On a summer night in 2017, Chen Zhanqi made a curious find in his lab in China’s Yunnan province. In an artificial nest, he spotted a juvenile jumping spider attached to its mother in a way that reminded him of a baby mammal sucking its mother’s teats. On closer inspection, the spider mom really seemed to be doting on her young, he says. “She had to invest so much in caring for the baby.” Further study by Chen and Quan Rui-Chang, behavioral ecologists at the Chinese Academy of Sciences’s Center for Integrative Conservation in Menglunzhen, confirmed the jumping spider females...