It was an arachnid mystery: Scientists studying a particular type of spider couldn't figure out how the babies kept growing in the nest even though mom wasn't bringing back any food to them. Then a researcher spotted a baby clinging to its mother's abdomen. "I had this radical idea that maybe spider moms feed their babies with something they produced," Zhanqi Chen of the Chinese Academy of Science tells New Scientist of the jumping spider Toxeus magnus. He was spot on: It turns out mothers feed their offspring "milk," similar to mammals, the researchers report in Science. Technically, the secretion from the abdomen might not be milk—spiders don't have the special glands required—but it's at least milk-ish, perhaps made up of partially digested eggs that went unfertilized, explains Popular Science.
And when researchers describe it as a "nutritious milk-like substance," they're not kidding. The stuff has four times the protein of cow's milk. Researchers found that the mother continues feeding her offspring for about 40 days, though the spiders are able to leave the nest at 20 days and begin supplementing their diet with insects. Generally, females were allowed to return more frequently to feed than their male counterparts, reports Gizmodo. When the scientists blocked the mother's secretion glands, her baby spiders died after about 10 days. And if they removed her from the nest at 20 days, the babies had a significantly lower rate of survival. When it comes to spiders, "this type of maternal care may be more widespread than has been assumed," the researchers conclude. (Spider fans will want to visit this Greek town.)