Mechanosensory appendages and giant interneurons in the firebrat (Thermobia domestica, Thysanura): a prototype system for terrestrial predator evasion.

Aspects of the structure and function of the abdominal cerci and caudal filament sensory systems, and associated giant interneurons of the thysanuran insect Thermobia domestica, the firebrat, extend comparative studies of a widespread predator evasion system. All elements of the cercal system, which is well known from diverse orthopteroid insects, are present in the primitively wingless thysanuran. In addition, a median terminal sensory appendage, the caudal filament, projects to the same general regions of the terminal ganglion but shows limited overlap of synaptic regions with cercal input. A segmental series of giant interneurons appears to be homologous with those of the orthopteroid insects. The cercal system, which may have evolved with the first terrestrial hexapods, reaches its zenith in the orthopteroid insects, but was replaced in holometabolan insects by visual startle mechanisms with descending giant interneurons.