Caterpillar ID needed, please

[Hector]

Can anyone hep with the ID of this Caterpillar?
(I assume it's a caterpillar)!
There were several feeding on my Alder tree throughout October. If it is not obvious from the photo they rest/feed very flat to the leaf.
Hoping that someone can help,
Hector.

[Trev Sawyer]

Hi Hector,
I think this is the unusual flattened larva of a species of Sawfly with the latin name Platycampus luridiventris.

Trev

[Hector]

Thanks Trev,
and apologies for thinking it was a caterpillar!
Hector