Three species of extremely primitive spider discovered in China

Three new species of an ancient, secretive group of spiders, all native to Hunan province in China, have been described. These “mesothelean” spiders diverged from other arachnid families about 300 million years ago and have strange, primitive features not found in the vast majority of living spiders.

Most spider species on Earth today belong to one of two groups: the heavy-bodied mygalomorph spiders – such as tarantulas and the notoriously venomous funnel-web spiders – and the tens of thousands of araneomorph spider species, many of which spin intricate, sticky webs.

Approximately 100 spider species belong to a poorly understood third group that fall under the suborder Mesothelae. The mesothelean spiders diverged from other spiders back when the planet’s rainforests were full of giant arthropods and the very first reptiles. Today, the sole remaining mesothelean spider family retains some physical features of the first spiders. Unlike all other spiders, mesotheleans have a segmented abdomen with plates on top, much like a shrimp tail or a bee’s rump. Their silk-spewing spinnerets are uniquely slung below the centre of their abdomen, rather than positioned at the rearmost tip.

All modern mesothelean spiders live in East and South-East Asia, are about 1 to 2 centimetres long and ambush prey from tube-shaped lairs.

Xin Xu at Hunan Normal University in China and her colleagues gathered young, mesothelean spiders in parks and villages in western Hunan province and reared them to adulthood in the lab.

Based on differences in the shape of the spiders’ sexual structures, which can be used to distinguish between closely related species, the researchers determined they were looking at three previously undescribed species, all in the genus Songthela. The species – Songthela anhua, Songthela longhui and Songthela zhongpo – are named after locations in the province where they were found.

“[Mesotheleans] are living fossils, rare in general,” says Sarah Crews at the California Academy of Sciences in San Francisco, who wasn’t involved with the research. “We are learning they are more diverse than once thought.”

Cataloguing this diversity is crucial for understanding potential environmental impacts from human activity such as building or mining.

“If you don’t know what you are looking at, you can’t determine anything about its range or rarity,” says Crews.

Many mesothelean spiders are found in very small geographic areas, possibly due to their travel-averse, burrow-dwelling lifestyle, which has led to different species inhabiting isolated areas along the folds of mountainous, forested terrain. These narrow ranges may make some mesotheleans – such as the Batu Caves trapdoor spider (Liphistius batuensis) from Malaysia, known from only a handful of limestone caves – particularly vulnerable to extinction.

But there is more spider life to discover. New species of spider are described every day, says Crews, with 977 species added to the list known by scientists in 2020.

“There are 50,979 currently valid species. Spiders are one of the most diverse terrestrial arthropod orders,” she says.