All spiders are predators, but most of them are small and have rudimentary defences against larger animals that in turn prey on them. Spiders have thus evolved a range of predatory behaviours that, at the same time, allow them to evade the threat of predation. Some of the most effective strategies involve deceiving ants.

More than 300 species of spiders are known to mimic the outward appearance of ants, a phenomenon called myrmecomorphy.
Aggressively territorial, ants are typically avoided by several predators, thus making them the perfect creatures to impersonate. Most ant-mimicking spiders have a "false waist" and are covered with reflective hairs to simulate the shiny, three-segmented bodies of ants. They have coloured patches around their eyes to make their simple eyes look more like an ant's compound eyes.

The spiders also behave like ants by waving their front pair of legs near their heads like antennae, and adopting an erratic zig-zag pattern of movement that is more like ants than spiders.

There are two reasons why a spider would want to mimic an ant: to eat them, and to avoid being eaten by them.

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http://phys.org/news/2014-11-spiders...prey.html#nRlv