FUNCTIONS OF CHORDOTONAL ORGANS 145 



thorax or the base of the abdomen in different families, while 

 they are totally wanting in many others. The tympanal organs 

 (Fig. 61) eonsist of a pair of vesicles— one on each side of the 

 body. Each vesicle is lined by tracheal epithelium and is lodged 

 in a cavity formed by the invagination of the integument of its 

 segment. It communicates with the exterior by means of an 

 opening on the side of the body, and this aperture is guarded by 

 an integumentary flap or shield. Just within the opening there 

 is found the glistening membrane of the true tympanum, and 

 closely associated with it there is a group of chordotonal sensillae 

 connected with a special tympanic nerve derived from the meta- 

 thoracic ganglion. Lying more deeply within the tympanic vesicle 

 there is, in many species, an accessory tympanum which is not, 

 however, associated with any sensillae. 



Tympanal organs are also described by Hagemann (1910) in 

 Coricca and its allies. They are located on either side of the 

 metathorax and in close relation with the second pair of spiracles. 



The Functions of the Chordotonal Organs. No definite proof 

 exists with regard to the functions of the simple ligamentous 

 chordotonal organs, and the subject is highly problematical. 

 Snodgrass (1926) lays stress upon Eggers' view (1923) that they 

 function as " rhythmometers." The last-mentioned authority 

 claims that most of the movements of insects result in rhythms. 

 In the case of the wings, for example, the action of these organs 

 sets the whole body into rapid vibration. He points out that the 

 movements of the antennas and legs tend to become rhythmic, 

 and the same applies to the pulsations of the dorsal vessel and the 

 movements of respiration. The body-movements of locomotion 

 in certain aquatic insects, the motion of the gills and of other 

 parts likewise tend to become rhythmic. Eggers suggests, there- 

 fore, that the chordotonal sensillge are organs for the purpose 

 of co-ordinating and regulating muscular movements of this 

 character. Notwithstanding this hypothesis, we must conclude 

 that similar kinds of sensillae in association with a tympanum, or 

 other cuticular receptive organ or membrane, are capable of 

 perceiving vibrations in the surrounding medium wherein the 

 insects live. As will be mentioned later, evidence in favour of 



