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The world's longest insect and a "ninja" slug which fires love darts at its mate are among dozens of new species recently discovered in a stretch of "irreplaceable" rainforest, conservationists said today.

The three governments with jurisdiction over the island of Borneo pledged three years ago to conserve a tract of tropical forest almost as big as the UK that is home to pygmy elephants, orang-utans, rhinoceros and clouded leopards.

In the three years since the "Heart of Borneo" conservation plan was drawn up, more than 120 new species have been discovered in the 220,000 square kilometre (85,000 square mile) area of tropical rainforest, according to a report by conservation charity WWF.

The new discoveries include a flying frog which changes colour at night, a flame-coloured snake, a "spectacled flowerpecker" bird which is thought to rarely descend from the canopy, 29 invertebrates, 17 fish and 37 new species of orchid.

A stick insect which is more than half a metre long was also discovered, as was a long-tailed slug which wraps its tail around its body when resting.

The yellow-green "ninja" slug also fires harpoon-like "love darts" at its mate which inject hormones that may increase the chances of reproduction.

And scientists have discovered that an already-known species of frog, the Bornean flat-headed frog, has no lungs, the Borneo's New World report said.

According to the study, the region is home to 10 species of primate, more than 350 birds, 150 reptiles and amphibians and 10,000 types of plants.

In the three years since the start of the Heart of Borneo project, which is supported by WWF, scientists have 123 new species - with an average of three new plants and animals being found a month.

Adam Tomasek, leader of WWF's Heart of Borneo initiative, said: "As the past three years of independent scientific discovery have proven, new forms of life are constantly being discovered in the Heart of Borneo.

"If this stretch of irreplaceable rainforest can be conserved for our children, the promise of more discoveries must be a tantalising one for the next generation of researchers to contemplate."

He added: "The discovery of these new species in the Heart of Borneo underlines the incredible diversity of this remarkable area and emphasises the importance of the commitments already made by Brunei Darussalam, Indonesia and Malaysia to protect it."

The scheme by the three governments aims to conserve the region's wildlife, enhance the management of protected areas, support sustainable use of resources, and develop eco-tourism.

WWF said the discoveries also highlighted the need to increase financial and technical support for the region to ensure the wildlife survives.