MECHANORECEPTION 59 



Johnston's Organ 



In 1855 Johnston discovered in the antennae of cuhcine mosquitoes 

 the compound chordotonal organ that now bears his name. Its 

 structure was not investigated in any detail until nearly forty years 

 later (Child, 1894). Since then it has been found to be almost a con- 

 stant feature of insect antennae, and much intensive study has been 

 given to its structure (e.g., Lehr, 1914; Eggers, 1923, 1924, 1928; 

 Debauche, 1936; Richard, 1956, 1957; Urvoy, 1958). In its classical 

 form it is basically a hollow cylinder or truncated sphere in the second 

 antennal segment (pedicel), where it stretches from the base of the 

 segment distally to the synovial membrane of segment three. The 

 sensilla comprising the organ vary in number from tens to hundreds in 

 the different species. Although the sensilla differ from those in other 

 chordotonal organs in that they lack caps or apical bodies, both 

 Eggers (1923, 1928) and Snodgrass (1926, 1935) agreed that they are 

 chordotonal sensilla. 



Each sensillum consists of a bipolar neuron, a scolopoid sheath, a 

 cap cell, and an enveloping cell (Eggers, 1928). As seen with the hght 

 microscope, the scolopoid sheath, which is ribbed like most, becomes 

 attenuated distally into a bundle of thin fibres. The fibres from a 

 number of sensilla are grouped together for common insertion into a 

 pore, cleft, or some other attachment on the synovial membrane. In 

 the simpler Johnston's organs, where the total number of sensilla is 

 small, each bundle with a common point of attachment is clearly 

 separate from every other bundle. This arrangement suggests that the 

 Johnston's organ is really a compound structure composed of many 

 chordotonal organs. The closed cylinders of the complex Johnston's 

 organs are a consequence of the extraordinarily large number of 

 sensilla that must terminate in the limited space of the pedicle 

 (Eggers, 1928). 



The organ is enormously developed in Culicidae and Chironomidae. 

 So many sensilla are crowded into the pedicel that the neurons lie in 

 several layers, and the cap and enveloping cells are so forced out of 

 position that they were originally described as supporting rods (Child, 

 1894). Furthermore, there is a corresponding modification of the 

 synovial membrane to accommodate the large number of sensilla. The 

 base of the third segment in male culicine mosquitos, for example, 

 flares to form a reinforced circular plate from which rigid spines 

 extend radially far into the pedicel. The sensilla are attached to these 

 spinous processes. In these and other species where the organ is a 



