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Insect From the Underground

Natural History, Feb, 2001 by Alan Burdick

In The Time Machine, published in 1895, H. G. Wells envisions a human race divided. Aboveground, in a city of daylight, live the Eloi, while the bloodthirsty Morlocks toil in tunnels far below the ground. A century later, two real-life British geneticists have discovered an entomological analogue: a mosquito that is closely related to the one inhabiting the daytime streets of London but that lives exclusively in the tunnels of the London Underground. Unlike their upstairs brethren, which bite only birds, members of this subterranean race show a distinct affinity for human blood.

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"They're quite voracious biters," says Richard Nichols, of Queen Mary and Westfield College at the University of London. "It's not that any one bite is all that bad, it's just that they all seem to want to get a bit of you."

The subterranean mosquitoes have been known for many decades; during World War II, Londoners who sought belowground shelter from the Blitz found themselves assaulted by the insects. Biologists named the attackers Culex molestus, presuming that they belonged to a species distinct from the aboveground C. pipiens. But until Nichols came along, the genetics had never been sorted out. He decided to unravel the mystery.

The research legwork fell to Nichols's then-doctoral student Katherine Byrne, now a geneticist at the Institute of Zoology in London. After midnight, when the trains had shut down, Byrne followed maintenance workers into the subway's bustling underworld, where she soon discovered an ideal environment for mosquitoes. "The temperature is warm and virtually constant," Byrne says. "There's no rain, no snow: it's great for a year-round life cycle." The polluted pools of water, ranging from "pretty nasty" to "hideous," are prime mosquito-breeding sites. As with all mosquitoes, only the females--the egg layers--seek out blood, feeding on rats and pinstriped rat-racers alike. The males get by on what Byrne calls soluble nutrition: decaying rubbish, perspired salts, bits of hair, and "fluff," a thin haze of flaked-off human skin so pervasive in the Underground that specialized workers ("defluffers") must regularly clean it from the train's conduction rail.

By night, Byrne scavenged grim subterranean puddles for mosquito larvae. By day, she lurked aboveground in tony gardens and backyards, collecting specimens from water-filled buckets and beery vats of compost. Taking her larval captives--C. pipiens from upstairs and C. molestus from downstairs--she returned to the lab to raise them.

As adults, Byrne's subjects proved fascinating. Although the two varieties look identical, their habits differ sharply. C. pipiens hibernates in winter; C. molestus breeds year-round in the warm subway, but it cannot survive the cold. C. pipiens must swarm in the open before mating, whereas C. molestus thrives in confined spaces. When Byrne crossbred the two varieties, none produced viable eggs--suggesting that C. molestus is reproductively isolated, the traditional signature of a new species. (Given the great differences in their behavior, Byrne notes, C. pipiens and C. molestus probably rarely meet or mate "in the wild.")

Analyzing the DNA, Byrne and Nichols found that different colonies of the underground mosquitoes are more genetically similar to one another than to their aboveground brethren. A specimen of C. molestus discovered at Euston, on the Victoria line, is more closely related to a specimen found at Finsbury Park, miles away on the same line, than it is to a visibly identical specimen of C. pipiens captured just upstairs at Euston. In short, subterranean sites are being established by other underground mosquitoes, not by mosquitoes from above. How do the mosquitoes spread through the Underground? "We think that the trains act as pistons, pushing cushions of air"--and mosquitoes--"ahead of them," Nichols says.

And the insects continue to evolve. Byrne and Nichols have identified three genetically distinct subvarieties of C. molestus, each one unique to a different subway line: Victoria, Bakerloo, and Central. The other lines of the Underground probably harbor subvarieties of their own, Byrne adds, "and I'm sure they exist in sewers, though I've not been to look."

Alan Burdick is a writer/producer for AMNH's Science Bulletins.

Insect From the Underground

Natural History, Feb, 2001 by Alan Burdick

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