For the antennae: Unroll the licorice and cut into twelve 1 1/4-inch-long pieces. Separate the two strands of each licorice piece, leaving them attached at one end. Roll 2 small (1/4-inch) balls of green fondant for each pair of antennae and attach the balls to one end of the licorice. Enclose the remaining fondant in plastic wrap so it does not dry out. Repeat with the remaining pairs of antennae. Set aside to dry.
For the head and body: Roll seven 1/2-inch balls of green fondant. Take one ball and attach a pair of antennae by sticking the licorice into it. With a small brush, dab a little water to moisten the top and attach 2 yellow sprinkles for eyes. Add pupils to the eyes by dipping a skewer into the melted chocolate and make small dots on the yellow sprinkles. Gently press the 6 remaining balls together to form the body of the caterpillar, and curve them to make it look wiggly. Moisten the top of an end ball and affix the caterpillar's head. Repeat to make 11 more caterpillars. Allow them to dry 20 minutes.
To assemble: Frost each cupcake with about 2 tablespoons buttercream. Place the cookie crumbs in a small bowl; invert the cupcakes into the crumbs and press gently to adhere. Top each cupcake with a caterpillar and press them gently into the "cookie dirt" to adhere. Garnish with fondant grass if desired.