A diurnal moth superposition eye with high resolution Phalaenoides tristifica (Agaristidae).

Abstract

This common Australian moth flies slowly and only in bright sunlight, but the compound eye has a sharply focused superposition image, to which 120-150 facets contribute. A distant point source is focused to form a blur circle on the retina. The diameter of this blur circle at 50% intensity, calculated from the width of the angular distribution of eyeshine, is 1.5-2.0° subtended at the centre of the eye, matching the interommatidial angle. The compromise between acuity and sensitivity, which is dependent on rhabdom spacing in relation to the blur circle diameter, has been struck with approximately 50% of the converging light from a distant point source falling on a single rhabdom. The F value of the focusing arrangement, defined as (focal length/aperture) is approximately 0.9. The structure of the eye, which undergoes negligible change on adaptation to light, is described by electron microscopy. The optical system is examined. The cornea and crystalline cone taken together constitute an afocal combination of lenses. Although the retinula cell columns act as light guides, a negligible contribution passes down them because rays are not focused upon their distal ends. The rhabdom columns are isolated from each other by complete sleeves of specialized tracheae backed by screening pigment. The e. r. g. is dominated by a single visual pigment resembling a rhodopsin with peak near 530 nm but adaptation studies suggest a small amount of a second pigment peaking near 360 nm. There are 14-16 retinula cells per ommatidium. An explanation for this great number, together with the use of a superposition image in daylight, cannot be offered until the visual behaviour is better understood. Apparently the eye is adapted for extreme sensitivity compatible with good resolution, and possibly this moth can see small contrast differences in small objects. For reasons that are discussed, a similar sharp focus is not to be expected in the eyes of moths that fly in dim light.