Nobody in France uses the outdated curse Sacre bleu! anymore, but perhaps it’s time to bring it back. In a paper published in the journal PeerJ on Tuesday, scientists revealed that France’s metropolises and overseas territories are being invaded by giant, vaguely penis-shaped flatworms that, in some cases, are a dark, glittering shade of blue. They didn’t need to go far to collect evidence: Citizen scientists, snapping photos of their scary new neighbors, did all the terrifying legwork. The strangest thing, however, is that scientists haven’t paid them much notice until now.

These huge hammerhead flatworms, known as bipaliines, appear to have been invading France and its territories for almost 20 years, and somehow, they’ve crawled right past scientists. “At the beginning of our study, we were intrigued by the almost total absence of published information about the presence of bipaliines in France,” the team, led by Jean-Lou Justine, Ph.D. of the Muséum National d’Histoire Naturelle in Paris, writes in the paper. Bilpaliines are native to the warm parts of Asia, but increased globalization has made it easy for them to hitch rides to cooler zones.

It turns out that other scientists had shrugged off reports from terrified citizens over the years. One kindergarten reported in 2013 that hundreds of “small snakes” had terrified the students; others who went to local scientists with specimens were told they were just big leeches or “plain, uninteresting animals.” One woman, who went to Justine’s team, said she’d found one of the flatworms in her cat’s fur but had been told by her veterinarian that it was just a tapeworm.

blue giant flatworm
Diversibipalium sp. "blue" are native to Mayotte, a French island off the African East Coast. 

Denial will get you nowhere, as the team found. In their four-year survey of 111 pieces of evidence collected by citizen scientists between 1999 and 2017, they discovered that there are two species of the giant hammerhead flatworms present in Metropolitan France. These species can grow to be utter monsters: Bipalium kewense and Diversibipalium multilineatum can grow to be 15 inches long.

In the French overseas territories in the Caribbean, South America, Africa and Oceania, there were even more giants. Bipalium vagum gets so big it can eat molluscs. Then, there are the two striking unnamed species that the researchers temporarily filed under the broad Diversibipalium family, under the subtypes “black” and the striking, iridescent “blue.” Diversibipalium sp. “blue,” the team writes, is only found on Mayotte, a French island off the African East Coast.

mayotte map
The bright blue worms were found on Mayotte, a tiny French overseas territory north of Madagascar. 

The invasive worms, the paper suggests, show the dark, sometimes glittering blue downside of globalization. As the team found through partial DNA sequencing, three of the species — B. kewense, B. vagum and D. multilineatum — had no variability in their genomes, suggesting that all of the specimens they looked at were just clones of each other and therefore all likely came from the same individual invaders. The team didn’t investigate how the species got to France and its territories, but they warn that it’s high time they find out, since these giant worms are much more than a visual nuisance.

Thanks to their giant size, these hammerhead flatworms are predators of other animals in the soil, which include the much smaller native earthworms of France. It’s not entirely clear what ecological effect the invaders have had yet, but there’s no question that the small native animals that they prey on are important to maintaining the natural ecology of French soil. The team minces no words in shaming the scientific officials who scoffed at French citizens that raised concerns about the predatory worms in the fast.

“Recently, a tendency to deny the risks posed by non-native species has emerged,” the team writes. “In opposition to this ‘denialism,’ we strongly believe that invasive flatworms, as active predators, constitute a danger to native fauna wherever they are introduced.”