Oak sawfly larva, Caliroa sp, along with feeding damage on the underside of an oak leaf. If you're so inclined, you can easily follow the larva's food as it moves through the digestive system. The leaf tissue is taken into the body and remains green prior to digestion. During digestion the food turns brown as it's broken down by enzymes and nutrients are absorbed from the gut. Finally the food residue becomes darker and pellitized (water is reabsorbed) prior to excretion from the posterior end. check

HymenopteraTenthredinidaeCaliroa spsawflylarva