Otoacoustic emissions in bushcricket ears: general characteristics and the influence of the neuroactive insecticide pymetrozine

The tympanal organ of the bushcricket Mecopoda elongata emits pronounced distortion-product otoacoustic emissions (DPOAEs). Their characteristics are comparable to those measured in other insects, such as locusts and moths, with the 2f1–f2 emission being the most prominent one. Yet the site of their generation is still unclear. The spatial separation between the sound receiving spiracle and the hearing organ in this species allows manipulations of the sensory cells without interfering with the acoustical measurements. We tried to interfere with the DPOAE generation by pharmacologically influencing the tympanal organ using the insecticide pymetrozine. The compound appears to act selectively on scolopidia, i.e., the mechanosensor type characteristically constituting tympanal organs. Pymetrozine solutions were applied as closely as possible to the scolopidia via a cuticle opening in the tibia, distally to the organ. Applications of pymetrozine at concentrations between 10⁻³ and 10⁻⁷ M to the tympanal organ led to a pronounced and irreversible decrease of the DPOAE amplitudes.     

The midline metathoracic ear of the praying mantis,Mantis religiosa

Summary The praying mantis, Mantis religiosa, is unique in possessing a single, tympanal auditory organ located in the ventral midline of its body between the metathoracic coxae. The ear is in a deep groove and consists of two tympana facing each other and backed by large air sacs. Neural transduction takes place in a structure at the anterior end of the groove. This tympanal organ contains 32 chordotonal sensilla organized into three groups, two of which are 180° out of line with the one attaching directly to the tympanum. Innervation is provided by Nerve root 7 from the metathoracic ganglion. Cobalt backfills show that the auditory neuropile is a series of finger-like projections terminating ipsilaterally near the midline, primarily near DC III and SMC. The auditory neuropile thus differs from the pattern common to all other insects previously studied.     

Postembryonic development of the complex tibial organ in the foreleg of the bushcricket Ephippiger ephippiger (Orthoptera,Tettigoniidae)

Summary The postembryonic development of the morphology and anatomy of the complex tibial organ in the foreleg of the bushcricket Ephippiger ephippiger is described. All the receptor cells are present in the subgenual organ, the intermediate organ and the crista acustica in the 1st larval instar. Generally, even in the 1st instar, the arrangement of the scolopidia in the three organs resembles the adult structure. The acoustic trachea, the tympana, the tympanal covers and the acoustic spiracle develop step by step in subsequent instars. The acoustic trachea resembles the adult structure for the first time in the 4th instar, although its volume is still small. The auditory threshold curves recorded from the tympanal nerve in instars 4, 5 and 6 show the same frequency maxima as those in the adult. The overall sensitivity significantly increases after the final moult. The dimensions of structures that lie within the crista acustica and that are probably involved in stimulus transduction and in frequency tuning have been analysed. The dorsal wall of the anterior trachea, the tectorial membrane and the cap cells have similar dimensions, especially in the last three instars and in adults.     

Functional morphology and development of tibial organs in the legs I,II and III of the bushcricketEphippiger ephippiger (Insecta,Ensifera)

Summary The anatomy of the complex tibial organs in the pro-, meso- and metathoracic legs of adults and larvae of the bushcricketEphippiger ephippiger is described comparatively. The subgenual organ and the intermediate organ are differentiated in the same way in legs I, II and III; the anatomy of the crista acustica and the tracheal morphology are significantly different. The final number of scolopidia in the tibial organ of each leg is present at the time of hatching. In the subgenual organ, the number of scolopidia is the same in all legs; in the intermediate organ, and especially in the crista acustica, the number of scolopidia decreases from leg I to legs II and III. In the first larval instar, the morphology of the tibia, the course of the trachea and the anatomy of accessory structures are developed in the same way in each leg. The specific differentiations forming the auditory receptor organ in leg I, such as the acoustic trachea, the tympana and tympanal cavities, develop step by step in subsequent instars. The auditory threshold recorded from the tympanal nerve in the prothoracic leg of adults is remarkably lower than in the meso- and metathoracic legs. Morphometrical analyses of structures that are suggested to play a role in stimulus transduction on scolopidia of the crista acustica reveal significant differences in the three legs.     

