Morphology of the visual system ofChaoborus crystallinus (Diptera, Chaoboridae)

Abstract

In this paper we present findings on the fine structure of the ommatidia and stemmata [lateral ocelli] of larvalChaoborus. The organization of these photoreceptors is compared with that of imaginai compound eyes and stemmata of related taxa; their homology is elucidated. Peculiar attributes of the larval ommatidium include the lack of a corneal lens, the presence of a eucone crystalline cone which is composed of four Semper cells, and the location of most retinula cell nuclei proximal the basal lamina. Two primary and 10–12 accessory pigment cells are present whose nuclei are situated distally along with the nuclei of the cone cells. The retinula cells are arranged in a pattern common to most Diptera. Surrounding the central retinula cell (R8) are retinula cells R1–R7. R7 distally sends a process to the center of the ommatidium. Rhabdomeres of R7 and R8 are in a tandem position. The rhabdom belongs to the fused type since the extracellular space between neighboring rhabdomeres and/or retinula cells is very small. The two stemmata are referred to here as primary and accessory stemma. The primary stemma is a complex formation of three fused units. Each of these units is itself a stemma. A dioptric apparatus can be found which is composed of a single-cell crystalline cone and a group of refractive cells. The secondary stemma possesses neither a dioptric apparatus nor pigment granules. Its rhabdom is voluminous but its arrangement is not fixed. Though these stemmata represent a highly derived situation, where several ommatidia or stemma units have been fused, arguments supporting the homology of stemmata and ommatidia can be furthered. The crystalline cone, though made of a single Semper cell, can be homologized with the cones of ommatidia since the fine structure is identical. Cones of both primary stemma and compound eye are of the eucone type, and centrally contain granules which exhibit a crystalline arrangement. Apart from refraction, Semper cells are involved in cuticle formation. Refractive cells may be homologous to either cone or pigment cells. The pattern in which refractive and cone cells of the primary stemma are arranged appears to be induced by retinula cells. Some of stemmata's characters which are very uncommon for ommatidia may be based on constraints due to complex stemma formation or to functional changes. Among many taxa related to Chaoboridae, a homologous group of stemmata with similar arrangement of units can be found. It is revealed that well-developed imaginal compound eyes within larval stages represent a synapomorphy of Culicidae and Chaoboridae.