Though I wish I had a parent or reared voucher specimen photo to show (I trust my adult ID, but evidence is great), I believe these caterpillars reared from May 11, 2004 eggs match well with the description on page 195 of Geometroid Caterpillars (Wagner, et al.), (though they hedge their bets there, too). Not absolutely certain these are in the right order but represent photos taken on June 2 at about 19 mm and June 4 at about 25 mm. By June 6 several were prepupal. Adults eclosed June 21 and 23. PLEASE NOTE that since this was a batch rearing I don't know if both images represent the same caterpillar. Notes don't say what I reared them on either, but maybe dandelion.