Milk comes from mammals. It's kind of a very mammalian thing. Even our government knows that. However, Chinese scientists have documented jumping spiders that give their young with nutrient-rich liquid droplets from strains on the mother's body. This is the only food for spiderling until they start looking for food, and even then they still drink it until they are a little more mature. The results are reported in Science.
Roaches and pigeons also provide their children with substances described as "milk," because they come from their bodies and provide an exclusive source of food for young people. Cockroach mothers store this substance into a brooding bag where their embryo develops.
Pigeon parents – both mother and father – produce plant milk and feed the baby birds for the first few days of their lives, until the babies can digest real food. Plant milk consists of cells filled with nutrients that are peeled off in fragments from the inside of the mother plant, which is below their neck.
Toxeus magnus are jumping spiders that are learned here, and they look like ants. Their childcare methods also seem to be different from the two examples above. They have special milk-making organs, which researchers suggest are made from irreversible eggs that are converted into food for surviving offspring.
These spiders also live in small nests like nuclear nests where mothers care for children even after they reach maturity. For a while, young businesses are looking for food but still go home, making them more like mammals.
So this is definitely not breastfeeding, but these researchers think that "functionally compares and behaviors for breastfeeding." This also serves similar evolutionary needs: compensating for uncertain food access and keeping young children in the nest as long as possible to protect it from predators.
Jumping spider milk has a protein content of 123.9 mg / ml, four times that of cow's milk. Vegan isn't it.
Science, 2018. DOI: 10.1126 / science.aat3692 (About DOI).