In the early September days of 1941, the German Luftwaffe started a prolonged bombardment of London which would later become known as ‘the Blitz‘. As the bombs continued to drop for months, thousands of Londoners sought refuge in the underground railway system.
During the most intense periods of bombardment, the London Underground was home to over sixty-thousand Londoners. With so many people living and sleeping on overcrowded platforms, filth and stench quickly filled these makeshift shelters. As if the horrors of continued bombardment weren’t enough, the shelter seekers were plagued by ravenous mosquitoes that fed on their blood. P.G. Shute, Assistant Malaria Officer of the Ministry of Health, noted:
We now come to the time of the 1940 blitz on London when the Under ground Railways began to be used for sleeping quarters. [..] Many people had complained of having been bitten at night while resting in the Liverpool Street Underground Station. I visited the station and caught many mosquitoes in the act of biting and satisfied myself that they were Culex molestus.
~ P.G. Shute, Culex Molestus, Trans. R. Ent. Soc. Lond. 102. Part 7. (Nov. 1951)
Londoners seeking shelter in the Elephant and Castle underground station during the Blitz. Photo by Bill Brandt.
Shute found the mosquito larvae in the smallest of puddles and quickly identified them as Culex Molestus. Other scientists later rejected the status of Culex Molestus as a separate species. The biggest problem? Culex Molestus looks identical to the Culex Pipiens mosquito that normally lives on the surface,
There are some large behavioural differences between these two subspecies though. Firstly, the underground Molestus mosquito doesn’t go into diapause like Culex Pipiens does. The railway tunnels are humid and warm all year long, making such hibernation unnecessary. Secondly, Culex Pipiens feeds on the blood of birds which they need for the development of their eggs, while the underground mosquito can lay a batch of eggs without a fresh meal of blood. Instead of terrorizing birds, they happily feed on mammal species, which in the London Underground, naturally boils down to humans and rats.
Even if Molestus is not an officially recognized species today, the question how they made the London Underground their home is still a valid one. Did these underground mosquitoes simply descend from local Culex Pipiens populations that invaded the Underground from the surface?
A Culex Pipiens male. Source.
In 1998, Katharine Byrne and Richard Nichols wanted to tackle this question by comparing the genetic makeup of subterranean and surface mosquito populations from 20 locations in London. They found that underground mosquitoes genetically didn’t match up with their immediate local surface neighbours and that crosses between surface and underground mosquitoes were unsuccessful.
Based on these results, Byrne and Nichols concluded that Culex Molestus did not repeatedly evolve from local surface populations. Instead they proposed that all the underground mosquitoes are descendants from a single colonization event of a local Culex Pipiens population. In their paper they list a number of explanations of how after that colonization, the Molestus mosquitoes could have became reproductively isolated from the surface Culex Pipiens populations.
There are a number of issues with this conclusion though. Since the London Underground opened up in 1863, this would mean that the underground and surface mosquitoes have almost become separate species in less than 200 generations, making it one of the fastest speciations known in the natural world. Moreover, mosquitoes much like those in London have popped up in other underground railways of Europe, including the Amsterdam Metro line which opened in 1977. This is difficult to explain if colonization events are indeed as rare as Byrne and Nichols proposed.
In 2004 a team of scientists decided to resolve these discrepancies. They compared 8 microsatellite regions from different mosquito populations around the world. As you can see in the picture below, mosquito individuals in Northern Europe either belong to the Molestus (red) or Pipiens (blue) subspecies. The subterranean Molestus mosquitoes are genetically most similar to mosquito populations found in Asia and Africa. The easiest explanation for this observation is that the London Underground mosquitoes aren’t locals looking for a change of scenery, but descendants from far away colonists!
An analysis of mosquitoes all over the world, where every individual is represented by one vertical line. The colours represent the probabilities that this individual belongs to a particular subspecies of mosquito.
The figure also shows that many mosquitoes in the United States have a hybrid ancestry, carrying microsatellites that are associated with both Molestus and Pipiens varieties. This is important to know, because in Europe it is the Pipiens variety that sometimes carries the West Nile Virus. Since these mosquitoes usually bite birds instead of humans, outbreaks of WNV have been sporadic in Europe. The existence of hybrid forms that bite mammals could explain the more epidemic spread of WNV in the US. Interestingly, it is unclear what keeps the mosquitoes from intermixing in Europe!
Even in the most urban of environments, we humans have created new niches that life will fill. If we manage to keep our underground railways in operation for a couple of thousand years, the European Culex Molestus and Culex Pipiens will definitely have become separate species. That we humans are contributing to speciation below our cities might be a comforting thought, the next time I’m bitten while on a platform!
Byrne, K., & Nichols, R. (1999). Culex pipiens in London Underground tunnels: differentiation between surface and subterranean populations Heredity, 82 (1), 7-15 DOI: 10.1038/sj.hdy.6884120
Fonseca, D. (2004). Emerging Vectors in the Culex pipiens Complex Science, 303 (5663), 1535-1538 DOI: 10.1126/science.1094247
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