S3 and S4 sensilla on <i>M. melolontha</i> larval antennae.

<p>A: SEM. Tip of the S3 sensillum. B–F: TEM. B: Transverse sections of the apical part of S3. The central lumen contains an empty dendritic sheath. C: Transverse section of the middle part of S3. The empty dendritic sheath follows a furrow along the inner surface of the shaft. D: Transverse section of the socket of S3 revealing several ensheathed outer dendritic segments. E: Below the socket, the dendrites are surrounded by the receptor lymph producing tormogen cell. F: Some dendritic profiles show microtubules. Note the small profiles (arrowheads). G: SEM. Tip of a S4 sensillum with a terminal pore (arrowhead). H: Oblique section of the pore area of S4 (inset: 3.5× magnification of the pore). I: Subterminal transverse section of the same sensillum. The narrow lumen (4× magnification see inset) contains a thin dendritic sheath but no observable dendrites. J: Transverse section of the center of the sensillum shaft. The still narrow lumen (inset: 3× magnification) houses at least one outer dendritic segment. K: Oblique transverse section of the area where the socket (top left) extends into the shaft (lower right). The dendritic sheath contacts the shaft cuticle (inset: 2× magnification of dendrites). Two cuticular lamellae (arrowheads) flank the dendritic sheath. L, M: Transverse section of the socket, containing a dendritic sheath attached to the cuticle. One dendrite terminates in a tubular body. Not all dendrites exhibit clear microtubules. N: Oblique longitudinal section of the innervation of a S4 sensillum. The dendritic sheath is adjacent to the extensions of its origin, the thecogen cell. Abbr.: Cu, cuticle; dS, dendritic sheath; fCu, flexible cuticle; Fo, fold; Mv, microvilli; oD, outer dendritic segment; RLy, receptor lymph; shC, sheath producing cell; tB, tubular body; toC, tormogen cell; tP, terminal pore.