Watching this with 'annotations on' makes it easier to figure out this really is a spider, not an ant! (or just read the summary below :) )YouTube
At first glance this spider looks very similar to an ant. Jumping spiders of the genus Myrmarachne may imitate ants in both appearance and behavior. By mimicking an ant the spider might better avoid predators like spider wasps. Some ant mimic spiders may even prey on ants or the homopteran bugs they tend.
Ants have three body segments while spiders have just two. Note how this spider's forward body segment has been adapted to give the overall body an appearance of three segments. The spider also waves it's front pair of legs giving the appearance of antenna. The spider's huge forward-facing eyes are a dead giveaway that it is a jumping spider. Jumping spiders (family Salticidae) have remarkable binocular vision which they use to locate, stalk and ambush prey. Though they are "jumping" spiders these spiders may only rarely engage in such non ant-like behavior. Ant-mimic jumping spiders are nevertheless very active and it was pure luck that I encountered this spider as it rested while feeding. In the past six years I have only encountered this type of spider three times in the forests where I walk.
This spider was encountered and filmed at an altitude of roughly 600 feet above sea-level and amidst the dense green forests of Nippondaira in central Japan. The Nippondaira is a small, isolated range of low coastal mountains between the cities of Shizuoka and Shimizu.