs also make me skeptical of the scorpion idea. A crab can look very much like a spider, especially the small ones, yet when a person encounters a crab in their yard or even their house they're probably not going to have as strong a reaction as if it were a giant spider.