Figure 4. The response of B. betularia larvae to a gradient in dowel luminance.

(A) Representative final instar B. betularia larvae from each luminance treatment resting on their corresponding dowel. Dowel treatments shown from left to right: Black (Bl), Dark grey (BW1), Mid grey (BW2), Light grey (BW3), White (Wh). (B) Average luminance of final instar B. betularia larvae reared under the five luminance treatments, as perceived by a blue tit (Cyanistes caeruleus) under bright daylight conditions. Solid line is the fitted cubic polynomial; dotted and dashed lines, provided for comparison, represent the linear (idealised continuous reaction norm) and stepped (two-state polyphenism) responses, respectively. Photo credit: Arjèn Van’t Hof.