Post-larval development of compound eyes and stemmata of Chaoborus crystallinus (De Geer, 1776) (Diptera : Chaoboridae): Stage-specific reconstructions within individual organs of vision

Abstract

During metamorphosis, the dioptric apparatus of the larval compound eye of Chaoborus crystallinus (Diptera : Nematocera) is radically reconstructed. The thin larval cornea of the ommatidia is replaced by strongly curved corneal lenses, and the eucone larval cone is replaced by an imaginal cone of the acone type. Curvature of the future lens is already apparent in very young pupae, in which the cornea consists only of a thin epicuticle with corneal nipples. Fibrillary cuticle is secreted by cone and primary pigment cells throughout pupal development. Lens formation is accompanied by movement of the nuclei of the accessory pigment cells. The larval cone disintegrates unexpectedly late in young, images. During late pupal development, 7 cone cell projections emerge. In contrast to the dioptric apparatus, the retinula cells and rhabdom remain almost unchanged during metamorphosis. The main refractive element of the larval ommatidium appears to be the cone, while that of the imaginal ommatidium is the corneal lens. In addition to the compound eyes, the pairs of stemmata are retained during the whole post-larval development. Pupal stemmata show no structural differences from the larval stemmata. The stemmata are still present in 2-day-old images (“retained stemmata”), but the primary stemma loses its dioptric apparatus and is proximally relocated to the basal region of the compound eye. 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 needs of vision.