Studies on the developing insect visual system.

Investigations on the postembryonic development of the (hemimetabolous) insect visual system were carried out using the following techniques: light and electron microscopy, selective neuronal staining, and surgical transplantations of retinal material. Animals of the following species were used for experimentation: Periplaneta americana, Gromphadorhina portentosa, Leucophaea maderae, and Schistocerca gregaria. Processes of determination and differentiation in the developing insect retina are described. The structure of the lamina ganglionaris and aspects of its morphogenesis have been analysed. Rules governing the formation of connections be tween the retina and the lamina have been elucidated. Two major findings of the thesis are: a) In the postembryonic growth of the retina, new ommatidia are formed only from cells generated from a "budding zone" lying along the anterior, dorsal, antero-dorsal and ventral retinal margins. This is contrary to recent suggestions that recruitment of head epidermal cells occurs, followed by their transformation into ommatidial cells. b) The establishment of neuronal connections between the retina and lamina does not depend on neuronal specificity. The topographic mapping of retinula terminals within the lamina results from a programmed growth sequence. The outgrowing axons of newly-formed retinula cells depend upon contact guidance by previously established connections to reach their target cells in the lamina. The interpretation of these two findings is that the budding zone always forms at the same positional value within the head capsule, and cells with this value are determined to connect with newly-formed second order cells. Interspecific grafts between retinae of G. portentosa and L. maderae allow the following conclusions: a) Ommatidium-forming mechanisms operating in the two different genera are identical or very similar. This is apparent from the formation of ommatidia composed of cells from both sources. b) For the first time it is possible to show that none of the cone cells within an ommatidium are necessarily related by their mitotic history. This supports the argument that cells within the differentiating ommatidium are determined by cellular interaction and not by cell lineage. c) Growing retinula axons elongate until they reach the outer optic anlage of the lamina. Axons are not programmed to grow particular distances. The thesis also includes anatomical descriptions of the locust and cockroach visual systems at various stages of development.