POSTLARVAL DEVELOPMENT OF COMPOUND EYES AND STEMMATA OF CHAOBORUS-CRYSTALLINUS (DE GEER, 1776) (DIPTERA, CHAOBORIDAE) - STAGE-SPECIFIC RECONSTRUCTIONS WITHIN INDIVIDUAL ORGANS OF VISION

During metamorphosis, the dioptric apparatus of the larval compound ey e of Chaoborus crystallinus (Diptera : Nematocera) is radically recons tructed. The thin larval cornea of the ommatidia is replaced by strong ly curved corneal lenses, and the eucone larval cone is replaced by an imaginal cone of the acone type. Curvature of the future lens is alre ady apparent in very young pupae, in which the cornea consists only of a thin epicuticle with corneal nipples. Fibrillary cuticle is secrete d by cone and primary pigment cells throughout pupal development. Lens formation is accompanied by movement of the nuclei of the accessory p igment cells. The larval cone disintegrates unexpectedly late in young imagos. During late pupal development, 7 cone cell projections emerge . In contrast to the dioptric apparatus, the retinula cells and rhabdo m remain almost unchanged during metamorphosis. The main refractive el ement of the larval ommatidium appears to be the cone, while that of t he imaginal ommatidium is the corneal lens. In addition to the compoun d eyes, the pairs of stemmata are retained during the whole post-larva l development. Pupal stemmata show no structural differences from the larval stemmata. The stemmata are still present in 2-day-old imagos (' 'retained stemmata''), but the primary stemma loses its dioptric appar atus and is proximally relocated to the basal region of the compound e ye. The reconstructions in the visual system of Chaoborus, which occur during ontogeny, are probably connected with the change from aquatic living larvae to aerial adults, and appear to fulfill stage-specific n eeds of vision.