Abstract
Using the combined Golgi-electron microscopy technique, we have determined the three-dimensional dendritic fields of the short visual fibres (svf 1–3) and first-order interneurons or L-fibres (L1-4) within the first optic ganglion (lamina) of the nocturnal bee Megalopta genalis. Serial cross sections have revealed that the svf type 2 branches into one adjacent neural unit (cartridge) in layer A, the most distal of the three lamina layers A, B and C. All L-fibres, except L1-a, exhibit wide lateral branching into several neighbouring cartridges. L1-b shows a dendritic field of seven cartridges in layers A and C, dendrites of L2 target 13 cartridges in layer A, L3 branches over a total of 12 cartridges in layer A and three in layer C and L4 has the largest dendritic field size of 18 cartridges in layer C. The number of cartridges reached by the respective L-fibres is distinctly greater in the nocturnal bee than in the worker honeybee and is larger than could be estimated from our previous Golgi-light microscopy study. The extreme dorso-ventrally oriented dendritic field of L4 in M. genalis may, in addition to its potential role in spatial summation, be involved in edge detection. Thus, we have shown that the amount of lateral spreading present in the lamina provides the anatomical basis for the required spatial summation. Theoretical and future physiological work should further elucidate the roles that this lateral spreading plays to improve dim-light vision in nocturnal insects.




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Acknowledgements
We thank Megan O'Connor and two anonymous reviewers for critically reading the manuscript, Victor Gonzales, Andre Riveros and Mark Holdstock for assistance with fieldwork, William Wcislo and the staff of the Smithsonian Tropical Research Institute (STRI) for their help and the Autoridad Nacional del Ambiente of the Republic of Panama for permission to export bees. The histological work was partly carried out by B.G. at the Center for Visual Sciences, Research School of Biological Sciences, Australian National University, Canberra.
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B.G. is grateful for grants from the Royal Physiographic Society, the Per Westlings Fond, the Foundation of Dagny and Eilert Ekvall and the Royal Swedish Academy of Sciences. E.J.W. would like to thank the Smithsonian Tropical Research Insitute, the Swedish Research Council, the Crafoord Foundation, the Wenner-Gren Foundation and the Royal Physiographic Society of Lund for their ongoing support.
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Greiner, B., Ribi, W.A. & Warrant, E.J. A neural network to improve dim-light vision? Dendritic fields of first-order interneurons in the nocturnal bee Megalopta genalis . Cell Tissue Res 322, 313–320 (2005). https://doi.org/10.1007/s00441-005-0034-y
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DOI: https://doi.org/10.1007/s00441-005-0034-y