Post-embryonic photoreceptor development and dark/light adaptation in the spittle bug Philaenus spumarius (L.) (Homoptera, Cercopidae)

Abstract

The aims of this paper have been to describe (1) the general structure of the compound eye of the spittle bug Philaenus spumarius, (2) the eye's post-embryonic development, (3) photomechanical changes upon dark/light adaptation in the eye, and (4) how leaving the semi-aquatic foam bubble and turning into an adult affects the organization of the eye. Spittle bugs, irrespective of size or sex, possess apposition type compound eyes. The eye's major components (i.e. facet, cornea, cone and rhabdom) grow rather isometrically from the smallest nymph to the adult. Photomechanical changes can occur during both nymphal and adult phases and manifest themselves through pigment granules and mitochondria migrating to and away from the rhabdom, and rhabdom diameters varying with time of day and ambient light level. When a nymph transforms into an adult, its compound eyes' dorsoventral axes widen, facet diameters increase, facet shapes turn from circular to pentagonal and hexagonal, the cornea thickens and the rhabdoms become thinner. The agile adults, free from the foam that surrounds the nymphs, can be expected to need their vision more than the nymphs, and the changes in eye structure do, indeed, indicate that the adults have superior visual acuity. A thicker cornea in the adults reduces water loss and protects the compound eye from mechanical and light-induced damage: protection given to the nymphs by their foam bubbles.