It’s an arachnophobe’s worst nightmare.

A child-size scorpion that roamed the UK 400 million years ago is thought to be largest of its kind to ever exist, per a skin-crawling study in the journal Paleontology.

“So we can safely say it’s extremely large and there’s no other scorpion in the fossil record that has claws anywhere near that size,” declared the study’s lead author Dr. Richard Howard of the Natural History Museum in London, the BBC reported.

Dubbed Praearcturus, this colossal arthropod scuttled across what is now England and Wales during the Devonian period circa 415 million years ago.

The species measured over 3 feet long with six-inch pincers, meaning its appendages were the same size as the bodies of the largest scorpions living today.

Despite its hulking proportions, this sting king took a long time to identify. The fossils were first discovered in Herefordshire in the 1870s, when they were initially thought to belong to a type of giant crustacean such as a woodlouse due to the lack of a tail or other identifying feature.

It wasn’t until 150 years later that researchers were able to put together the pieces. Howard said they were able to glean the ID through modern techniques like CT scans and 3D modeling as well as better-preserved fossils, most notably a sternum that matched remains found in Canada that were described in 2015.

Dr. Russell Garwood, a paleontologist at The University of Manchester, explained, “by bringing together material from several collections and using cutting edge imaging techniques, we’ve been able to build a clearer picture of the animal than was previously possible, which is really exciting.”

Meanwhile, Howard found the critter’s size unique given that it lived around 50 million years earlier than the “giant millipedes or dragonfly-like insects” of the Carboniferous period, when Earth had developed swamps, jungles and other advanced terrestrial habitats.

“Confirming that this animal is a scorpion fundamentally changes our understanding of how and when these creatures evolved to such extraordinary sizes,” he said.

One theory is that the animal grew huge because it faced little competition in its environment, allowing it to dominate the ecosystem in a manner that wouldn’t be possible later on

Another possible reason? Praearcturus spent part of its time in the water. Some of the fossils sported flap-like structures known as epimera — similar to features on the carapaces of lobsters and crabs — which suggested that these scorpions were semiaquatic, Livescience reported.

This would explain their behemoth proportions as water can support larger bodies.

More importantly, the bodaceous bug represents a transitional period when aquatic critters were making inroads on land.

“Praearcturus gives us a fascinating glimpse into how early animals adapted to these changing environments,” said Dr Greg Edgecombe, Merit Researcher at the Natural History Musuem, London, and co-author of the study. “It may even represent a lineage that returned to the water after earlier ancestors had already begun living on land.”