Monika J. B. Eberhard

Structure and sensory physiology of the leg scolopidial organs in Mantophasmatodea and their role in vibrational communication.

Individuals of the insect order Mantophasmatodea use species-specific substrate vibration signals for mate recognition and location. In insects, substrate vibration is detected by mechanoreceptors in the legs, the scolopidial organs. In this study we give a first detailed overview of the structure, sensory sensitivity, and function of the leg scolopidial organs in two species of Mantophasmatodea and discuss their significance for vibrational communication. The structure and number of the organs are documented using light microscopy, SEM, and x-ray microtomography. Five scolopidial organs were found in each leg of male and female Mantophasmatodea: a femoral chordotonal organ, subgenual organ, tibial distal organ, tibio-tarsal scolopidial organ, and tarso-pretarsal scolopidial organ. The femoral chordotonal organ, consisting of two separate scoloparia, corresponds anatomically to the organ of a stonefly (Nemoura variegata) while the subgenual organ complex resembles the very sensitive organs of the cockroach Periplatena americana (Blattodea). Extracellular recordings from the leg nerve revealed that the leg scolopidial organs of Mantophasmatodea are very sensitive vibration receptors, especially for low-frequency vibrations. The dominant frequencies of the vibratory communication signals of Mantophasmatodea, acquired from an individual drumming on eight different substrates, fall in the frequency range where the scolopidial organs are most sensitive.

Structure and function of the arolium of Mantophasmatodea (Insecta).

All species of the insect order Mantophasmatodea characteristically keep the 5th tarsomere and pretarsus (arolium plus two claws) turned upwards and off the substrate. The unusually large arolium was studied in two species of Mantophasmatodea using bright field light microscopy, reflection microscopy, fluorescence microscopy, TEM, SEM, and Cryo‐SEM. It contains an epithelial gland, numerous tracheoles, and nerves. The gland consists of enlarged epithelial cells with large nuclei, mitochondria, RER, golgi complexes, microtubules, and numerous secretion vesicles. Evidence for exocytosis of the vesicles into the gland reservoir between the epithelial gland and the thick cuticle could be observed. Cryo‐SEM revealed that the ventral side of the arolium and distal part of its dorsal side are covered with a liquid film. Fluid footprints of arolia of individuals walking on a glass plate also indicate the presence of secretory fluid on the arolium surface. Behavioral experiments using animals with ablated arolia showed that representatives of Mantophasmatodea do not need their arolia to detect and respond to vibratory communication signals nor to catch small to medium‐sized prey. Individuals with ablated arolia were not able to move upside down on a smooth glass plate. We conclude that Mantophasmatodea use their arolia for attachment when additional adhesion force is required (e.g. windy conditions, handling large prey, mating). They can bring their arolia in contact with the surface in a very fast reflex (18.0 ± 9.9 ms). The secretory fluid found on the surface is produced by the gland and transported to the outside, presumably through small pore channels, to enhance adhesion to the substrate. J. Morphol., 2009.

Vibrational Communication in Two Sympatric Species of Mantophasmatodea (Heelwalkers)

The communication via percussion of the abdomen on the substrate for species recognition and mate location of males and females of two sympatric species of the recently described insect order Mantophasmatodea (Heelwalkers) was investigated. Each sex produced a single and distinctive call. The female call consisted of repeated single pulses, whereas the more complex male call comprised repeated pulse trains. The calls of males and females of the two species were of similar general structure, but differed in most temporal characters such as pulse and pulse train repetition time. In behavioral playback experiments females reacted to the call of conspecific males by calling and decreasing locomotion. When stimulated with the call of the heterospecific, sympatric male, females showed no reaction. Males exhibited abdominal rubbing, high tapping rates, increased activity (both movement and active searching) as well as characteristic searching behavior at branch nodes, when presented with the conspecific female call. Being stimulated with the playback of the heterospecific female call and the conspecific male call respectively, males responded with less intense locomotor and searching behavior. The drumming behavior in the control situation (no playback) suggests that males sometimes call in the absence of other individuals.

Sympatry in Mantophasmatodea, with the description of a new species and phylogenetic considerations

We describe a new genus of Mantophasmatodea, Viridiphasma gen. n. (Austrophasmatidae), represented by one new species, V. clanwilliamense sp. n. The new species differs from previously described species in features of the male and female postabdomen including the genitalia, in morphometrics and details of colouration. The new species occurs syntopically with another austrophasmatid, Karoophasma biedouwense Klass et al., 2003; this is the first well-documented case of sympatry of two mantophasmatodean species. We therefore survey the morphological differences between these two species, document the absence of any morphological evidence of hybridisation, and also report on differences in life history. While a previous molecular phylogeny using COI and 16S genes ambiguously placed V. clanwilliamense sp. n. near the base of Austrophasmatidae (but not as sister to all other Austrophasmatidae), morphological characters strongly support V. clanwilliamense sp. n. to be the sister taxon of a clade comprising all remaining Austrophasmatidae. This phylogenetic placement challenges the current hypothesis of a linear north-to-south diversification of Austrophasmatidae.

