Structure and putative function of dark- and light-adapted as well as UV-exposed eyes of the food store pest Psyllipsocus ramburi Sélys-longchamps (Insecta: Psocoptera: Psyllipsocidae)

The psocopteran Psyllipsocus ramburi Sélys-Longchamps can render food stuffs unpalatable and may serve as an intermediate host for cestodes. Its two circular compound eyes consist of about 26 facets, capped by strongly convexly curved corneae of 10–18 μm in diameter. Corneal nipples or interommatidial hairs are not developed. Beneath each corneal lens a cluster of four cone cells, enveloped by two primary pigment cells, separates an ommatidial group of eight retinula cells from the inner corneal surface. Membrane specializations of the retinula cells, known as the microvilli, measure 60 nm in diameter, and collectively make up the rhabdom, which is columnar in shape and has a distal diameter of 4 or 5 μm, depending on whether it is day- or night-adapted. Cone cell lengths measure 4.5 μm during the day and 8.5 μm at night and retinula cell screening pigments closely approach the edge of the rhabdom during the day. A 1-h exposure to UV-A (λmax=351 nm) of ca. 1200 lx causes an almost total destruction of the photoreceptive membranes of the rhabdom and bleached all retinula cell screening pigments, but not the pigment grains of the primary pigment cells. Calculations, based on the anatomical data, suggest that the eyes are adapted to function under dim light levels, but cannot produce sharp images since their best possible acceptance angles are 22° and 28° in light- and dark-adapted states, respectively. Destruction of vision, likely affecting biorhythm and reproduction, by exposing the insects to UV-A may offer an alternative to the use of chemicals in controlling these insects.