​Inside London’s Underground Mosquito Lab

Underneath a busy street in central London, a former coal bunker and bomb shelter is home to some unusual occupants. This is where the London School of Hygiene and Tropical Medicine keeps its insectaries, which are kept in tropical-like conditions and buzz with the hum of thousands of mosquitoes.

"It's a pretty hot environment to work in, but the mosquitoes like it," says Sarah Kelly, a scientific officer at ARCTEC who spends a lot of time in the underground vaults tending to the insect test subjects.

The Arthropod Control Product Test Centre (ARCTEC) is responsible for testing products such as insect repellents and insecticides to make sure they work before they hit consumer shelves. There's only one real way to test: find a volunteer to try out the product, expose their body to some hungry mozzies, and see if they bite.

Kelly showed us round the insects' lodgings and I volunteered to demonstrate a couple of tests by sticking my arm into a box of mosquitoes hungry for blood and donning some insecticide-impregnated clothing to test in the "free flight room," where the mosquitoes get free reign to fly about in search of a human host to feed on. If the product's working, it should mask the tell-tale chemicals and odours that lead a mosquito to human skin.

Everyone reacts differently to mosquito bites. Having undergone a precautionary bite test before filming, I knew what to expect: while the bites left only the tiniest red pin-prick on my control arm (the one without the repellent) at first, they fully revealed themselves the day after filming.

The mosquitoes used in these tests were of the species Stegomyia aegypti, which originally hails from Africa and is known to spread diseases including dengue fever and yellow fever, both of which can be life-threatening. That, of course, is why it's particularly important to test the efficacy of products that aim to protect against mosquito bites. The insects in the lab, however, are totally clean; they're raised from generation to generation in the lab and never exposed to infection. For the same reason, once the mosquitoes have been exposed to a human, they can't be used again—sadly, they have to go to the morgue.

"We get bitten by them quite frequently, so it feels like a little bit of revenge when we get to test on them," she says.