Precursor structures and evolution of tympanal organs in Lepidoptera (Insecta, Pterygota)

 The patterns of scolopal organs and their innervation were studied by the methylene blue method in larvae, pupae and adults of an Yponomeuta species (Yponomeutidae) and of tympanate adult representatives of the Noctuoidea, Geometridae, Drepanidae and Pyraloidea. The studies were focused mainly on the mesothorax, the metathorax and some anterior abdominal segments. In the abdominal tympanal organs of Geometridae, Drepanidae and Pyraloidea, the auditory scolopidia are homologous with the lateral scolopal organs of the first abdominal segment; however, the hearing organs as such evolved independently in the three taxa. The studies confirm that the tympanal organ in the Noctuoidea is derived from the caudal dorsolateral region of the metathorax including its dorsal scolopal organ and the B-cell. The adult scolopal organs are present already in the larvae and are maintained nearly unchanged during metamorphosis to the adult. Only in the Noctuoidea are the three sensory cells of the larval scolopal organs, which become part of the tympanal organs, reduced to one (in Notodontidae) or two (in other Noctuoidea) during metamorphosis. A hypothetical scenario of the evolution of the tympanal organs is outlined. Accepted: 12 March 1997     

Mechanics of the transduction of sound in the tympanal organ of adults and larvae of locusts

Summary The mechanical transmission of sound in the tympanal organ of adults and 5th instar larvae ofLocusta migratoria andSchistocerca gregaria has been investigated by means of stroboscopic measurements within a frequency range from 1–20 kHz.Frequency dependent spatial distributions of amplitudes and phases of oscillation on the tympanal membrane and the Müller's organ could be demonstrated. Cuticular structures on the membrane may act as a lever arm (e.g. elevated process) and cause a transformation of the (unidimensional) membrane motion into components of displacements in the Müller's organ perpendicular, as well as even parallel, to the membrane.Sites of maximum relative displacements at distinct frequencies are found to be correlated to the course of the dendrites of the acoustic receptor cells. Differences in morphology of the tympanal organ between the two species as well as between adults and larvae always correspond to differences in the mechanical properties (resonances etc.). Consequently, differences or changes in the neurophysiological response characteristics of the different receptor cells have been found.Based upon these findings a correlation between the anatomical and physiological classification of the receptor cell groups is presented.Abbreviations T1, T2, T3, T6, T7 reference points on the tympanal membrane - M1, M4 reference points on the ganglion of the Müller's organ - K1, K2 reference points on the elevated process     

Mechanical response of the tympanal membranes of the tree cricket <Emphasis Type="Italic">Oecanthus henryi</Emphasis>

Crickets have two tympanal membranes on the tibiae of each foreleg. Among several field cricket species of the genus Gryllus (Gryllinae), the posterior tympanal membrane (PTM) is significantly larger than the anterior membrane (ATM). Laser Doppler vibrometric measurements have shown that the smaller ATM does not respond as much as the PTM to sound. Hence the PTM has been suggested to be the principal tympanal acoustic input to the auditory organ. In tree crickets (Oecanthinae), the ATM is slightly larger than the PTM. Both membranes are structurally complex, presenting a series of transverse folds on their surface, which are more pronounced on the ATM than on the PTM. The mechanical response of both membranes to acoustic stimulation was investigated using microscanning laser Doppler vibrometry. Only a small portion of the membrane surface deflects in response to sound. Both membranes exhibit similar frequency responses, and move out of phase with each other, producing compressions and rarefactions of the tracheal volume backing the tympanum. Therefore, unlike field crickets, tree crickets may have four instead of two functional tympanal membranes. This is interesting in the context of the outstanding question of the role of spiracular inputs in the auditory system of tree crickets.     

