Ant colonies > Internal structure of ants > Chordotonal organs

Chordotonal organs

The Chordotonal Organs

These structures, which are present in a great many insects, even in the larval stages, are typically compact, spindle-shaped bundles of sensillae, each consisting of a chitin-secreting gland and a nerve cell. These cells are arranged in a series at an angle to the integument and are stretched, like a tendon, across a cavity between opposite points in the cuticle, or between a point in the cuticle and some internal organ. The gland cell secretes and retains within its cytoplasm a peculiar cone or rod, known as the scolopal body.

The chordotonal organs are supposed to be auditory in function, because they are most elaborately developed in the stridulating Orthoptera (crickets and katydids), and because their structure would seem to be adapted to respond­ing like the chords of a musical instrument to delicate vibrations.

In ants the development of these sense-organs is greatly inferior to that of the Orthoptera just mentioned, but they are nevertheless very easily seen when one knows exactly where to look for them. They were first detected by Lub­bock (1877) in the proximal portion of the fore tibiae of Lasius flavus, Myrmica rugi­nodis and Pheidole me­gacephala. He pointed out their resemblance to the subgenual chordo­tonal organs of Orthop­tera, discovered by von Siebold in 1844, but al­though he fancied he could discern some of their minute structure, his account and figure are very primitive.

The matter was re-investi­gated by Graber (1882), who found the organs in Solenopsis, Myrme­cina and Tetramoriun2, and showed that they occur not only in the fore but also in the middle and hind tibiae, that they contain scolopal bodies and are also in other respects typical chordotonai organs.

FIG. 33. Chordotonal organs in tibiae of Myrmica rubra worker. (Janet.) A, Longitudinal section of fore tibia ; B, cross-section of same ; C, cross-section of middle tibia; D, cross-section of hind tibia; a, chordotonal organ ; b, internal f ossa ; c, small; d, large trachea; e, nerve ; f, muscle ; g, septum ; h, scolopal bodies ; i, ganglion cells ; k, distal nuclei.

Janet (1904) has studied their structure with great care, and has not only added many details to those seen by his pre­decessors, but has also discovered a number of less conspicuous chordotonal organs in other parts of the ant's body. He finds a pair in the head at the base of the antennae (Fig. 27, acho), one in the prosternum, just under the prothoracic ganglion (cho), with which it is connected by short nerves, a similar pair in the metasternum and two pairs, one in the petiole and another in the postpetiole, which lie near the tracheal stigmata and are innervated by the ganglia of their respective segments. Eight pairs of chordotonal organs have, therefore, been seen in the ant's body, but it is not improbable, as Janet suggests, that others exist, for such minute and recondite objects are very easily overlooked even in well-prepared sections.

I find that the tibial organs (Fig. 33) are very easily seen in light-colored ants that have been simply mounted in alcohol, and that they are clearer in males than in workers or females. In clove oil, or Canada balsam, however, the structures are seen only with difficulty and after they have been located in alcoholic specimens.