This Common White Wave (Cabera pusaria) has emerged on 17th June. The caterpillar came from birch on Penistone Hill, near Haworth VC63. It was found on a rainy day during the first week of September 2009.
I had worked out the species from the well-documented description of the brown-form larva; having a greenish central part of the flattened head.
At the same wind-swept location, on the same day, I found an Iron Prominent caterpillar (also on birch) and a Narrow-winged Pug caterpillar on the flowering heather (smaller photograph, also featured on page 105 of Yorkshire Butterflies & Moths 2009).