Fine-structure of the compound eye of the buprestid beetle Curis caloptera (Coleoptera, Buprestidae)

Curis caloptera is a buprestid beetle, which is active in bright sunlight. Its eye, like that of many other diurnal arthropods, is of the apposition type, in which dioptric apparatus and receptor layer are not separated by a region devoid of pigment. Perhaps to prevent damage by U. V.-radiation, the cornea is relatively thick (approximately 90 micron) and crystalline cones are of the "eucone-type". In each ommatidium the cone cell extensions occupy regular positions between the 8 retinula cells and reach down to the basement membrane where they end in bulbous swellings and contain grains of screening pigment. Pigment grains, slightly smaller than those present in the primary pigment cells, are also found within the retinula cells. Although the rhabdom possesses a uniform diameter of approximately 2 micron over its entire length of almost 300 micron, the number of rhabdomeres contributing to the rhabdom varies and depends on the level at which the rhabdom is sectioned. At the distal end, only one retinula cell possesses a rhabdomere; the same holds true for the proximal end, where a different rhabdomere (with microvilli at right angles to those of the distal cell) dominates. One retinula cell, of darker appearance in electron micrographs, occupies a distal position in each ommatidium and remains preferentially oriented within a sector of 60 degrees irrespective of the ommatidial axis. The ommatidial axis itself was found to vary 235 degrees. We provide circumstantial evidence for the view that the cell in question could be a U. V.-receptor with a role to play in an unambiguous determination of the E-vector. Separate bundles, each containing 8 axons, pass through the basement membrane together with 1 or 2 tracheoles. A traceheal tapetum is not developed.