Hairier skin may be the key to avoiding being bitten by bed bugs, claim Sheffield academics.
Hungry bugs placed on shaved arms were more likely to try to feed compared with those on unshaved arms, the journal Biology Letters reported.
Researchers say the hair slows down the bed bugs and warns the victim.
Pest controllers say the UK is currently experiencing a steep rise in the number of bed bug infestations.
Prof Michael Siva-Jothy, from Sheffield University's Department of Animal and Plant Sciences, recruited 29 brave volunteers to test the theory further, watching the bedbugs as they found a place to feed and removing them only as they were about to bite.
He found that more layers of both longer visible hairs and finer, "vellus" hairs near the surface appeared to work as a deterrent to the insects, with the finer hairs also acting as an early warning system.
Prof Siva-Jothy said: "Our findings show that more body hairs mean better detection of parasites - the hairs have nerves attached to them and provide us with the ability to detect displacement."
He said they also slowed down the insect as it searched for a tasty spot to bite.
"The results have implications for understanding why we look the way we do, what selective forces might have driven us to look the way we do, and may even provide insight for better understanding of how to reduce biting insects' impact on humans."
However, even though men are naturally hairier than women, they do not appear to be bitten less often.
Professor Siva-Jothy suggested this pointed to an evolutionary battle between bed bugs and their prey, with the insects adapting to automatically head for relatively hairless bits of the body, such as wrists and ankles.
He added that extreme hairiness might also be more of a disadvantage than an advantage.
"If you have a heavy coat of long thick hairs it is easier for parasites to hide, even if you can detect them.
"Our proposal is that we retain the fine covering because it aids detection and if we lost all hair, even the relatively invisible fine hair, our detection ability goes right down."
This tallies with other studies which look at how humans came to be relatively less hairy than apes.
Other scientists have suggested that swapping thicker fur for clothes was a way of making insect bites and parasitic infestations less likely.
Prof Mark Pagel, an evolutionary biologist at the University of Reading, said that biting parasites remain a major cause of disease and death worldwide, making them a potentially enormous evolutionary pressure on early man.
He said: "This vellus hair is certainly no use for anything else, so it is a reasonable hypothesis that it developed in response to a strong selective pressure in our past.
"Mammals are unique in developing this wonderful fur, and humans are the only mammals to jettison it, so there must have been a very good reason to do so."