The jointed limb quite literally defines the phylum to which insects belong. It's right there in the name Arthropoda (from Greek arthros (jointed) and poda (foot)). But in insects, these members form more than just limbs. They are the basis for everything from antennae to mouthparts [sources: Fleury, Wigglesworth].
Insects likely evolved from a critter made up of numerous similar leg-bearing segments, not unlike a simple centipede. Over time, these parts fused to form the familiar head, abdomen and thorax of modern insects, while their connected legs changed to take on roles from sensing to stinging. An insect's head, for example, consists of six primitive segments, the modified legs of which now serve as antennae and specialized mouthparts [sources: Fleury, Wigglesworth].
And we do mean specialized. If you think a shrimp fork is a specific utensil, try mouthparts with cutting edges, or with tubes and grooves that assist both bloodsuckers and nectar-feeders [source: Wigglesworth; Mitchell and Scott].