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Abstract:
The thermo-/hygrosensitive sensilla styloconica of the silk moth Bombyx mori were studied using cryofixation and freez-substitution. These sensilla are characterized by a short poreless cuticular peg, which is double-walled in its distal part. The central lumen is innervated by the unbranched outer dendritic segments of the two presumed hygroreceptor cells. The presumed thermoreceptor cell displays lamellae below the peg base. Within the peg lumen, the dendrites are surrounded by the peridendritic dense coat and the lowdensity matrix. Below the peg base, these structures continue as the dendrite sheath, which is separated from the outer sensillum-lymph space by a layer of the trichogen cell. The central lumen, therefore, is only connected with the inner sensillum-lymph space, but the appearance of the low-density matrix, within the peg, differs from that of the sensillum lymph below the peg. In moist-adapted (24 h) sensilla, the two hygroreceptor dendrites invade the peg for three quarters and one half of its length, respectively, and fill the cross-sectional area of the lumen by 50–80%. In dry-adapted (24 h) sensilla, the dendrites terminate more proximally and fill the cross-section by ∼35%. The volume of the low-density matrix increases under dry conditions and decreases under humid conditions. At intermediate ambient humidity, the morphology of these sensilla is halfway between the dry-adapted and the moist-adapted state. The effect of dry-adaptation is reversible, so that sensilla that were first dry-adapted and then moist-adapted (24 h each) before cryofixation cannot be distinguished from moist-adapted sensilla. The reduction of the exposed length of the dendrites is interpreted as a shift of the working range of the receptors and/or protection against desiccation. The current theories of sensory transduction in hygroreceptors, in particular the hygrometer and evaporimeter hypotheses, are discussed with respect to the present findings.
