Green Larva on well eaten Yellow Flag Iris leaf growing in pond. Has double rows of lighter coloured spots.
No interactions present.
The number of fleshy 'prolegs' under the body (behind the three pairs of true insect legs) will show this to be a sawfly (moths have five pairs of prolegs at most, sawflies have at least six). And in this case the foodplant is a very good clue - there is only one sawfly that feeds on it, Rhadinoceraea micans, and it looks just like your photo!
Entomologist and biological recorder
I'd agree with Iris Sawfly. I've also seen them on Yellow Flag recently: http://www.ispot.org.uk/node/2074
Hi JaseJ, thanks for posting the link to your previous observation, I'd overlooked that one. Have just been back to it and given it a Spot-on!
Lat/Lng: 53.208352246428, -0.11858582496643
OS grid ref: TF257695
Banovallum House - HQ of Lincolnshire Wildlife Trust, Garden, wood, grassland, River bank