Evolution of the metathoracic tympanal ear and its mesothoracic homologue in the Macrolepidoptera (Insecta)

Two independent methods of comparison, serial homology and phylogenetic character mapping, are employed to investigate the evolutionary origin of the noctuoid moth (Noctuoidea) ear sensory organ. First, neurobiotin and Janus green B staining techniques are used to describe a novel mesothoracic chordotonal organ in the hawkmoth, Manduca sexta, which is shown to be serially homologous to the noctuoid metathoracic tympanal organ. This chordotonal organ comprises a proximal scolopidial region with three bipolar sensory cells, and a long flexible strand (composed of attachment cells) that connects peripherally to an unspecialized membrane ventral to the axillary cord of the fore-wing. Homology to the tympanal chordotonal organ in the Noctuoidea is proposed from anatomical comparisons of the meso- and metathoracic nerve branches and their corresponding peripheral attachment sites. Second, the general structure (noting sensory cell numbers, gross anatomy, and location of peripheral attachment sites) of both meso- and metathoracic organs is surveyed in 23 species representing seven superfamilies of the Lepidoptera. The structure of the wing-hinge chordotonal organ in both thoracic segments was found to be remarkably conserved in all superfamilies of the Macrolepidoptera examined except the Noctuoidea, where fewer than three cells occur in the metathoracic ear (one cell in representatives of the Notodontidae and two cells in those of other families examined), and at the mesothoracic wing-hinge (two cells) in the Notodontidae only. By mapping cell numbers onto current phylogenies of the Macrolepidoptera, we demonstrate that the three-celled wing-hinge chordotonal organ, believed to be a wing proprioceptor, represents the plesiomorphic state from which the tympanal organ in the Noctuoidea evolved. This ’trend toward simplicity’ in the noctuoid ear contrasts an apparent ’trend toward complexity’ in several other insect hearing organs where atympanate homologues have been studied. The advantages to having fewer rather than more cells in the moth ear, which functions primarily to detect the echolocation calls of bats, is discussed. Accepted: 18 June 1999     

The accessory organ,a scolopidial sensory organ,in the cave cricket Troglophilus neglectus (Orthoptera: Ensifera: Rhaphidophoridae)

Mechanoreceptor organs occur in great diversity in insect legs. This study investigates sensory organs in the leg of atympanate cave crickets (Troglophilus neglectus KRAUSS, 1879) by neuronal tracing. Previously, the subgenual and the intermediate organs were recognised in the subgenual organ complex, lacking the tympanal membranes present for example in the tibial hearing organs of Gryllidae and Tettigoniidae. We document the presence of the accessory organ in T. neglectus. This scolopidial organ is located in the posterior tibia close to the subgenual organ and can be identified by position, innervation and orientation of the dendrites of sensory neurons. The main motor nerve in the leg innervates a part of the subgenual organ and the accessory organ. The dendrites of sensory neurons in the accessory organ are characteristically bent in proximo‐dorsal direction, while the subgenual organ dendrites run distally along the longitudinal axis of the leg. The accessory organ contains 6–10 scolopidial sensilla, and no differences in neuroanatomy occur between the three thoracic leg pairs. Hence, the subgenual organ complex in cave crickets is more complex than previously known. The wider taxonomic distribution of the accessory scolopidial organ among orthopteroid insects is inconsistent, indicating its repeated losses or convergent evolution.     

Structure of the green lacewing tympanal organ (Chrysopa carnea,neuroptera)

Small swellings near the base of the radial vein in each fore wing of the green lacewing, Chrysopa carnea, resemble typical insect tympanal organs, but some important differences are apparent. The swellings are bounded dorsally and laterally by thick cuticle and ventrally by thin, membranous cuticle. The ventral membrane is formed by a single, thin sheet of exocuticle with flattened hypodermis internally, but lacks the tracheal component that forms part of the tympanum in the typical insect tympanal organ. The portion of the membrane beneath each swelling is rippled while proximally it is smooth. In contrast to typical insect tympanal organs, the swellings in C. carnea are largely fluid-filled since an unexpanded trachea runs through each organ. A distal and a proximal chordotonal organ composed of typical chordotonal sensory units are associated with each swelling. The distal organ contains from five to seven units while the proximal organ is composed of from 18 to 20 units. Each sensory unit is composed of three readily identifiable cells. Distally, an attachment cell unites with the membrane and is contiguous with the scolopale cell, which surrounds the dendrite of the bipolar neuron. On the basis of the morphological evidence, one would not expect these swellings to function as sound receptors. However, the results of physiological and behavioral experiments, presented elsewhere, show that these organs are receptors for ultrasound.     

