Denizen's of London's Underground Logo

Denizen’s of London’s Underground


The British constructed the world’s first underground passenger railway in London in 1863. The first tunnels built for the Metropolitan Railway used the cut-and-cover method, forming conduits just below the surface. Circular holes at deeper levels soon became the preferred method. The round tunnels provided a nickname by which locals still lovingly refer to their subway: the Tube. Today, the expansive system is known officially as the London Underground and provides up to 5 million fares daily.

Over the years, the Tube has provided Londoners with more utility than transport. During both World Wars, the German military targeted the English capital with bombing campaigns. Few spots in the metropolis were as effective as the subway system for air raids. The Underground did not provide comfortable safety, but desperate times often do not allow convenience.

Subway systems worldwide do not garner reputations for cleanliness. The mental picture conjured by many when it comes to living organisms underground often veers toward rats. But the humans who sheltered in the Tube during bombings began to encounter something other than rodents. Something insatiable and ferocious.

The London Underground Logo - photo by Jack Taylor
A map of the London Underground
Londoners sheltering in Aldwych Station in 1940 - photo by AP/London Transport Musuem

Head of Environment for the London Underground, Steven Judd, said, “The Tube then was a very different place than it is now.” He noted the citizens would have encountered standing pools of water, which attracted nasties such as flies, ticks, lice, and fleas. But reports of one particularly painful pest garnered urban-legend status: mosquitoes.

We usually associate these blood-sucking insects with the outdoors. Indeed, mosquitoes dine mostly on birds and non-human mammals. However, anyone who spends any amount of time outside knows they will not hesitate to attack humans. Scientists estimate mosquitoes still kill over 1 million people each year, thanks to the transmission of a plethora of diseases. They are often cited as the most deadly animal to humans.

Although certainly small numbers of avians likely found their ways into the underground, these mosquitoes were not there to feast on birds. And, of course, humans didn’t spend most of their time in the tunnels. So, why were there so many mosquitoes hanging out in the Tube?

Anatomy of an adult mosquito - graphic by Mariana Ruiz Villarreal

In 1999, an entomologist named Katharine Byrne decided to study the mosquitoes in the Underground. She compared them to every day, aboveground mosquitoes and discovered something incredible: it was nearly impossible for the two groups to mate. The potential implication there is that the underground mosquitoes were somehow a different species than those living above!

Additionally, the subway skeeters exhibited different behaviors and featured different gene frequencies. Those living underground bred around the calendar and never hibernated; the other group took the winters off. Normal mosquitoes fed mostly on birds, but those in the subway also attacked mice, rats, and, during extended stays, humans. The genetic differences suggested a possible “founder event.” Essentially, a group of mosquitoes might have seeded the system and drifted genetically until they represented a new species.

They dubbed the pests the London Underground mosquito

Culex pipiens molestus - image by Walkabout12

Subsequent testing prompted some scientists to question whether Culex molestus (great name) is a species separate from Culex pipiens. Instead, some biologists believe the London Underground mosquito to be a subspecies of C. pipiens because, though significant differences exist, their morphologies are identical.

Even if just a subspecies, the question remains: how did the mosquitoes in London’s subway system evolve differently than those above? Unfortunately, our story has a bit of a buzzkill plot twist. Fortunately, this wrench adds further mystery!

Though we now give them London Underground nomenclature, the British mosquitoes are not even the first discovery of this (sub)species. Biologist Peter Forsskål spotted the difference in Egypt all the way back in the 18th century. There, like in England, C. molestus lived in subterranean environments, while C. pipiens terrorized the surface. In 2011, the London Underground mosquito showed up in New York City sewers. In fact, nearly every continent on the planet has pockets of C. molestus.

If they thrive in human-made, underground settings, how is the same form of mosquito all over the world?

The blue areas represent the range of C. pipiens, the normal mosquitoes - graphic from ResearchGate

Genetic sequencing does indicate that the worldwide enclaves of the London Underground mosquito are separated by insignificant differences, meaning they are not simply groups that ended up in subterranean areas individually by happenstance. They are the same mosquitoes. How did they disperse?

The bug jury is out. The leading theories involve world trade routes. One particular hypothesis revolves around automobile tires. Apparently, the rubber circles are fantastic vectors for mosquito larvae. Though hitchhiking mosquitoes on global goods makes a lot of sense in the modern world, it’s also possible the insects predate widespread trade. Some scientists believe the split between C. pipiens and C. molestus might have occurred in eons past, with the London Underground variety adapting to new digs sometime afterward, perhaps relatively recently.

The science might not yet be settled, but this story already checks all the satisfying nature boxes. In the past, we learned about how physical barriers or features can affect evolution. Usually, that refers to mountains, canyons, or islands. Can we add human-made structures to the list now? If we did not prompt this new species of mosquito, how did they end up living underground? Mother Earth never fails to write wonderful mysteries!

The next time you take the Tube (or any other subway), make sure to mind the gap and the mosquitoes!

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