78 THE PHYSIOLOGY OF INSECT SENSES 



origins, insects respond aggressively to sounds produced by prey (e.g., 

 the antlion to its prey) and defensively to sounds produced by pred- 

 ators (e.g., moths to the sounds of bats). Insects can also detect a 

 certain amount of the white noise that fills the physical environment. 

 So far as is known, only two kinds of mechanoreceptive sensilla 

 are sensitive to sound, sensilla trichodea and chordotonal sensilla, 

 although there is nothing about the structure of campaniform sensilla 

 or stretch receptors that precludes the possibility of their also respond- 

 ing to sound. Indeed, stretch receptors accurately signal phasic 

 stimulation up to 5 c/s (Lowenstein and Finlayson, 1960). Hairs 

 sensitive to sound do not differ fundamentally from those concerned 

 with tactile and proprioceptive functions (hair plates on the legs of 

 Periplaneta respond to loudspeaker sounds [Pringle, 1938c] ) nor do 

 the chordotonal sound receptors differ appreciably in structure from 

 proprioceptive chordotonal sensilla. The more highly specialized 

 chordotonal receptors, however, are grouped together and associated 

 with elaborate accessory structures enhancing their sensitivity to 

 sound. These organs include: tympanic organs, Johnston's organ in 

 culicine mosquitoes, and subgenual organs. 



TYMPANIC ORGANS 

 Morphology 



Few external structural details escaped the eyes of nineteenth-century 

 taxonomists, so it is not surprising that the occurrence of tympana was 

 noticed in a number of insects. In some cases the structures were 

 examined only with a view of their taxonomic usefulness ; in others, 

 guesses were made regarding their function. Those who guessed 

 correctly believed the organ to be an instrument of sound production. 

 Miiller (1826), studying the paired tympana on the first abdominal 

 segments of Acrididae, decided that they were organs of hearing. 

 Tympanic organs were then discovered in the tibiae of the prothoracic 

 legs of Tettigoniidae and GryUidae (Siebold, 1844) and later in moths 

 and butterflies. Among the Lepidoptera tympanic organs occur in the 

 abdomen of adults in the super-families Geometroidea and Pyraloidea 

 and in the metathorax of Noctuidea (Swinton, 1877; Jordan, 1905; 

 Deegener, 1909; Eggers, 1911, 1919; von Kennel, 1912). Abdominal 

 tympanic organs occur in the Cicadidae (Vogel, 1923 a) and in Corixa 

 andafewother Hemiptera (Graber, 1882 a, 1822 b; Hagemann, 1910; 

 Wefelscheid, 1912; Schaller, 1951), although in some of these forms 

 the variation from an obvious tympanic organ makes those sensilla 

 associated with tracheae look suspiciously like the proprioceptive 