The tympanal hearing organ of the parasitoid fly Ormia ochracea (Diptera, Tachinidae, Ormiini)

Tympanate hearing has evolved in at least 6 different orders of insects, but had not been reported until recently in the Diptera. This study presents a newly discovered tympanal hearing organ, in the parasitoid tachinid fly, Ormia ochracea. The hearing organ is described in terms of external and internal morphology, cellular organization of the sensory organ and preliminary neuroanatomy of the primary auditory afferents. The ear is located on the frontal face of the prothorax, directly behind the head capsule. Conspicuously visible are a pair of thin cuticular membranes specialized for audition, the prosternal tympanal membranes. Directly attached to these membranes, within the enlarged prosternal chamber, are a pair of auditory sensory organs, the bulbae acusticae. These sensory organs are unique among all auditory organs known so far because both are contained within an unpartitioned acoustic chamber. The prosternal chamber is connected to the outside by a pair of tracheae. The cellular anatomy of the fly's scolopophorous organ was investigated by light and electron microscopy. The bulba acustica is a typical chordotonal organ and it contains approximately 70 receptor cells. It is similar to other insect sensory organs associated with tympanal ears. The similarity of the cellular organization and tympanal morphology of the ormiine ear to the ears of other tympanate insects suggests that there are potent constraints in the design features of tympanal hearing organs, which must function to detect high frequency auditory signals over long distances. Each sensory organ is innervated by a branch of the frontal nerve of the fused thoracic ganglia. The primary auditory afferents project to each of the pro-, meso-, and metathoracic neuropils. The fly's hearing organ is sexually dimorphic, whereby the tympanal membranes are larger in females and the spiracles larger in males. The dimorphism presumably reflects differences in the acoustic behavior in the two sexes.     

The fine structure of the cockroach subgenual organ

This paper describes the fine structure of the cockroach subgenual organ, a complex ciliated mechanoreceptor that detects vibrations in the substrate upon which the animal stands. Located beneath the knee in each walking leg, the cockroach subgenual organ is a thin, fan-shaped flap of tissue slung across the dorsal blood space of the tibia at right angles to the leg's long axis. It is innervated by approximately 50 chordotonal sensilla. The fine structure of the chordotonal sensilla is is described in detail ; possible transducer sites are discussed.     

Complex tibial organs in fore-, mid-, and hindlegs of the bushcricket Gampsocleis gratiosa (Tettigoniidae): Comparison of morphology of the organs

The structure of the complex tibial organs in the fore-, mid-, and hindlegs of the East Asian bushcricket Gampsocleis gratiosa (Tettigoniidae, Decticinae) is described comparatively. In each leg the tibial organs consist of three scolopale organs: the subgenual organ, the intermediate organ, and the crista acoustica. Only in the forelegs are the tibial organs differentiated as tympanal organs, and sound transmitting structures (acoustic trachea, tympana, and tympanal covers) are present. The morphology of the tracheae in the mid- and hindlegs is significantly different from that found in the forelegs. The number of scolopidia in the subgenual organ is highest in the midleg and lowest in the foreleg; in the intermediate organ the number is also highest in the midleg, and the fore- and hindleg contain 40% fewer scolopidia. In the crista acoustica, the number of scolopidia decreases from, the fore- to the mid- and hindlegs. The morphology and the dimensions of the scolopidia and the attachment structures within the crista acoustica of the mid- and hindlegs differ strongly from those in the foreleg. The results indicate that, in addition to the presence of a sound transmitting system, the specific differentiations within the crista acoustica are important for the high auditory sensitivity of the tibial organs in the forelegs. © 1994 Wiley-Liss, Inc.     

