Sydney, July 10 : Sydney mattresses are home to thousands of dust mites, who indulge in sex marathons lasting as long as 24 hours, says an expert.

Dr. Matt Colloff, a CSIRO Entomology scientist who has spent 25 years studying mites, has described their bizarre sexual behaviour in his 600-page, 150 dollars book, ''Dust Mites'', which was launched on July 9.

"You can''t see them and you can''t feel them. But almost every house has them. A lot of Sydney mattresses would have over a million," the Sydney Morning Herald quoted Colloff as saying.

Thanks to its warm, moist climate, "Sydney is one of the world''s top 10 dust mite capitals," he added.

These less than half a millimetre long organisms inhabit beds, carpets, upholstered furniture, clothes, and curtains as they thrive on skin scales that have fallen from their human housemates.

"Their sexual behaviour is absolutely bizarre. They mate back-to-back. The male has a penis shaped liked an old-fashioned coffee-pot spout and locks onto the female with a pair of suckers," said Colloff.

As the dust mite''s penis is so narrow, compared with its sperm, "the sperm has to travel in single file,'' thus making mating slower than other organisms.

"They remain in that position for 24 hours. She continues on with everyday life and the little male hangs on," said Colloff.

The mites are often found in beds because the heat of human bodies between the sheets triggers sweating, creating the humidity and warmth the mites crave.

They spend their days "eating and copulating and defecating," said Colloff. (ANI)