This explains why arachnophobics, who make up around one in ten of the population, often focus their anxiety on tarantulas which are usually found in more tropical climes.
Higher panic levels are registered in the brain if the object of concern is unfamiliar, according to a new report.
A study of 20 people with a medium to low fear of spiders found the human brain responds to threats based on proximity, trajectory and expectations.
Dr Dean Mobbs, a neuroscientist from Cambridge University, said: "There are several regions of the brain that are triggered when we see a tarantula - particularly the panic area which is situated in the centre.
"The UK has one of the highest rates for phobias about spiders and snakes because we don't come across them very much."
Dr Mobbs hopes to develop an aversion therapy to prevent the "cascade of events in in the brain" which causes spider-induced terror.
During the study, published in Proceedings of the National Academy of Sciences, participants placed their foot into a box and rated their level of fear as they watched via a video link as a tarantula was placed closer and closer to them.
The world's most poisonous spider, which is unlikely to be found in Britain, is the Brazilian huntsman. Just a 50,000th of an ounce of venom can kill an adult human.