The strangest thing about Toxeus magnus jumping spiders is not their resemblance to ants, though it’s a great impression — skinny legs and all. Spilling their most amazing secret requires a little pressure. Press gently on a female spider’s abdomen and she will produce yellowish fluid from the opening where she laid her eggs.

Those droplets are loaded with sugar, fat and lots of protein. This, in the simplest terms, is her milk. Spider milk, drop for drop, contains four times more protein than a cow’s, per a new report published Thursday in the journal Science. It’s the first time scientists have documented a milk-like substance in an arachnid.

Young spiders drink the milk to survive, according to scientists at the Xishuangbanna Tropical Botanical Garden, a research institution run by the Chinese Academy of Sciences. “Spiders usually do not provide extended parental care after spiderlings emerge from the egg-sac,” said Zhanqi Chen, an expert in arachnids at the botanical garden and an author of the paper. By comparison, female T. magnus spiders are downright motherly.

Infant spiders utterly depend on milk for their first three weeks. The scientists know this because when they blocked the milk by painting Wite-Out on the mother’s abdomen, spiderlings did not live past 20 days old. Between 20 to 40 days old, the spiderlings begin to forage while continuing to drink milk. After 40 days, mothers no longer provide milk, but the spiderlings continue to live at home. (For a thriving brood, the authors suggest mother spiders keep their nests clean and free of parasites.) Offspring only leave the nest at about three months, long after they are mature adults.

“This is a very cool study!” said biologist Ximena Nelson, who studies jumping spiders at New Zealand’s University of Canterbury and was not involved with this research. It was “absolutely” the first evidence of milk in a spider species, she said.

At its most basic, producing milk is “feeding your offspring some aspect of yourself,” said Sasha Dall, a University of Exeter biologist who has studied milk production and is not a member of the research team. If the definition of milk requires it to contain lactose, what Dall called an “extremely narrowly focused” definition, then what these spiders produce does not pass muster. But it has almost all other hallmarks of milk, he said.

Julie Sharp, who studies lactation at Deakin University in Australia and was not involved with this research, said “it is fair to call this substance ‘milk,’ as it provides sole nourishment from the mother to the young.” Sharp wants to know what else the fluid contains. “Bioactives to aid growth and development, similar to mammalian milk,” may exist along with the proteins and fats, she said.

Chen and his co-authors likened the spider behavior to lactation in mammals. But Sharp pointed to a crucial difference: Lactation, she said, is normally associated with a specialized organ. The study authors have not found the equivalent of a spider mammary gland.

All mammals, from hamsters to hippos, produce milk. Taxonomist Carl Linnaeus had this attribute on his mind when he coined “mammalia” in 1758, borrowing from the Latin word for “teat.” But these spiders join the rare cluster of non-mammal parents that produce milk and other nourishment.

Discus fish secrete a mucouslike substance on their scales. Pigeons and doves make “crop milk,” regurgitating fluid from pouches in their throats. Cockroaches produce milk crystals to feed embryos. A pregnant tsetse fly nourishes the maggot growing within by pumping a milk-like substance into her uterus.

Other invertebrate mothers, including some spiders, feed their offspring unfertilized eggs. Chen and his colleagues suspect that the T. magnus milk may have evolved from this egg-feeding behavior.

Dall, in a 2004 study, determined a trade-off must occur in the evolution of milk. Milk is an investment. Imperfect biological reactions, including the production of milk, lose energy or nutrients along the way. “Each time you do this, there are inefficiencies,” Dall said. To make this investment worthwhile, the alternatives — bringing prey back to the nest, for example — must be too dangerous. Dall predicted something in this spiders' niche must make foraging for mothers “very risky,” he said.

Spider experts know very little about these animals, though, beyond their mimicry. About 250 closely related species exist, and all resemble ants. The mimicry tricks other animals into avoiding the spiders, fearful of bites and stings from aggressive ants.

“Some are thin and orange, others thick and black, exactly in tune with the particular species of ant that they are mimicking,” said Wayne Maddison, who studies the evolutionary history of jumping spiders at the University of British Columbia. Otherwise, “as for most jumping spiders, Toxeus magnus is not well known at all.”

“Predation seems to me to be the most likely — or strongest — selective pressure in this species,” Nelson said. Humans may be too quick to claim parental care for “higher” animals, like ourselves, she said. But this contributes to a growing body of evidence invertebrates care for their young.

These spiders, particularly the tiny spiderlings, are vulnerable to the very ants that they resemble. But ants rarely venture into the nests, especially if a mother spider is guarding.

“It pays to stay in your nest for as long as possible,” Nelson said. With mom and milk, the nest has just what a growing spider needs.

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