Based on your photos this looks like Mexican bean beetle larvae, an insect  pest of beans. Look for adult beetles that look like copper colored  lady bugs with black spots and bright yellow Eggs in clusters of 40-60  on the lower leaves.   Larvae and adults feed on the underside of leaves between the veins, removing the lower epidermis of the leaf. Crush egg clusters, larvae, and adults by hand.  
Badly infested plants should be stuffed into plastic bags and sealed. One week later you can compost the plants and dead beetles.
See our pest profile for prevention and control information http://extension.umd.edu/growit/insects/mexican-bean-beetle
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