SOUND RECEPTION 103 



from electrophysiological records of activity in segmental nerves 

 known to carry fibres from other types of sensilla, particularly chordo- 

 tonal sensilla. Pumphrey and Rawdon-Smith (1936 c), studying the 

 locust, believed that the responses originated in trichoid sensilla. Later 

 Pumphrey (1940) suggested that the segmental chordotonal organs 

 were perhaps involved (see also Hughes, 1952). Working with a num- 

 ber of acridid species, Haskell (1956 a) inclined to the view that res- 

 ponses did indeed emanate from hairs because smearing the abdomen 

 with vaseHne abolished the response. In any event, the receptors 

 involved fired asynchronously at all stimulus frequencies and required 

 high stimulus intensities (+ 7 dynes/sq. cm.). Even after smearing with 

 vaseline, however, a residual sensitivity to sound occurred, and 

 Haskell (1956 a) suggested that in this instance the chordotonal organs 

 might be involved. Other sensilla on the abdomen of grasshoppers, 

 presumed to be sensitive to vibrations of the substrate, will be dis- 

 cussed later. 



Little is known about the functions of hair sensilla sensitive to 

 sound. It is unlikely that those in Orthoptera are concerned with 

 reception of noises produced by stridulation (Haskell, 1952 a). In the 

 cockroach stimulation of cereal hairs with a puff of air evokes the 

 'evasion response'. This kind of response is not well developed in 

 locusts. In Orthoptera in general mechanical stimulation of the cerci 

 seems to be of great importance during copulation, and perhaps this 

 is the principal role of cereal hairs. 



Hairs most highly developed for the reception of air-borne sounds 

 differ in performance from tympanic organs in several respects : they 

 respond over much lower frequency ranges; they respond synchro- 

 nously with stimulus frequency over certain ranges and thus do 

 exhibit a limited frequency discrimination; they fatigue fairly rapidly; 

 they tend to equilibrate. Like tympanic organs, however, they are 

 displacement receivers. 



THE JOHNSTON'S ORGAN OF CULICIDAE 



In the males of Culicidae and Chironomidae the Johnston's organ 

 attains an extraordinary complexity manifested by such an enormous 

 multiplication of sensilla that the pedicel is bulbous and a special 

 internal cuticular framework has developed to serve as points of 

 attachment for the many sensilla. This exceptional organ is demons- 

 trably able to be stimulated by air-borne sound waves as Johnston 

 (1855) and Child (1894) originally surmised. 

 When Mayer (1874) demonstrated that the hair (or fibrillae) 



H 



