Providing milk and long-term care together is virtually unheard of in insects and other invertebrates. And with the exception of mammals, it’s not even that common among vertebrates. As such, the results “help increase our understanding of the evolutionary origins of complex forms of parental care,” says Nick Royle, a behavioral ecologist at the University of Exeter in the United Kingdom who was not involved with the work. They suggest prolonged mothering may not require the complex brain power that researchers have assumed, he says.

Females of this jumping spider species (Toxeus magnus) lay between two and 36 eggs at a time. As soon as the eggs hatch, the mother begins to deposit tiny milky droplets around the nest, Chen and colleagues observed in the lab. When the team members analyzed the liquid, they discovered it contained four times the protein of cow’s milk, as well as fat and sugar.

In their first couple of days, the baby spiders sipped droplets of this spider milk around the nest, the researchers observed. But soon they began to line up at the entrance of the mother’s birth canal to suckle. At 20 days, they began to hunt outside the nest, but they still supplemented their diet with mother’s milk until they were sexually mature—another 20 days.

When Chen painted over the mothers’ birth canals to cut off the milk supply, spiders younger than 20 days all died. When he removed the mother from the nest, older spiders grew more slowly, left the nest sooner, and were more likely to die before adulthood, he and his colleagues report today in Science. Other spiders may hang around their young for a few days but rarely feed them.

The “milk” may be liquified eggs that are passed out of the birth canal prematurely, Quan says. Some amphibians and other invertebrates lay similar “trophic eggs” for offspring to eat, he notes, although only when those offspring are really young. Cockroaches also produce “milk,” but that nourishment is simply absorbed passively through the eggshell of their embryos and is not part of the hatched roachlings’ diets.

Please refer to Spider moms spotted nursing their offspring with milk to read more.