From the start, Zhanqi Chen realized that something was odd about the spiders. He had first spotted the species, known as Toxeus magnus , in a park in Singapore, and whenever he’d peer into their silken nests, he’d usually find a centimeter-long adult female surrounded by several smaller youngsters. That was weird. Most spiders are solitary, and even cannibalistic toward their own kind. There are a few kinds of sociable spiders that live in colonies , but Toxeus magnus shouldn’t have been one of them. It’s a jumping spider, a group generally known for being loners. And yet, there it was, apparently living in family groups, where the mothers cared for their young—another rarity among spiders. The mystery deepened when Chen collected several of the spiders and reared them in his lab at the Xishuangbanna Tropical Botanical Garden. He noticed that after hatching, the spiderlings would stay in their home nests for at least three weeks. During that time, they never left, and their mother...