Central projections of fibers in the auditory and tensor nerves of cicadas (Homoptera: Cicadidae)

On the morphology and evolution of cicadomorphan tymbal organs

Cicadas and many of their relatives (Hemiptera: Cicadomorpha) generate vibroacoustic signals using tymbal organs located on their first two abdominal segments. Although tymbals are well-studied in Cicadidae, their systematic distribution in other Cicadomorpha and their possible homologies to the vibroacoustic mechanisms of other Hemiptera have been debated for more than a century. In the present study, we re-examine the morphology of the musculoskeletal system of cicadomorphan vibroacoustic organs, and we document their systematic distribution in 78 species drawn from across the phylogeny of Cicadomorpha. We also compare their morphology to the recently-described snapping organ of planthoppers (Fulgoromorpha). Based on the structure and innervation of the metathoracic and abdominal musculoskeletal system, we find that several key elements of cicadomorphan vibroacoustic organs that have previously been assigned to the first abdominal segment in fact belong to the second. We find that tymbal organs are nearly ubiquitous in Cicadomorpha, and conclude based on their phylogenetic distribution, that they are likely to be synapomorphic. The unusual tymbal-like organs of the Deltocephalinae and Typhlocybinae, represent derived modifications. Finally, we propose a standardised terminology for sternal components of the cicadomorphan vibrational organs, which can be used in future taxonomic descriptions.

Evolutionary escalation: the bat-moth arms race

Echolocation in bats and high-frequency hearing in their insect prey make bats and insects an ideal system for studying the sensory ecology and neuroethology of predator-prey interactions. Here, we review the evolutionary history of bats and eared insects, focusing on the insect order Lepidoptera, and consider the evidence for antipredator adaptations and predator counter-adaptations. Ears evolved in a remarkable number of body locations across insects, with the original selection pressure for ears differing between groups. Although cause and effect are difficult to determine, correlations between hearing and life history strategies in moths provide evidence for how these two variables influence each other. We consider life history variables such as size, sex, circadian and seasonal activity patterns, geographic range and the composition of sympatric bat communities. We also review hypotheses on the neural basis for anti-predator behaviours (such as evasive flight and sound production) in moths. It is assumed that these prey adaptations would select for counter-adaptations in predatory bats. We suggest two levels of support for classifying bat traits as counter-adaptations: traits that allow bats to eat more eared prey than expected based on their availability in the environment provide a low level of support for counter-adaptations, whereas traits that have no other plausible explanation for their origination and maintenance than capturing defended prey constitute a high level of support. Specific predator counter-adaptations include calling at frequencies outside the sensitivity range of most eared prey, changing the pattern and frequency of echolocation calls during prey pursuit, and quiet, or 'stealth', echolocation.

Chordotonal organs in hemipteran insects: unique peripheral structures but conserved central organization revealed by comparative neuroanatomy

Hemipteran insects use sophisticated vibrational communications by striking body appendages on the substrate or by oscillating the abdominal tymbal. There has been, however, little investigation of sensory channels for processing vibrational signals. Using sensory nerve stainings and low invasive confocal analyses, we demonstrate the comprehensive neuronal mapping of putative vibration-responsive chordotonal organs (COs) in stink bugs (Pentatomidae and Cydinidae) and cicadas (Cicadidae). The femoral CO (FCO) in stink bugs consists of ventral and dorsal scoloparia, homologous to distal and proximal scoloparia in locusts, which are implicated in joint movement detection and vibration detection, respectively. The ligament of the dorsal scoloparium is distally attached to the accessory extensor muscle, whereas that of the ventral scoloparium is attached to a specialized tendon. Their afferents project to the dorso-lateral neuropil and the central region of the medial ventral association center (mVAC) in the ipsilateral neuromere, where presumed dorsal scoloparium afferents and subgenual organ afferents are largely intermingled. In contrast, FCOs in cicadas have decreased dorsal scoloparium neurons and lack projections to the mVAC. The tymbal CO of stink bugs contains four sensory neurons that are distally attached to fat body cells via a ligament. Their axons project intersegmentally to the dorsal region of mVACs in all neuromeres. Together with comparisons of COs in different insect groups, the results suggest that hemipteran COs have undergone structural modification for achieving faster signaling of resonating peripheral tissues. The conserved projection patterns of COs suggest functional importance of the FCO and subgenual organ for vibrational communications.

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.

Power and control muscles of cicada song: structural and contractile heterogeneity

Sound production in cicadas is powered by a pair of large muscles whose contractions cause buckling of cuticular tymbals and thereby create sound pulses. Sound is modulated by control muscles that alter the stiffness of the tymbals or change the shape of the abdominal resonance chamber. Muscle ultrastructure and contractile properties were characterized for the tymbal muscle and two control muscles, the ventral longitudinal muscle and the tymbal tensor, of the periodical cicada Magicicada septendecim. The tymbal muscle is a fast muscle that is innervated by a single motoraxon. The control muscles are an order of magnitude less massive than the tymbal muscles, but their innervation patterns were considerably more complex. The tensor muscle is innervated by two axons, each of which evokes rather slow twitches, and the ventral muscle is innervated by at least six axons, some of which produce fast and the others slow contractions. Muscle contraction kinetics correlated well with ultrastructure. Fibers of the tymbal muscle and the portions of the ventral muscle thought to be fast were richly supplied with transverse tubules (T-tubules) and sarcoplasmic reticulum (SR); slow portions of the ventral muscle and the tensor muscle had relatively little SR.

