Where, exactly, does the sand flea have sex? On the dusty ground, where it spends the first half of its life? Or already nestled snugly in its host—such as in a human foot—where it can suck the blood it needs to nourish its eggs? The answer to this question, which has long puzzled entomologists and tropical health experts, seems to be the latter. A new study, in which a researcher let a sand flea grow inside her skin, concludes that the parasites most likely copulate when the females are already inside their hosts.

Tunga penetrans, also known as the chigger flea, sand flea, chigoe, jigger, nigua, pique, or bicho de pé, is widespread in the Caribbean, South America, and sub-Saharan Africa. The immature female burrows permanently into the skin of a warm-blooded host—it also attacks dogs, rats, cattle, and other mammals—where over 2 weeks it swells up to many times its original size, reaching a diameter of up to 10 mm. Through a small opening at the end of its abdominal cone, the insect breathes, defecates, and expels eggs. The female usually dies after 4 to 6 weeks, still embedded in the skin.

Native to the Caribbean, sand fleas infected crewmen sailing with Columbus on the Santa Maria after they were shipwrecked on Haiti. They and others brought the parasite back to the Old World, where it eventually became endemic across sub-Saharan Africa. Even today it is an occasional stowaway, showing up in European and North American travel clinics in the feet of tourists who have gone barefoot on tropical beaches.

For people living in infested regions, however, the flea is a serious public health issue. What starts as a pale circle in the skin turns red and then black, becoming painful, itchy—and often infected, a condition called tungiasis. One flea seems to attract others, and people can be infested with dozens at once . The infection makes it hard for people to walk and can lead to permanently deformed feet. There is no drug treatment; the only way to get rid of the parasites is digging them out. People often use unsterile instruments to do this, causing other infections, including tetanus. Tungiasis affects the poorest of the poor, says Jörg Heukelbach, who studies the disease at the Federal University of Ceará, Fortaleza, in Brazil, and is neglected—and often unrecognized—even by health officials in endemic regions.

Marlene Thielecke, a Ph.D. student at Charité University Medicine in Berlin, was studying ways to prevent tungiasis infection in Madagascar when she noticed she was hosting a flea herself. Because part of her job was to evaluate and record sand flea developmental stages in local villagers’ feet, she says, “I thought it might be interesting” to observe one in her own foot. So she began to take regular photographs and videos to keep track of what happened.

At first, the flea didn’t bother Thielecke and she noted that it seemed to grow normally. But she soon realized it wasn’t laying any eggs—unusual for an embedded and otherwise apparently mature flea. It also lived much longer than usual; after 2 months, it was still regularly expelling liquid from its abdomen, a sign it was still alive—but still no eggs. At that point, Thielecke says, the spot was itchy, painful, and prevented her from walking normally. “I started to get uneasy” about leaving it in for so long, she says, so she extracted it.

Months later, back in Berlin, she talked about the observations with her supervisor at the Charité, tungiasis expert Hermann Feldmeier. The lack of eggs and the unusually long life of Thielecke’s flea were likely because it was never fertilized , the researchers conclude in a paper published this month in Travel Medicine and Infectious Disease. As soon as she found the flea, Thielecke began to wear socks and closed shoes to prevent other fleas from entering. (“One was enough,” she says.) She and Feldmeier think that perhaps the flea lived so long because it was in a sort of developmental “waiting state,” expecting a male to find it and fertilize its mature eggs.

The fact that her flea was apparently a virgin suggests an answer to the long-standing puzzle of whether fleas are fertilized before or after they embed. More than a century ago, researchers reported observing male fleas mating with unembedded females; others have argued that it’s more likely that females first embed, then are fertilized by males who find an infected mammal and mate with embedded females through her abdominal opening. (Males don’t embed themselves in skin.) That would make evolutionary sense, Feldmeier says. Because females tend to embed in clusters, any male that comes along will have a chance to mate with multiple females—and a group of females will have a higher chance of being found by a male than a single one.

The insight into the flea’s sex life doesn’t have immediate implications for fighting tungiasis in areas where it is endemic. But it could help doctors recognize the parasite in travelers who return home with a flea. Physicians should be aware that single fleas “don’t necessarily develop normally” and can live longer than the textbooks predict, Feldmeier says.

And although the single observation doesn’t prove that females are always fertilized after embedding, “it’s very interesting,” says Ingela Krantz, a professor emeritus of infectious disease epidemiology at Umeå University in Sweden. “This is the type of knowledge you usually skip because you say ‘Sand fleas, they are a nuisance, we treat them.’ But to start from the beginning and try to understand these strange parasites is important.”