Blue biliprotein as an effective factor for cryptic colouration in Rhodinia fugax larvae

The fifth instar larva of the saturniid silkworm, Rhodinia fugax, is light yellowish-green on its dorsal surface and dark green on the ventral surface with a lateral demarcation between the two colours. The larva of R. fugax closely resembles the leaves of the host plant, Quercus serrata, in colour and shape. The spectral reflectance of the larval integument of R. fugax corresponds to that of the Q. serrata leaf. In the larval integument, there is more blue biliproteins (BPs) on the ventral surface than on the dorsal surface. Light intensity influences larval colouration. The larval integuments are green under light conditions (1000 lux), whereas larvae kept in dark conditions (10 lux) turn yellow. The BP-I content of the haemolymph is also affected by light intensity. The quantities of BP-I and its blue chromophore are higher under light conditions than under dark conditions. In contrast, there is little difference in the yellow chromophore content between the two light intensities. When larvae are kept in the light, the BP-I content in the cuticle is higher than under dark conditions in both the ventral and dorsal surfaces, and its chromophore content parallels the BP content. However, the amounts of BP-II and its chromophore in the epidermis show no change with the light intensity. Moreover, the quantity of yellow chromophore in the integument is also not affected by light intensity. Therefore, light stimulates the accumulation of BP-I and its chromophore in the haemolymph and cuticle, whereas the accumulation of BP-II and its chromophore in the epidermis are not influenced by light intensity. These results suggest that BPs and their chromophores determine the larval colouration and may play an important role in the cryptic colouration of R. fugax larvae.