Heterarthrus Sawfly

Heterarthrus Sawfly.                                                               

Small fingernail in size.  Note scalloped edges.  Circular almost flat appearance, like ravioli!  Transluscent 'body' or 'casing'. Green body (?) inside?  Tiny black bar on edge.  Constant flick flacking across the gravel.  One side appeared lighter in colour.  Initially I thought I detected mouth piece and legs making me think it may be a tick.  Now I am thinking it looks more like a silk chrysalis perhaps?

Found in Titchmarsh Nature Reserve car park, surrounded by trees (not sure what type), blackberry bushes and elder flowers, wheat growing in adjoining field, surrounded by typical countryside hedgerows and close to River Nene and lakes.

UPDATE:  Thank you Scarlett for Heterarthrus Sawfly identification! The larva lives in a mine on Sycamore then makes a silk cocoon with the upper surface of the leaf.  This cocoon drops to the ground and jumps about!