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Abstract

The fine structure of caeloconic sensilla of Bombyx mori was studied in cryofixed specimens, These sensilla belong to the category of double-walled wall-pore sensilla. The pegs are similar to 10 mu m long, located in pits on the dorsal side of the antennal branches, and longitudinally grooved in their distal half (grooved surface similar to 30 mu m(2)), The central lumen contains the outer dendritic segments of usually five receptor cells, and is surrounded by up to 15 partially fused cuticular fingers, The peripheral lumina of these cuticular fingers are filled with material resembling wax-canal filaments, Radial spoke channels (similar to 600 per peg), each 10-20 nm wide, connect the central lumen with the longitudinal groove channels. Groove and spoke channels are assumed to mediate the transport of odorant molecules from the outer epicuticular surface layers to the sensory dendrites, Thus the double-walled wall-pore sensilla represent a bauplan essentially different from single-walled wall-pore sensilla; the reason, however, why the two types are found together throughout the insect orders remains enigmatic. Other peculiar features of the coeloconic sensilla of the silkmoth ape invaginations of the outer dendritic segments and direct contacts between the receptor cell somata, The latter may be the structural correlate to electrophysiological observations indicative of peripheral interaction between the receptor neurons. All three auxiliary cells have elaborately folded apical plasma membranes studded with portasomes and associated with an abundance of mitochondria; basally they often contact tracheal branches, As compared to the auxiliary cells of the single-walled olfactory sensilla of the same species, ail the mentioned features are much more prominent and hint to a higher ion pumping activity at the border to the sensilium-lymph cavities.