Why does a hunting praying mantis often wait motionless rather than chase prey?

Answer: It avoids alerting prey until the insect comes within reach

Its front legs require a long rest before every strike — A mandatory long rest before every strike does not explain this hunting strategy. A mantis waits for prey to enter the reach of its grasping front legs. Waiting saves it from having to chase a target that is still too far away.

Its eyes can detect prey only while its body is still — A mantis is not unable to detect prey whenever it moves. It can approach prey as well as wait for it. Remaining still during an ambush chiefly helps keep the prey from noticing the hunter.

It avoids alerting prey until the insect comes within reach ✓ — Waiting quietly helps an ambush-hunting mantis remain unnoticed until prey comes close. It then grasps the insect with its spiny front legs. Mantises can also move to find or approach prey; they are not always motionless.