Figure 5. The response of B. betularia larvae to a gradient in dowel colour and luminance.

(A) Photograph of final instar B. betularia larvae from each colour treatment resting on their corresponding dowel. Dowel treatments shown from left to right: Brown (Br), More brown (BG1), Brown-green (BG2), More green (BG3), Green (Gr). (B) Average greenness of dowels vs. B. betularia larvae exposed to dowels from each of the five treatment groups, as perceived by a blue tit (Cyanistes caeruleus) under bright daylight conditions. Solid line is the fitted quadratic polynomial; dotted and dashed lines, provided for comparison, represent the linear (idealised continuous reaction norm) and stepped (two-state polyphenism) responses, respectively. Photo credit: Lukasz Lukomski.