He does have 6 legs, but in this photo his front two are tucked up against his thorax - no idea why but maybe he's eating.
Here's some info I found while Googling:
The front legs have rows of sharp spines which are used to grip prey. Mantids have a triangular shaped head with a large eye on each side that can see the slightest movement up to 35m away. It is the only insect that can turn its head 180º. It has excellent hearing. Mantids have straight leathery wings and powerful jaws.
It grabs the prey with its strong front legs, bites the head off and eats its prey. Mantids eat beetles, spiders, grasshoppers, crickets, small vertebrates such as tree frogs, lizards and mice. They also eat other mantids!
After mating with a male, a female mantid lays groups of 14-100 eggs, in a froth that hardens to protect the eggs through the winter. The young mantids, called nymphs, hatch in the spring. Often their first meal is one of the other young just hatching. Nymphs eat leafhoppers, aphids or small flies. All through the summer they shed their skin many times as they grow into adults.