THE VISUAL ECOLOGY OF PUPILLARY ACTION IN SUPERPOSITION EYES

The two most common mechanisms of pupillary screening-pigment migratio n in arthropod superposition eyes are the cane and longitudinal pigmen t migration mechanisms. The dynamics of each were investigated by opti cal modelling and by determining experimentally the relationship betwe en eye glow brightness and screening pigment position within the eyes of two representative insect species: the noctuid moth Agrotis infusa and the dung beetle Copris elphenor. During dark adaptation, in both m echanisms, the screening pigment is contracted distally to expose the proximal half of each crystalline cone. During light adaptation the pi gment migrates proximally and reduces light flux in the retina. In the longitudinal mechanism, pigment migrates into the clear zone of the e ye. In the cone mechanism, pigment never enters the clear zone and is instead restricted to the proximal half of each crystalline cone: a mi grating sleeve of pigment creates a small aperture at the end of the c rystalline cone, the area of which depends on the degree of light adap tation. According to the model, the cone mechanism provides a limited range of light attenuation (ca. 0.6 log units) for which both good spa tial resolution and accuracy of control are maintained, and within thi s range attenuation is controlled very finely. Beyond this range, whil st attenuation is still possible, diffraction at the pigment aperture and increasing coarseness of control worsen visual performance signifi cantly. In contrast, the longitudinal mechanism provides a much larger useful range of light attenuation (up to several log units) and maint ains reasonable fineness of attenuation control over the entire range (although not as fine as the cone mechanism). The experimental results support the model. An extensive survey of arthropods with superpositi on eyes reveals that the cone mechanism is almost exclusively possesse d by those animals experiencing a narrow range of light intensities, a nd the longitudinal mechanism by those experiencing a wide range.