Development of leg chordotonal sensory organs in normal and heat shocked embryos of the cricket Teleogryllus commodus (Walker)

This paper describes the embryonic development of some parts of the sensory peripheral nervous system in the leg anlagen of the cricket Teleogryllus commodus in normal and heat shocked embryos. The first peripheral neurons appear at the 30% stage of embryogenesis. These tibial pioneer neurons grow on a stereotyped path to the central nervous system and form a nerve which is joined by the growth cones of axons that arise later, including those from the femoral chordotonal organ, subgenual organ and tympanal organ. The development of these organs is described with respect to the increase in number of sensory receptor cells and the shape and position of the organs. At the 100% stage of embryogenesis all three organs have completed their development in terms of the number of sense cells and have achieved an adult shape. To study the function of the tibial pioneer neurons during embryogenesis a heat shock was used to prevent their development. Absence of these neurons has no effect on the development of other neurons and organs proximal to them. However, the development of distal neurons and organs guided by them is impaired. The tibial pioneer neurons grow across the segmental boundary between femur and tibia early in development, and the path they form seems to be essential for establishing the correct connections of the distal sense organs with the central nervous system.     

The tympanal hearing organ of the parasitoid fly Ormia ochracea (Diptera,Tachinidae, Ormiini)

Tympanate hearing has evolved in at least 6 different orders of insects, but had not been reported until recently in the Diptera. This study presents a newly discovered tympanal hearing organ, in the parasitoid tachinid fly, Ormia ochracea. The hearing organ is described in terms of external and internal morphology, cellular organization of the sensory organ and preliminary neuroanatomy of the primary auditory afferents. The ear is located on the frontal face of the prothorax, directly behind the head capsule. Conspicuously visible are a pair of thin cuticular membranes specialized for audition, the prosternal tympanal membranes. Directly attached to these membranes, within the enlarged prosternal chamber, are a pair of auditory sensory organs, the bulbae acusticae. These sensory organs are unique among all auditory organs known so far because both are contained within an unpartitioned acoustic chamber. The prosternal chamber is connected to the outside by a pair of tracheae. The cellular anatomy of the fly's scolopophorous organ was investigated by light and electron microscopy. The bulba acustica is a typical chordotonal organ and it contains approximately 70 receptor cells. It is similar to other insect sensory organs associated with tympanal ears.The similarity of the cellular organization and tympanal morphology of the ormiine ear to the ears of other tympanate insects suggests that there are potent constraints in the design features of tympanal hearing organs, which must function to detect high frequency auditory signals over long distances. Each sensory organ is innervated by a branch of the frontal nerve of the fused thoracic ganglia. The primary auditory afferents project to each of the pro-, meso-, and metathoracic neuropils. The fly's hearing organ is sexually dimorphic, whereby the tympanal membranes are larger in females and the spiracles larger in males. The dimorphism presumably reflects differences in the acoustic behavior in the two sexes.     

Functional morphology of bushcricket ears: comparison between two species belonging to the Phaneropterinae and Decticinae (Insecta,Ensifera)

Summary The morphology of the complex tibial organs in the forelegs of two bushcricket species belonging to the Phaneropterinae and Decticinae (Tettigoniidae) is described comparatively. In both species the tibial organs are made up of the subgenual organ, the intermediate organ and the crista acustica; the latter are parts of the tympanal organs and serve as auditory receptors. The very thin tympana in the forelegs ofPholidoptera griseoaptera (Decticinae) are protected by tympanal covers whereas inLeptophyes punctatissima (Phaneropterinae) the tympana are thicker and fully exposed. The overall auditory sensitivity ofL. punctatissima is lower and the sensitivity maximum of the hearing threshold lies at higher frequencies compared toP. griseoaptera. The number of scolopidia in the three scolopale organs and the dimensions of parts of the sound conducting system differs in the two species. In the crista acustica ofL. punctatissima a higher number of scolopidia is distributed in a smaller range than inP. griseoaptera; the scolopidia are especially concentrated in the distal part. Morphometrical analyses indicate that the dimensions of the spiracles, the acoustic trachea and the tympana determine the overall auditory sensitivity and that the arrangement of the scolopidia and the dimensions of structures in the crista acustica affect the frequency tuning of the hearing threshold.     

