structure called a cilium, a hair-like projection built from a ring of nine paired microtubules. Unlike the cilia that beat back and forth on the surface of cells in the lungs or airway, this cilium is immobile and acts instead as a sensitive mechanical antenna: when it bends or stretches, channels in its membrane open and generate an electrical signal.

Most scolopidia contain between one and three sensory neurons. In the fruit fly Drosophila melanogaster, for example, the sensory organs of the larval body wall each contain a single neuron,