OMMATIDIUM OF LACEWING CHRYSOPA (NEUROPTERA)

The eye is a clear zone eye with extensive movement of retinula cells on adaptation to light. The ommatidium has 3 types of rhabdomere at different levels, so that the eye necessarily abstracts at least 3 kinds of information simultaneously from the incoming rays. In the light-adapted state light can enter each ommatidium only via a crystalline tract that is surrounded by dense pigment grains. A small distal rhabdomere (cell 7) always lies at the end of this tract. In the dark-adapted eye the retinula cell nuclei and distal rhabdomere move to the cone tip and the crystalline tract is drawn into the cone. There is then a region of the retinula cell column between cone tip and proximal rhabdoms, across which there is no structure that could act as a light guide. A key question is how the light is focused across this clear zone in the dark-adapted state. As shown by the wide angular distribution of eyeshine when a parallel beam is incident on the dark-adapted eye, rays are poorly focused upon the columns of the large rhabdoms. The wide visual fields of receptors 1-6 in the dark-adapted eye, inferred from the observation of eyeshine, are seen as a way of narrowing the bandwidth of spatial frequencies, so that only the largest objects in the visual field contribute to motion-detection. This would improve the signal-to-noise ratio not in the receptors themselves, but in the neural mechanism by simplifying the incoming signal.