This tiny slug-like creature is the larva of a sawfly in the genus Caliroa. I found several on the underside of red oak leaves. Despite their common name of sawfly, these insects are not flies (Diptera), but rather members of the Hymenoptera (wasps, sawflies, bees, ants). The adults of this genus look like dark wasps. This is a new species for our insect list at the garden.