Some of the specimens are incredibly fragile

The London museum says the move, to a new building due to be completed in 2008, will help safeguard the specimens for future generations to view.

The move is being carried out with extreme care, many of the examples in the collection are irreplaceable.

One aim is to make the collection more accessible for the viewing public.

It will take 18 weeks to move the specimens out of about 140,000 drawers in the 1950s entomology building, which is to be demolished. They will be moved into temporary homes in other parts of the museum and at a storage facility in Wandsworth, south London.

Unique specimens

Darwin Centre Phase Two, which will occupy the same site as the entomology building, will keep the collections safe in a seven-storey environmentally controlled "cocoon".

The museum's collection is described as a "library of life"

"People come to the museum and they love it, but they don't see that much of it," said Nigel Fergusson, an entomologist at the museum who is helping co-ordinate the move.

"We haven't really told the story of this collection that is used for research all the time."

The insects range in size from the barely visible fairy fly, Alaptus magnanimus, (with a wingspan of 0.02cm) to the world's largest moth, Thysania agrippina, from Central and South America (with a wingspan of 30cm).

The largest specimen to move will be a 105cm by 42cm hornet nest (Vespa crabro crabroniformis) from China.

Library of life

The insects have been collected over three centuries by the naturalists Charles Darwin, Hans Sloane and Sir Joseph Banks.

"The insect collections form an irreplaceable 'library of life' supporting research on human health, biodiversity, conservation and the environment around the world," Dr Fergusson said.

He added: "What we do need to take care of is that we move it all slowly and carefully."

Some of the small flies in the collection were the most fragile specimens, said Dr Fergusson.

The antiquated conditions in the old building, designed in the 1930s and completed in the 1950s, were putting the collections at risk from attack by pests, humidity, temperature and fire, says the museum.

But Dr Fergusson said that inspections of the collection show the specimens are generally in excellent condition apart from the occasional cracks in the glass display covers.