Ant Mimic Jumping Spider

Though it looks very much like an ant, it is in fact a spider. Related to jumping spiders they aren't very skilled at jumping. They are however accomplished mimics.

Myrmarachnea japonica

They move in an ant-like manner, and wave their front pair of legs in the air to imitate antenea.

Spiders of course have two main body parts, the head (cephalothorax) and abdomen, while ants have three: head, thorax, abdomen. For these spiders a light coloured section of hair near the mid-part of the head and a slight constriction serves to create the illusion that there is a separate head and thorax.

This species does have a degree of sexual dimorphism the most obvious of these features are the jaws of the male spiders being much larger than the female's. These jaws are hinged and as such are folded in half, stratched out, they would be nearly as long as the entire cephalothorax of the spider itself!:

While most jumping spider males, which are not ant-mimics, have bright colours and/or enlarged front legs and palps. Those adaptations would break the illusion of being an ant. Somehow, it seems that having jaws as large as the head doesn't break this illusion, perhaps ants have very poor depth perception.

The best way to spot these particular spiders is to look at the plants and walls of building where you'd likely find ants, the spider's prefered prey. If you spot an "ant" that isn't moving in quite an ant-like manner: lots of stops and longer pauses (ants hardly ever stop for a very long time), did it notice you in turn? Take a closer look; count its legs. You've likely found an ant mimic spider.