Comparison of auditory sense organs in parasitoid Tachinidae (Diptera) hosted by Tettigoniidae (Orthoptera) and homologous structures in a non-hearing Phoridae (Diptera)

The dipteran parasitoids Therobia leonidei and Homotrixa alleni (Tachinidae) use acoustic cues to locate their calling tettigoniid (Ensifera, Orthoptera) hosts. The sexually dimorphic tympanal organs of both fly species are located at the prosternum. For comparison a homologous chordotonal organ in the non-hearing fly Phormia regina, Meigen (Phoridae) is also described. The scolopidial sense organs of the ears have approximately 180 sensory cells in Th. leonidei and 250 cells in H. alleni. Interspecific analysis indicates that the cell number and arrangement might be genus specific in Tachinidae. The mononematic scolopidia, each with one sensory cell, are of different sizes and insert at the tympanal membrane. Large scolopidial units (diameter of sensory cells up to 50 μm) extend longitudinally from the centre of the sensory organ towards the ligament, whereas small units (sensory cell diameter up to 10 μm) are arranged sequentially within the sensory organ. This arrangement is discussed to be a possible basis for frequency discrimination. The ultrastructure of the scolopidia is similar in the hearing and non-hearing flies. In both groups, the majority of scolopales has a diameter from 2 to 2.9 μm, although hearing species have additionally wider scolopales. The homologous chordotonal organ of Ph. regina consists of approximately 55 sensory cells of uniform direction. The data are discussed in comparison to the ears of other Diptera.     

Physiology and tonotopic organization of auditory receptors in the cricketGryllus bimaculatus DeGeer

Summary Physiological recordings were obtained from identified receptors in the tympanal organ ofGryllus bimaculatus. By immersing the prothoracic leg in Ringer solution and removing the anterior tympanic membrane the auditory receptors were exposed without significantly altering the frequency response of the auditory organ (Fig. 1). Each receptor was tuned to a specific sound frequency. For sound frequencies below this characteristic frequency the roll-off in sensitivity decreased from 20–30 dB/octave to 10–15 dB/octave as the characteristic frequency of receptors increased from 3–11 kHz (Fig. 4A). For each individual receptor the slope, dynamic range and maximum spike response were similar for different sound frequencies (Fig. 9A). The receptors were tonotopically organized with the characteristic frequency of the receptors increasing from the proximal to the distal end of the array (Figs. 5, 6). Several receptors had characteristic frequencies of 5 kHz. These receptors were divided into two groups on the basis of their maximum spike response produced in response to pure tones of increasing intensity (Fig. 7). Independent of the tuning of the receptor no two-tone inhibition was observed in the periphery, thus confirming that such interactions are a property of central integration.     

Postembryonic development of the auditory system of the cicada Okanagana rimosa (Say) (Homoptera: Auchenorrhyncha: Cicadidae)

Cicadas (Homoptera: Auchenorrhyncha: Cicadidae) use acoustic signalling for mate attraction and perceive auditory signals by a tympanal organ in the second abdominal segment. The main structural features of the ear are the tympanum, the sensory organ consisting of numerous scolopidial cells, and the cuticular link between sensory neurones and tympanum (tympanal ridge and apodeme). Here, a first investigation of the postembryonic development of the auditory system is presented. In insects, sensory neurones usually differentiate during embryogenesis, and sound-perceiving structures form during postembryogenesis. Cicadas have an elongated and subterranian postembryogenesis which can take several years until the final moult. The neuroanatomy and functional morphology of the auditory system of the cicada Okanagana rimosa (Say) are documented for the adult and the three last larval stages. The sensory organ and the projection of sensory afferents to the CNS are present in the earliest stages investigated. The cuticular structures of the tympanum, the tympanal frame holding the tympanum, and the tympanal ridge differentiate in the later stages of postembryogenesis. Thus, despite the different life styles of larvae and adults, the neuronal components of the cicada auditory system develop already during embryogenesis or early postembryogenesis, and sound-perceiving structures like tympana are elaborated later in postembryogenesis. The life cycle allows comparison of cicada development to other hemimetabolous insects with respect to the influence of specially adapted life cycle stages on auditory maturation. The neuronal development of the auditory system conforms to the timing in other hemimetabolous insects.