It looked like milk. And for all intents and purposes, it was.
When a brood of spiderlings first hatch, their mother starts secreting the liquid from her epigastric furrow, a fold on her underside that she also uses to lay eggs. For the first week, she dabs the droplets onto the walls of her nest, and the youngsters scurry over to suck these up. After that, they drink from the furrow directly. During their crucial early period, the spiderlings rely on the milk as their only source of sustenance. It doesn’t have a lot of fat or sugar, but it’s loaded with proteins—four times as much as the equivalent amount of cow milk. When Chen stopped the spiderlings from drinking this fluid, by blocking their mother’s epigastric furrow with a dab of correction fluid, all of them died within 10 days. (The correction fluid itself didn’t affect them.)
Whether the liquid truly counts as milk depends on how you define the term. Traditionally, milk is defined as a nutritious liquid secreted by the mammary gland, and mammary glands are found only in mammals such as ourselves. But if you stretch the description to include any parental secretion that nourishes and provides for the young, then milklike stuff starts cropping up in many unexpected corners of the animal kingdom.
The tsetse fly is an insect that does a good impression of a mammal: It gives birth to live young, which it feeds within the womb with a milklike fluid. The parasitic bat flies do something similar. One species of Pacific cockroach also gives birth to live young, which it nourishes with a yellow milk, full of glittering protein crystals. Pseudo-scorpion mothers carry their hatchlings in a sac attached to their belly, and feed them with a nutritious liquid secreted from their ovaries. And pigeons (fathers included) feed their relatively helpless chicks by coughing up a chunky, fatty liquid that they secrete from their throats.
Compared with these creatures, the jumping spiders are arguably closest to mammalian lactation, in that they produce milk from a specialized organ, from which the youngsters drink over a very long time. “It would be really interesting to dissect the spiders [to see if there] was some kind of identifiable gland or something like that,” says Laura Hernandez of the University of Wisconsin at Madison, who studies lactation. And Katie Hinde, another lactation expert at Arizona State University, wants to know if the spider’s liquid contains other components that are found in mammal milk, including hormones, immune chemicals, and bacteria.
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But in the meantime, “we can call it spider milk,” Hinde says. “I’m not hung up on it coming from a mammary gland. I’m interested in how it supports development.”
Twenty days after hatching, the spiderlings make their first forays out of the nest, and start hunting for small flies. But they still return to their mother to drink her milk, until finally weaning at 40 days of age. Even then, most of them stay in the nest for many weeks more, while the mother continues to care for them. She’ll throw out their molted exoskeletons, repair the nest, and evict parasites such as mites. Even when Chen dammed up the milk-producing furrow, he found that older spiderlings still benefit from their mother’s fastidiousness, and are less likely to survive in her absence.