The angular legs, dark colour and an unpredictable nature are what makes spiders so much scarier than other animals, scientists say.
Psychology professor Jon May says studies show people tend to prefer curved shapes over sharp ones, have bad associations with dark colours, and prefer creatures they feel they understand.
"Spiders just tick all these boxes, and like any phobia, when it builds up in someone's mind they can become scared even seeing a picture," says Prof May, from Plymouth University.
"We like bright-coloured butterflies and ladybirds, but spiders are dark coloured with long angular legs - and the shape and colour both have strong negative associations.
"We are also very sensitive to seeing things moving out of the corner of our eye and immediately notice it, and insects move quickly and unpredictably.
"People scared of spiders will often report them being bigger than they were or say they saw one crawl into someone's mouth, which spiders never do.
We don't understand their behaviour.'
Prof May says fear is also "socially conditioned", so if children see scared parents or siblings it's more likely they'll develop fear.
Arachnophobics can deal with their fears by trying to sympathise with the insects and learn about them, he says.
- DAILY MAIL