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Please could you ID this caterpillar found on a small species birch tree in the garden. Thank you
It is a sawfly, probably of the genus Arge, Craesus or Nematus.
At the larval stage, the species are difficult to ID with certainty.
Some of the British species of Argidae (incl. Arge)
and Tenthredinidae (incl. Craesus, Nematus)
One is tempted to say it is birch sawfly, which appears to be used a a common name for both Cimbex femoratus (http://www.naturespot.org.uk/species/birch-sawfly, different), and Arge Pectoralis (http://www.pbase.com/tmurray74/image/67293861, head not black), and Arge pullata (European birch sawfly, different), and American dusky birch sawfly (Craesus latitarsus, http://www.projectnoah.org/spottings/11534251, very similar but does not occur in the UK?).
But it is none of those.
Looking at some of the possibilities...
Arge cyanocrocea - probably not: wrong food plant association
Arge berberidis - ditto
Arge melanochra - ditto
Arge ochropus - ditto
Arge pagana - possibly ditto (and has orange head)
Nematus pavidus - ditto
Arge ustulata - wider range of food plants - maybe
Craesus septentrionalis - most likely to be yours