Fig. 9
Internal ultrastructure of peg-shaped sensilla based on TEM analysis. A Section of the cuticle revealing the peculiar ultrastructure of nano-ridges delimitating the cuticular scutes. Ridges appear rounded due to the presence of conspicuous bars (arrows) lying on and obscuring the groove-like structure established by the much more electron-dense exocuticle. B Distal aspect of a peg-shaped sensillum from the lateral group, longitudinal section. Note the coiled nature of the sensillum cell apparatus, which is cut at five different levels (from upper left to lower right). C Close-up of the peg cut longitudinally. Only parts of the central mass plugging the tip pore are visible (arrowhead). D Longitudinal section of a peg-shaped sensillum showing a long extracellular canal lined by a cuticle. Note that the tip-pore (arrowhead) is clogged by secretion, which is continuous with the layer on the peg shaft. E Close-up of profiles of two outer dendritic segments projecting up the extracellular canal. These profiles are suggested to be terminations as they exhibit characteristics of tubular bodies. F Longitudinal overview section of several peg-shaped sensilla cut at various levels. The transition zones of inner and outer dendritic segments are surrounded by the first (scolopale) sheath cell (asterisks). Cross profiles of more distally cut peg-shaped sensilla can be found at the upper end of the image (crosses). The inner sensillum lymph space only carries two outer dendritic segments. G Distal aspect of peg-shaped sensilla in detail. The two outer dendritic segments of biciliated receptor cell 1 (rc1) with elongated dendritic outer segments are enwrapped by a thickened dendritic sheath formed mainly by the second (median) sheath cell. H Close-up (dashed box in F) of the transition zone of inner dendritic segments and both long outer dendritic segments of receptor cell 1. One cilium is only represented by its most basal part (arrow). Note the low electron-density of the inner sensillum lymph space contrasting well with the strongly osmiophilic scolopale-like structure in the first (proximal) sheath cell. I Transverse-oblique section further proximally of (G) showing the region where the two short outer dendritic segments of receptor cell 2 (rc2) are attached to the dendritic sheath via tubular bodies. The long outer dendritic segments are cut in the opposite corner of the receptor lymph cavity. Note the extensive system of microtubules and filaments connecting the scolopale-like structure to the outer rim of the dendritic sheath. bb, basal body/bodies; encu, endocuticle; excu, exocuticle; ds, dendritic sheath; epc, interstitial epidermal cell; esp, extracellular space; fi, unspecified cytoplasmic filaments; ids, inner dendritic segment; isl, inner sensillum lymph space; lod, long outer dendritic segments); mi, mitochondrium; mt, microtubules; ods, outer dendritic segment(s); ovs, secretion layer; pe, peg-shaped sensillum; rc1, receptor cell 1 with long outer dendritic segments); sco, scolopale-like structure; shc1, first sheath cell (scolopale cell); shc2, second (median) sheath cell; shc3, third (distal) sheath cell; sod, short dendritic outer segments; tb ,tubular bodies