Structure, development, and evolution of insect auditory systems

This paper provides an overview of insect peripheral auditory systems focusing on tympanate ears (pressure detectors) and emphasizing research during the last 15 years. The theme throughout is the evolution of hearing in insects. Ears have appeared independently no fewer than 19 times in the class Insecta and are located on various thoracic and abdominal body segments, on legs, on wings, and on mouth parts. All have fundamentally similar structures-a tympanum backed by a tracheal sac and a tympanal chordotonal organ-though they vary widely in size, ancillary structures, and number of chordotonal sensilla. Novel ears have recently been discovered in praying mantids, two families of beetles, and two families of flies. The tachinid flies are especially notable because they use a previously unknown mechanism for sound localization. Developmental and comparative studies have identified the evolutionary precursors of the tympanal chordotonal organs in several insects; they are uniformly chordotonal proprioceptors. Tympanate species fall into clusters determined by which of the embryologically defined chordotonal organ groups in each body segment served as precursor for the tympanal organ. This suggests that the many appearances of hearing could arise from changes in a small number of developmental modules. The nature of those developmental changes that lead to a functional insect ear is not yet known.

Organization of the auditory pathway in the thoracic ganglia of noctuid moths

We describe the neuroarchitecture of the noctuid thoracic nerve cord and use this framework to interpret the organization of the auditory pathway responsible for escape behaviour in noctuid moths. Noctuid moths possess only two auditory receptors (A1, A2), in each ear. The axon of the A1 cell projects initially to a glomerulus located ventrally and medially in the metathoracic ganglion, where it bifurcates. One branch ascends in the ventral intermediate tract to the brain, the other descends in the ventral intermediate tract into abdominal neuromeres of the metathoracic ganglion. Both axons arborize in the median ventral and ring tracts in each neuromere. The central projections of the A2 cell remain largely within the metathoracic ganglion. The axon bifurcates at the midline and directs arborizations dorsally to the dorsal intermediate and median dorsal tracts, and ventrally into the ring tract where the arborizations overlap those of the A1 afferent. The afferent projections remain ipsilateral to the ear of origin. We describe a posterior auditory association area in the metathoracic ganglion in which the major arborizations of several identified interneurones overlap those of the A1 afferent and make monosynaptic connections with it. These interneurones all respond tonically to sound stimuli. We have also identified the projections of the A1 afferent, interneurones, and motor neurones in the segmentally equivalent anterior auditory association area of the mesothoracic ganglion. An interneurone with major arborizations in the same tracts as the A1 afferent, and receiving monosynaptic input from it, is described. The arborizations of higher order interneurones lie mainly in dorsal tracts along with those of flight motor neurones. All the interneurones in this anterior centre respond phasically or phasic/tonically to sound stimuli. The relevance of this anatomical organization for predator avoidance behaviour is considered and the organization of auditory pathways in tympanate insects compared.

Audition in the praying mantis, Mantis religiosa L.: identification of an interneuron mediating ultrasonic hearing

1. The praying mantis possesses a single ear located in the ventral midline of the metathorax. We have studied the mantis' auditory nervous system using both extracellular and intracellular techniques and have identified anatomically and physiologically a mirror-image pair of interneurons (MR-501-T3) in the metathoracic ganglion that mediates ultrasonic hearing. 2. MR-501-T3 is tuned broadly to ultrasound with best sensitivity (55-60 dB SPL) between 25 and 45 kHz. Its tuning matches closely that of the whole tympanal nerve. 3. The physiological responses of MR-501-T3 are characterized by: (1) a phasic-tonic firing pattern with a distinctive initial burst at 500-800 spikes/s; (2) minimum latencies of 8-12 ms; (3) no spontaneous activity; (4) sigmoid intensity response curves with a small (10 dB) dynamic range; (5) accurate coding of stimulus duration and of repetition rates up to 60 pps. 4. The ascending axon of MR-501-T3 conducts action potentials at 4 m/s, a rate comparable with some giant fiber systems. 5. MR-501-T3 shows no directional capability. Sound from right and left produce identical responses in both cells of the pair. Neither cutting one tympanal nerve nor removing one hemi-ear leads to different responses in the two cells indicating that they must receive a common input, either from the auditory afferents or from interneurons. We present evidence that the two cells are not directly connected. 6. MR-501-T3 is a large, symmetrical cell with its processes primarily in the intermediate neuropil (lateral ring tract). Its integration segment crosses the midline in the supramedian commissure, and the cell body lies dorsally near the entrance of the leg nerve. The axon travels in the dorsal lateral tract and is one of the largest (17 microns) in the connective. 7. Given the strong anatomical similarities between MR-501-T3 and the G and B cells of the locust, these cells may be homologous. 8. We present arguments based on our physiological results and existing behavioral data that MR-501-T3 is part of an ultrasonic warning/escape system in the mantis. As in moths, lacewings, and crickets, this system may provide a defense against nocturnally foraging bats.