The direction of incident light and its perception in the control of pupal melanization in Pieris brassicae

Abstract

The rôle of the direction of incident light in pupal melanization of the cabbage white butterfly, Pieris brassicae, was examined by exposing larvae during their sensitive period to various combinations of a light and dark background and of melanization inhibiting and promoting light. The results show that melanization is strongly controlled by the direction of the incident light. By blinding different eyes with black varnish, their rôle in the perception of the brightness contrast was studied. Nearly the same degrees of melanization are achieved as by corresponding arrangements of brightness contrast in the environment. Blinding the most ventrally located ocellus (No. 1) results in maximal melanization; blinding additional ocelli has no additional effect. On the other hand, no influence is observed if only the most dorsal ocellus (No. 6) is blinded. By coating the eyes with colours, their rôle in perception of melanization inhibiting and promoting light was investigated. Melanization is markedly influenced by light perceived via the ventral situated ocelli. In this case, the effect also depends on the number of ocelli involved. All effects may be explained by a mechanism which differentiates and transforms signals received through two or more ocelli, light perceived via the most ventral ocellus (No. 1) being the most important signal. An additional extraocular light receptor appears to be located dorsally in the head. This extraocular receptor can discriminate between different spectral ranges and perceive a brightness contrast (if the ocelli are blinded and the trunk is exposed to white illumination).