The prolegs of caterpillars have tiny hooks, termed "crochets," that provide a better grip on vegetation. The photo above (credit Satyrium) depicts a caterpillar on the far side of a leaf. A more detailed image is shown below:
Each proleg has a circle of hooks (or crochets), seen here in yellow and orange... Caterpillars have three pairs of true, jointed legs that remain in their adult moth form, whereas the prolegs disappear. The brilliant colours seen in this image were created using a technique called differential interference contrast (DIC) illumination.Lower image from the Wellcome collection, via The Daily Croissant.