For decades, London’s subway system was thought to harbor a unique species of mosquito that evolved to thrive in the city’s underground tunnels. This “London Underground mosquito” was famously believed to have adapted rapidly to the subterranean environment during World War II, when Londoners, sheltering from German air raids, were tormented by persistent mosquito bites. Scientists observed these pests’ unusual behavior and concluded they might have evolved in the underground confines of the city itself.
The mosquito in question, known scientifically as Culex pipiens, exists in two distinct behavioral forms, although they are visually identical. One, called Culex pipiens form pipiens, prefers birds and thrives in open-air environments. The other, known as Culex pipiens form molestus — from the Latin word for “annoying” — is drawn to humans and can survive in subterranean habitats. For years, biologists speculated that it was this latter form that had uniquely adapted to the Tube system.
The idea gained popularity after a 1999 genetic study suggested that the underground mosquitoes in London had diverged from their aboveground counterparts and developed traits suited for life below the surface. These traits included the ability to breed without a blood meal, indiscriminate biting behavior, and mating in confined spaces throughout the year. However, the study relied on a limited dataset, analyzing only a fraction of the mosquitoes’ genetic material, leaving many questions unanswered.
A new study, published this week in Science, overturns the long-held belief that London’s Tube mosquitoes evolved underground. Led by Yuki Haba, a postdoctoral researcher at Columbia University, the research team conducted a comprehensive genetic analysis of hundreds of mosquitoes from around the world, including historical specimens collected during the World War II era. Their findings suggest that Culex pipiens form molestus has a far older origin — in the Mediterranean region, particularly the Middle East — and that its adaptation to underground habitats occurred much later as the mosquitoes migrated north.
According to Haba, the split between the aboveground pipiens and the belowground molestus likely occurred between 3,000 and 2,000 years ago, though it could have happened as early as 10,000 years ago or as recently as 1,000 years ago. Early molestus populations were primarily aboveground and gradually dispersed to different parts of the world, eventually reaching northern Europe. Once these mosquitoes encountered colder climates where survival aboveground was impossible during winter, they found refuge in subterranean environments such as the London Underground.
The team’s research journey began in 2018, when they reached out to scientists worldwide to obtain mosquito samples. After thousands of emails and several years, they compiled specimens from over 50 countries, totaling approximately 800 samples, including both contemporary and historical specimens preserved in ethanol. Direct collection from the London Underground was not permitted, but historical samples housed at London’s Natural History Museum and analyzed by the Wellcome Sanger Institute provided crucial insights.
Senior author Lindy McBride, an associate professor at Princeton University, explained that the study revealed molestus descended directly from Mediterranean pipiens populations. The adaptation to human hosts likely coincided with the rise of irrigated agriculture in the Middle East, creating ideal breeding sites in otherwise arid regions. The species was first formally described in Egypt in 1775 by naturalist Peter Forsskål and had likely existed in the region for at least a millennium prior. By the 19th century, records showed molestus in southern Europe, including Croatia and Italy, before finally appearing belowground in northern Europe around 1920.
The research fundamentally changes our understanding of the London Underground mosquito, emphasizing that its subterranean habits in London are the result of pre-existing behavioral traits and environmental opportunity rather than rapid evolution in the city’s tunnels. Richard Nichols, a professor of genetics at Queen Mary University of London and co-author of the 1999 study, acknowledged the new findings as a testament to the evolving nature of science. “Our original results still stand, but the interpretation has changed,” Nichols said, noting that the new study benefits from larger datasets and full genome analyses rather than the limited gene surveys available in the late 1990s.
Cameron Webb, an associate professor of medical entomology at the University of Sydney, praised the study as “fascinating and comprehensive.” He emphasized that molestus mosquitoes are associated with subterranean habitats worldwide, not just in London, and that understanding their biology is critical for anticipating urban pest challenges. As cities adapt to climate change and expand underground infrastructure, the potential for mosquitoes to exploit these environments increases, highlighting the importance of proactive public health strategies.
The London Underground mosquito story is not just about an urban myth; it underscores the complexity of mosquito evolution, the interplay between human activity and insect habitats, and the importance of comprehensive scientific investigation. While the idea of a species evolving overnight in the Tube captured the public imagination, the reality reveals a far richer narrative: one that stretches across continents and millennia, showing how mosquitoes have adapted to humans, agriculture, and urban environments long before the construction of subway systems.
With over 3,000 species of mosquitoes inhabiting nearly every continent, including recent discoveries in Iceland, the study of Culex pipiens form molestus serves as a reminder that even well-known pests can hold surprises. As urban planners and public health officials design the cities of the future, understanding the evolutionary history and ecological flexibility of mosquitoes is more relevant than ever.
In the end, the “London Underground mosquito” is less a product of wartime adaptation and more a testament to the enduring resilience and adaptability of one of humanity’s oldest pests.