SOUND RECEPTION 105 



are not innervated in mosquitoes. Early workers postulated that the 

 vibrating fibrillae acted as mechanical amplifiers which set the flagellar 

 shaft in vibration. The movement of the shaft with respect to the 

 pedicel was presumed to be detected by Johnston's organ. In an 

 extensive series of experiments with Aedes aegypti Roth (1948) ob- 

 tained results that confirmed this hypothesis. The mating response of 

 males to sound is abolished by completely removing the flagellum, 

 joining the flagellum rigidly to the pedicel with shellac, or weighting 

 the tips of the antennae with large beads of shellac. In all cases res- 

 ponsiveness returns when the shellac is removed. When the fibrillae 

 are removed from the antennae greater intensities of sound are re- 

 quired in order to elicit responses. 



Although no electrophysiological studies of the Johnston's organ 

 of mosquitoes have yet been reported, these experiments of Roth, 

 taken in conjunction with Burkhardt's electrophysiological analysis of 

 the Johnston's organ of Calliphora, establish beyond reasonable doubt 

 that this organ in Culicidae is indeed adapted to respond to air-borne 

 sound waves. 



PERCEPTION OF VIBRATIONS IN SOLIDS 



It had been suspected for many decades that insects were sensitive to 

 vibrations in solids because they gave clear behavioural responses 

 when the substrate upon which they were standing or walking was 

 subjected to this form of motion. No very meaningful experiments 

 were performed, however, prior to the introduction of electro- 

 physiological methods of analysis. By recording from leg and seg- 

 mental nerves it has now been possible to demonstrate that there are 

 rather specific responses by some insects to vibration. A few clues as 

 to the identity of the sensilla involved have been obtained. 



The two types of sensilla suited to the task are trichoid sensilla and 

 chordotonal sensilla. It is to be expected that those parts of the body 

 immediately in contact with the substrate, that is, the legs and abdo- 

 men, would be especially equipped with the requisite organs ; however, 

 aside from a few experiments showing that in Locusta there are hairs on 

 the sternites which mediate responses to vibrations of the substrate 

 (Haskell, 1956 a), there is no extensive work on areas of the body other 

 than the legs. 



The legs certainly possess elaborate sensory equipment. In addition 

 to trichoid sensilla they invariably possess numbers of chordotonal 

 sensilla (Eggers, 1928; Debaisieux, 1935, 1938). Generally the legs 

 contain chordotonal organs at four locations ; the femur, the proximal 



