The visual system of insects has attracted histologists for a long time. We have detailed histological studies of the visual systems of Diptera, Hymenoptera and Odonata dating from the last century: Leydig's (1864) study on optical ganglia of insects, Ciaccio's (1876) on the fine structure of the first ganglion in the mosquitos and Hickson's (1885) giving for the first time an exact description of the three optical ganglia of Musca. From 1896 several papers appeared using neuro histological methods, mainly Golgi techniques and methylene blue staining. In 1896 Kenyon published his work on the...