Like nursing baby mammals, some jumping spiders spend their earliest days feeding on a nutritious "spider milk," secreted by their mothers. What's more, spider mothers continue to care for and feed their young offspring the milk-like fluid - which contains nearly four times the protein of cow's milk -long after they're able to forage for food on their own.

Arthropods, which include insects, arachnids and crustaceans, are the most diverse group of terrestrial species on the planet. And that might be why researchers have had such a difficult time estimating their numbers, especially in tropical forests where so many arthropod species are known to thrive.