Ant colonies > Internal structure of ants > Johnstonian organ

Johnstonian organ


The Johnstonian Organ

This peculiar structure, first described by Johnston in 1855, and since carefully investigated by Child (1894.), is very similar to the chordotonal organs. It is found only in the second antennal joint of insects and seems to reach its highest development in certain Orthorrhaphotts Diptera (gnats). Child found it also in the Hymenoptera (Forniica, Vespa, Bombus) and Berlese has published some good figures of it in the hornet.

I find that it is decidedly larger in male than in worker and female ants, especially in those genera like Pheidole and Solenopsis, in which the males have an unusually swollen or globular second antennal (first funicular) joint. Janet seems to have overlooked the Johnstonian organs in the Myrmica, which he has studied so exhaustively.

In section the organ is seen to consist of a variable but considerable number of sensillae differing but slightly from those of the chordotonal organs, and also containing scolopal bodies. These sensillae are stretched more or less parallel with the long axis of the funiculus, through the cavity of the second joint. Their distal or hypodermal ends are attached to the articular membrane between the second and third joints, while their proximal ends are innervated by a portion of the antennal nerve. They form a compact cylinder enclos­ing the remainder of the nerve which passes on into the more distal antennal joints. Both Johnston and Child are inclined to regard the sense-organs under discussion as auditory, although the latter believes that their more primitive function is tactile. Tthe auditory and tactile sensations of insects are not sharply distinguishable.