Johnston's organ is a collection of sensory cells found in the pedicel (the second segment) of the antennae in the class Insecta.[2] 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.[3][4] The number of scolopidia varies between species. In homopterans, the Johnston's organs contain 25–79 scolopidia.[5] 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 (1822-1891)[6] father of the physician and Assyriologist Christopher Johnston.