Johnston's organ is a collection of sensory cells found in the pedicel (the second segment) of the antennae

Antenna (biology)

Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....

 in the Class

Class (biology)

In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 Insecta. Johnston's organ detects motion

Motion perception

Motion perception is the process of inferring the speed and direction of elements in a scene based on visual, vestibular and proprioceptive inputs...

 in the flagellum

Flagellum

A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...

 (third and typically final antennal segment). It consists of over 200 scolopidia

Scolopidia

A scolopidia is the most fundamental unit of a mechanoreceptor organs in insects. It is a composition of three cells, a scolopale cap cell which caps the scolopale cell, and a bipolar sensory nerve cell...

 arrayed in a bowl shape, each of which contains a mechanosensory chordotonal neuron. The presence of Johnston's organ is a defining characteristic which separates the Class Insecta from the other hexapods

Hexapoda

The subphylum Hexapoda constitutes the largest grouping of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura . The Collembola are very abundant in terrestrial environments...

 belonging to the group Entognatha

Entognatha

The Entognatha is a class of ametabolous arthropods, which, together with insects, makes up the subphylum Hexapoda. Their mouthparts are entognathous, meaning they are retracted within the head. Entognatha are apterous, meaning they lack wings. The class contains three orders: Collembola , Diplura...

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Johnston's organ can be seen in Drosophila

Drosophila

Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...

. It consists of a sail-like flagellum which rotates in a structure called the funiculus. Air movement deforms the cuticle at the joint between segments 2 and 3 where the sensory units of Johnston’s organ attach. Johnston's organ reacts between 150 and 500 Hz. One function is for detecting the wing beat frequency of a mate. The third segment can also be deformed by gravity irrespective of head orientation and this enables it to sense gravity.

Johnston's organ was named after the physician Christopher Johnston, father of the physician and Assyriologist Christopher Johnston

Christopher Johnston

Christopher Johnston, M.D., Ph.D. was an American physician and Assyriologist, a scholar of ancient Mesopotamia.-Personal life:...

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Johnston's organ can also sense wind.