Ultrastructure and function of digitiform sensilla on the labial palp of a larval elaterid (Coleoptera)

There are typically six elongate pegs positioned in longitudinal grooves in the outer walls of the distal segment of the labial palps of larvae of Ctenicera destructor (Brown). They are termed digitiform sensilla on the basis of their form and function. Each has a subapical pore typical of a contact chemoreceptor in surface scan, and a terminally branched dendrite but not the cuticular pores typical of a chemoreceptive porous hair. The dendritic terminations are encased by the dendritic sheath, the subapical pore is plugged in the wall of the peg, and there is only one innervating neuron, all of which are typical of a tactile mechanoreceptor. These pegs respond electrophysiologically to contact and vibratory stimuli, but not to the amino acids, sugars, salts, and water tested, nor to a changed pressure in the cephalic hemocoel.