Many insects have a tough outer shell called an exoskeleton cellulose an endoskeleton chitin

An exoskeleton.

Explanation:

Tough outer shell of many insects is known as exoskeleton that represents an external skeleton. The exoskeleton provides protection and support to the insects body as they lack an internal skeletal system as higher organisms. In insects, it is made up of chitin.

Exoskeleton also plays role in sensing, feeding, excretion, and protection against desiccation in land insects.

Thus, the correct word to be filled in the blank is 'exoskeleton.'

the most logical answer would be a