This is the fly larvae resting between a 3rd instar Chrysomela aeneicollis prepupa (left) and a C. aeneicollis pupa (right). For more on these, see
RANK, N.E. and SMILEY, J.T. (1994), Host-plant effects on Parasyrphus melanderi (Diptera: Syrphidae) feeding on a willow leaf beetle Chrysomela aeneicollis (Coleoptera: Chrysomelidae). Ecological Entomology, 19: 31-38. https://doi.org/10.1111/j.1365-2311.1994.tb00387.x