The mechanosensory apparatus of the femoral tactile spine of the cockroach, Periplaneta americana

Abstract

Tactile spines are large cuticular sense organs that appear to provide insects with a sense of touch which is spatially coarse but of great sensitivity. Cockroach legs have a number of these spines on each leg and a particularly prominent spine on the end of each femur, the femoral tactile spine. The ease of recording afferent activity from this spine during mechanical stimulation has made it one of the most thoroughly studied insect mechanoreceptors and yet it has never been examined by electron microscopy. We report here the results of an examination of the femoral tactile spine by both scanning and transmission electron microscopy, as well as by light microscopy. The spine is shown to be innervated by a single sensory bipolar neuron with its soma located in the base of the spine. A canal through the wall of the spine leads to the outside and emerges just above the junction between the base of the spine and its articulating socket membrane. The sensory dendrite of the neuron passes from the soma through this canal and forms a modified ciliary sensory ending with the typical dendritic sheath and dense tubular body that is characteristic of insect mechanosensory cuticular sensilla. The tubular body is embedded in a cuticular terminal plug which closes the exterior end of the canal but appears to be fastened to the spine by a very flexible ring of cuticle. This plug is connected to the socket membrane by a specialized socket attachment which presumably serves to move the plug relative to the wall of the spine during movement of the spine within the socket. The morphology of this sensillum is discussed in terms of the possible ways in which it is stimulated by movements of the spine and also in light of the dynamic behaviour of the receptor which is now very well described.