Green colouration of cocoons in Antheraea yamamai (Lepidoptera: Saturniidae): light-induced production of blue bilin in the larval haemolymph

When the larvae of a saturniid silkmoth, Antheraea yamamai, are maintained under high intensity light (5000 lux), they produce green cocoons whereas the cocoons produced under light of low intensity (e.g., 50 lux) or in darkness are yellow. The green colour of the cocoon is due to the presence of a blue bilin pigment in combination with yellow pigment, and light stimulates the accumulation of blue bilin. In the present study, we show that two blue bilins, with similar characteristics to the sarpedobilin in the green cocoon, can be induced in larval haemolymph both in vivo and in vitro. In both conditions, the amount of these bilins increased with increasing intensity or duration of light exposure. Induction also occurred at 0 °C. In contrast, the chromophore of the constitutive biliprotein of the haemolymph did not change depending on light conditions. Size fractionation of the haemolymph indicates that the precursor of the blue bilins induced by light is bound to a protein with a molecular mass of 5000 Da or more. Thus, in these insects, the blue bilin responsible for green colouration is facultative under photochemical stimulation.