The theory I've heard most was that prehistoric humans learned to fear early arachnids which were more deadly on average. Letting a creepy crawly escape could have meant a tribe member dying later to a potent bite before the creation of anti-venoms. The instinct to swat is supposedly a reaction to avoid getting bitten. Mothers would have had a natural incentive to obliterate any dangerous invaders coming into her children's environment.
Plus the little egg legged bastards look intensely alien compared to the anatomy of other creatures we're used to. They have 8 legs, 8 eyes, large venomous fangs, and scurry about like walking detached hands.
I think jumping spiders and daddy long legs are harmlessly cute, but you have to be in deep denial to not recognize how the appearance of less docile arachnids could spark a shock response from most people. Especially when so many of them remember the first painful bite they got in childhood...
all that applies to snakes for me especially the looking alien part.
I just think it's funny so many people think spiders are that much worse than snakes. if you're in a country that has a lot of both(mine, australia) you realise a spiders chance of killing you these days is like 0.1%
Snakes is considerably higher
Oz hasn't had a death due to spider bite since like '79. ---
