Detailed view of the distribution of α-ARD immunoreactivity in the visual system of Calliphora. A Layered organization of α-ARD immunoreactivity in the medulla. The medulla layers in this horizontal section are denoted according to the two terminologies used in blowflies and fruitflies. On the left side layers are labeled with Strausfeld's [1,34] terminology, which is based on the termination sites of laminar neurons (L1-L3) and photoreceptor terminals (R7,R8) in Calliphora. Labels on the right side reflect the terminology developed by Fischbach & Dittrich  for Drosophila. The inner medulla shows strong immunoreactivity with layer 9 appearing to have the highest density of nAChRs. In the outer medulla the layers with the terminal arborizations of L1 and L3 lamina monopolar cells are immunopositive. The layers with the terminals of L2 and R8 show lower levels of α-ARD immunoreactivity (bottom is medial, top is lateral). B Lobula and lobula plate. The posterior part of the lobula is divided in two α-ARD immunoreactive layers. The peripheral layer of the lobula contains the dendrites of the columnar T5 neurons, which project to the lobula plate and are likely input elements of the lobula plate tangential cells. The lobula plate contains high levels of α-ARD immunoreactivity throughout. In the distal portion of the lobula plate (left in frame) two distinct major layers are apparent, which correspond to those ones labeled in 3H-Deoxyglucose studies  after stimulation with horizantal or vertical visual motion. A similar pattern in the lobula plate was also observed in a study with ChAT-immunohistochemistry . Note, the large holes in the sections at the posterior face of the lobula plate mark the main dendrites of the VS-cells. These show no immunoreactivity on their surface, indicating that ARD-containing nAChRs are not present on the main dendrites of VS-cells. C High magnification view of the lobula plate. Immunoreactivity is labeled with DAB. For better comparison with panels A and B, this image was digitally inverted so that positive immunoreactivity is also represented by bright pixels. Note the intense immunoreactivity indicated by arrows close to the main dendrites of VS-cells (white circles). This might indicate a high density of nAChRs on the higher order dendrites of VS cells. Scale bar in A: 40 μm; in B: 50 μm; in C: 16 μm.