VII] 



THE SENSE ORGANS 



155 



Chordotonal Organs (fig. 70). 



The peculiar organs known as chordotonal organs (fig. 70), and 

 supposed to be organs of hearing, occur on all the legs of Dragonfly 

 larvae, and probably also in the imago. The larva of Aeschna was 

 found by Zawarsin [204] to have three sets of these organs on each 

 leg, two on the trochanter and one on the tibia close to the knee- 

 joint. Each set consists of a number of minute organs called 

 otaria. Each otarium (fig. 70 B) is a small slit covered by smooth 



ot 



Fig. 69. Hypopharynx of Aeschna Fig. 70. A. Chordotonal organ 



brevistyla Ramb. (x 13). Original, in knee-joint, of larva of Aesch- 



chitin preparation. na ( x 40). B. Section through 



a single otarium (x 1000). 

 fe femur; nv nerve- endings; 

 ot otaria; pn pedal nerve; 

 th sensillae ; ti tibia. Methy- 

 len blue preparation, after 

 Zawarsin. 



chitin, under which is found a tiny rod or style. This rod is 

 hollow, and carries a bundle of excessively fine nerve-fibrils. The 

 total length of such an organ is only 0-01 mm. 



Chordotonal organs differ from tympanulae in that the auditory 

 vibration is received on a rod instead of on a stretched membrane 

 or tympanum. Possibly such an organ only registers vibrations of 

 much lower frequency than those which produce sounds. Their 

 presence on the legs of the larva suggests that they may respond 

 to currents set in motion by other animals in the water, and thus 



