Johnston's organ is a collection of sensory cells found in the
pedicel (the second segment) of the
antennae in the
Class Insecta. Johnston's organ
detects motion in the
flagellum (third and typically final antennal segment). It consists of
scolopidia arrayed in a bowl shape, each of which contains a mechanosensory chordotonal neuron. The number of scolopidia varies between species. In
homopterans, the Johnston’s organs contain 25 - 79 scolopidia. The presence of Johnston's organ is a defining characteristic which separates the Class
Insecta from the other
hexapods belonging to the group
Entognatha. Johnston's organ was named after the physician Christopher Johnston, father of the physician and Assyriologist
Christopher Johnston.