Combine the walnuts, garlic, arugula, and cilantro or basil in a food processor or blender. Whirl them just until they are coarsely chopped. While the machine is running, add the olive oil in a thin stream. Transfer the pesto to a bowl. (At this point the pesto can be frozen. Thaw it before proceeding.)
Stir the Parmesan cheese, salt, and cayenne into the pesto
Bring a large pot of salted water to a boil. Add the pasta, and cook it, stirring occasionally, until it is just tender. Drain the pasta, return it to the empty pot, and toss it with the pesto, adding a tablespoon or two of water if necessary to distribute the pesto evenly.
Transfer the pasta to a serving bowl or to individual plates, garnish with additional Parmesan cheese and serve.