It was a decent guess for the time, but it was also wrong. When Greg Hurst, an evolutionary biologist from the University of Liverpool, stumbled across these records in the 1990s, he knew exactly what was behind the heavy skew towards females. “It was clearly a male-killing bacterium,” he says.

In the 80 years since Simmonds and Hopkins first studied the blue moon, scientists had discovered several species of bacteria that have a special antagonism towards males. They mostly live in the cells of insects and other arthropods, and they pass from one generation to the next by stowing away in eggs. Sperm, however, are too small to house them, so if they end up in a male, they are stuck. Females are their ticket to the future; males are an evolutionary dead end. So these microbes have evolved ways of favoring female hosts at the expense of male ones.

Some do it by killing male embryos in their eggs, before they are even born. This is a suicidal act, since the male-killers go down with their hosts. But their genetically identical clones live on inside the female embryos, which now face less competition. They might even get a nutritious boost—in insects like ladybirds, surviving siblings will cannibalize the remains of their dead brothers, giving them a valuable boost of nutrition early in life.

These male-killers are not rarities. They plague many different groups of insects, including flies, beetles, bugs, wasps, mosquitoes and more. Wherever they exist, males become rare and sex ratios skew away from the neat 1:1 equity that we see in ourselves and most other animals.

Hurst had seen these effects first-hand. He had already spent years studying male-killers in ladybirds and African butterflies. In the latter, the females outnumber males by 25 to 1. “That’s a silly sex ratio,” he says. The blue moon butterfly was even more ludicrous. When Hurst heard about it, he thought, “A 100:1 sex ratio was too good to miss.”

In 1999, he travelled to Fiji with graduate student Emily Dyson to see the butterflies for himself. Once they arrived, they quickly saw signs of the same sex bias that Simmonds described. Male butterflies normally defend the plants in their territories, so you will typically see many more males than females when you walk along a flowery path. But on these islands, Dyson and Hurst could go for weeks without spotting a male blue moon.

Dyson collected and bred the butterflies and sure enough, around 60 percent of the broods were all-female—a lower proportion than in Simmonds’ day, but still significant. The team discovered that the culprit was a well-known male-killing bacterium called Wolbachia. It was there in the ovaries of every butterfly from an all-female brood; the team detected traces of its genes, and could see it down a microscope.

Dyson continued working in Fiji for a year, until she was interrupted by an inconvenient military coup. “They’re very peaceful, but health and safety rules don’t allow you to leave your graduate student in the same place as a military coup,” says Hurst. “So she flew to Samoa.” There, she found the same extremely skewed sex ratio that Hopkins described eight decades earlier—100 females (mostly virgins) for every male (mostly tired). And there too, Wolbachia was responsible, and exactly the same strain as the one in Fiji.