An arachnid with an erection nearly half the size of its body has been found in South-East Asia, preserved in amber for millions of years.

Key points: Arachnid found in amber believed to be 99 million years old

Arachnid found in amber believed to be 99 million years old First time the penis of a prehistoric harvestman has been captured outside the body

First time the penis of a prehistoric harvestman has been captured outside the body Discovery shows prehistoric harvestmen mated in the same way as their modern counterparts

The eight-legged creature was entombed in amber 99 million years ago while sitting on a tree in Myanmar.

The unfortunate fate of the arachnid — a relative to the daddy long-legs called the Halitherses grimaldiihad — was revealed in The Science of Nature journal.

Scientists from the Berlin Museum for Natural History said the find was the first to capture a penis of this nature in amber.

The arachnid is in the harvestman group — a diverse group with more than 6,600 species. Only 38 fossil species are known, the journal article said.

Besides being a long 99 million years for the erect arachnid, the fossilised phallus shows prehistoric harvestmen mated in the same way as animals do today.

"The penis has a slender, distally flattened truncus, a spatulate heart-shaped glans and a short distal stylus, twisted at the tip," the article stated.

"No living harvestman has a penis with exactly this shape.

"This is the first record of a male copulatory organ of this nature preserved in amber and is of special importance due to the age of the deposit."

Getting to the point in mating act

Unlike male spiders and scorpions, which use modified legs to transfer sperm to females in tidy packets, most harvestmen, including the one found in the amber fossil, insert into genital openings next to the females' mouthparts.

The research team, led by the museum's Jason Dunlop, said the penis of the harvestman was usually hidden inside the body when not in use.

"In the taxonomy of modern harvestmen, penis anatomy is extremely important: each species has its own unique structure," the research team said.

"Also the major groups (or families) of the harvestmen have different penis forms.

"So far only a few fossil harvestmen have revealed their penis, which makes it difficult to compare them effectively with living species."