Post-embryonic development of the principal retina of a jumping spider. II. The acquisition and reorganization of rhabdomeres and growth of the glial matrix

Abstract

An accompanying paper by Blest ( Phil. Trans. R. Soc. Lond. B 320, 489 (1988 b )) describes the conformational changes undergone by principal retinae of a salticid spider during post-embryonic development, and identifies the origins of the tiered receptor mosaics. We now discuss the ultrastructural morphogenesis of the individual receptive segments, with particular reference to the growth of microvilli, to the achievement of light-guide properties by rhabdomeres of the foveal mosaic of Layer I, and to putative mechanisms that ensure that inter-receptor spacings are optically apt. 1. Establishment of the tiered retina by conformational changes precedes the first appearance of plasmalemmal microvilli in all four Layers of receptive segments. Microvilli are first differentiated in the ventral retina. 2. Short, irregular microvilli, initially interdigitated, appear on the plasmalemmae of receptive segments around day 10 of post-embryonic development. They progressively lengthen, cease to be inter-digitated, and become more regular throughout subsequent development, during the moult to the second instar at day 12, and until day 15 when spiderlings become fully independent and principal retinae attain their final conformation. 3. Until late in development, after the moult to the second instar, foveal Layer I receptive segments are contiguous, and densely occupied by organelles. The six glial strands that flank each receptive segment are prominent. A transformation between days 14 and 15 variously reduces or eliminates the glial columns, and, by endocytosis, the plasmalemmal areas of the receptive segments. The Layer I cytoplasm is stripped of organelles other than microtubules. Consequently, receptive segments become separated from each other, and their rhabdomeres can perform as light-guides. 4. Over the same period, receptive segments of Layers II-IV slowly acquire the dense population of mitochondria that have been shown to equilibrate the refractive indices of rhabdomeres and their surrounds. 5. Final conformational changes to the retinae and the establishment of definitive spacings between receptive segments seem to be assured by the differential growth and local involution of the finely divided glial processes intercalated between them. Despite some caveats, it is argued that this protracted sequence of events is consistent with a limited degree of ontogenetic recapitulation. In particular, it is remarkable that the early states of the nascent microvilli so closely resemble those found in less advanced families of spiders.