The spectral sensitivities of identified receptors and the function of retinal tiering in the principal eyes of a jumping spider
The functional organisation of the central retina of the anterior median (AM) eyes of a jumping spider,Plexippus (Salticidae) is examined by anatomical, electrophysiological and optical methods. A model of the eye is derived from the data.
The anatomy of the AM eye is similar to that of salticid eyes described by Land (1969a) and Williams and McIntyre (1980). There are four tiers of receptors of which only the most proximal (Layer I) is a regular mosaic with rhabdoms designed to have light-guide properties. The receptor population of Layer I is homogeneous, whereas in Layers II–IV more than one receptor type can be considered to contribute to each layer.
Intracellular recordings from AM photoreceptors reveal only two spectral classes: green cells with peak responses at ca. 520 nm, and ultraviolet (UV) cells with peak responses at ca. 360 nm. ERGs from intact retinae exhibit similar peaks. Spectral sensitivities from pooled intracellular recordings from green cells and ERGs correspond reasonably closely. The comparison does not, therefore, support the possibility that the retina contains receptors with peak responses at longer wavelengths, although it does not exclude it.
Spectrally characterised cells were marked by the injection of Lucifer Yellow. From the results of 13 successful injections, (a) peripheral Layer I and peripheral and central Layer II cells are green receptors; (b) Layer IV cells are UV receptors. Central Layer I and Layer III receptors were not marked.
The chromatic aberration, focal length and other optical parameters of the corneal lens of the AM eye were measured directly. The lens functions essentially as a single-surface lens of refractive index 1.40, and, together with the curved interface between the anterior chamber of the eye and the receptor matrix, forms a telephoto system.
The spacing between receptor Layers I and IV is matched to the chromatic aberration of the eye; if green light from an object in front of the spider is focused on Layer I, UV light will be focused on Layer IV (and Layer III).
The distal ends of Layer I receptors form a staircase, those lying laterally being closer to Layer II than those lying medially. This staircase enables the spider to receive in-focus images from objects at distances between ca. 3 cm — ∞ in front of it. It is suggested that the scanning movements of the retinae described by Land (1969b) serve to sweep an image across the staircase so that it will be in focus on some part of Layer I, provided that the object is within that range of distances.
Retinal tiering (including the staircase of Layer I) compensates both for the chromatic aberration of the dioptrics of the eye and for its inability to accommodate.
anterior median (eye)