Johnston's organ

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.

In the fruit fly Drosophila melanogaster, the Johnston’s organ contains almost 480 sensory neurons. Distinct populations of neurons are activated differently by deflections of antennae caused by gravity or by vibrations caused by sound or air movement. This differential response allows the fly to distinguish between gravitational, mechanical, and acoustic stimuli.

The Johnston’s organ of fruit flies can be used to detect air vibrations caused by the wingbeat frequency or courtship song of a mate. One function of the Johnston’s organ is for detecting the wing beat frequency of a mate. Production of sound in air results in two energy components: the pressure component, which is changes in air pressure emanating away from the source of the sound; and the particle displacement component, which is the back and forth vibration of air particles oscillating in the direction of sound propagation. Particle displacement has greater energy loss than the pressure component, so the displacement component, called “near-field sound,” is detectable only within one wavelength of the source.

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