Evolution and Diversity of Vibrational Signals in Mantophasmatodea (Insecta)

Vibrational communication for species identification and mate location is widespread among insects. We investigated the vibrational communication signals of 13 species of the insect order Mantophasmatodea (Heelwalkers). Males and females produce percussive signals by tapping their abdomens on the substrate to locate conspecific mates. We show that male and female calls are of similar general structure but differ in temporal characteristics. Using a principal component analysis, we demonstrate that most species can be distinguished by their calls only. Mapping the calls onto an existing molecular phylogenetic tree reveals a slow diverging drift of male call pattern but no specific trend. For females, a trend from faster towards slower pulse repetition times is indicated. Two sympatric species, Karoophasma biedouwense and Viridiphasma clanwilliamense (Austrophasmatidae), exhibit very different call parameters. The latter species produces calls rather different from all other investigated species, which might hint at reproductive character displacement.

Cell-intrinsic mechanisms of temperature compensation in a grasshopper sensory receptor neuron

Changes in temperature affect biochemical reaction rates and, consequently, neural processing. The nervous systems of poikilothermic animals must have evolved mechanisms enabling them to retain their functionality under varying temperatures. Auditory receptor neurons of grasshoppers respond to sound in a surprisingly temperature-compensated manner: firing rates depend moderately on temperature, with average Q10 values around 1.5. Analysis of conductance-based neuron models reveals that temperature compensation of spike generation can be achieved solely relying on cell-intrinsic processes and despite a strong dependence of ion conductances on temperature. Remarkably, this type of temperature compensation need not come at an additional metabolic cost of spike generation. Firing rate-based information transfer is likely to increase with temperature and we derive predictions for an optimal temperature dependence of the tympanal transduction process fostering temperature compensation. The example of auditory receptor neurons demonstrates how neurons may exploit single-cell mechanisms to cope with multiple constraints in parallel. DOI: http://dx.doi.org/10.7554/eLife.02078.001

Variable Molecular Markers for the Order Mantophasmatodea (Insecta)

The recently discovered insect order Mantophasmatodea currently comprises 19 Southern African species. These mainly occur in allopatry, have high levels of color polymorphism and communicate via species- and gender-specific vibratory signals. High levels of interspecific morphological conservatism mean that cryptic species are likely to be uncovered. These aspects of Mantophasmatodean biology make them an ideal group in which to investigate population divergence due to habitat-specific adaptation, sexual selection, and potentially sensory speciation. Lack of appropriate genetic markers has thus far rendered such studies unfeasible. To address this need, the first microsatellite loci for this order were developed. Fifty polymorphic loci were designed specifically for Karoophasma biedouwense (Austrophasmatidae), out of which 23 were labeled and tested for amplification across the order using 2-3 individuals from 10 species, representing all 4 currently known families. A Bayesian mitochondrially encoded cytochrome c oxidase I (COI) topology was reconstructed and divergence dates within the order were estimated for the first time. Amplification success and levels of polymorphism were compared with genetic divergence and time since divergence. In agreement with studies on vertebrate taxa, both amplification and variability were negatively correlated with distance (temporal and genetic). The high number of informative loci will offer sufficient resolution for both broad level population genetic analysis and individual based pedigree or parentage analyses for most species in Austrophasmatidae, with at least some loci available for the other families. This resource will facilitate research into the evolutionary biology of this understudied but fascinating group.

Erratum to: Evolution and Diversity of Vibrational Signals in Mantophasmatodea (Insecta)

The original paper of this article unfortunately contains errors. In spite of thorough proofreading, in Table 1 there is a mistake concerning the collection sites of two species of Mantophasmatodea (the locations of the two Namibian species were mixed up and one GPS-data was incorrect). The authors are hereby publishing the correct Table 1. J Insect Behav (2014) 27:838–839 DOI 10.1007/s10905-014-9472-2

Cellular temperature compensation of sensory receptor neuron responses

Temperature is known to modulate ion channel kinetics and hence also action-potential generation. This poses a challenge for neural systems that need to retain their functionality also under conditions of varying temperature. Multiple strategies to counterbalance the effects of environmental temperature changes exist: mammals keep their body temperature approximately constant, while poikilothermic species need to implement temperature-compensation at the behavioral, systems, or cellular level. While mechanisms of behavioral and systems level have been identified [1], cellular mechanisms of temperature-compensation as well as their associated metabolic cost remain largely unknown. We investigated the effect of temperature on auditory processing in the grasshopper. We recorded intracellular responses of auditory receptor neurons to auditory broad-band noise stimuli at different intensities at two distinct behaviorally relevant temperatures. Interestingly, we found that changes in temperature did not have large effects on sound-intensity coding in receptor neurons. These neurons constitute the input layer of a feedforward network and hence do not receive network input. We concluded that the observed temperature robustness of receptor-neuron responses must arise from intrinsic, network unrelated effects. In general, the receptor-neuron response is shaped by two processing steps: mechanosensory transduction and spike generation. Both can contribute to temperature compensation. Either both transduction and spike generation are compensated (hypothesis I), or alternatively, their temperature dependencies can cancel each other (hypothesis II). To test hypothesis I we assumed a temperature-invariant transduction and asked, first, whether temperature-compensation could be achieved for a spike-generating mechanism with realistic temperature dependencies of the ionic conductances. The latter refers, in particular, to increases of gating kinetics by a factor of 2-4 with temperature increments of 10°C (defining a Q10 value of 2-4) as well as modest increases of peak conductances. Second, we explored whether temperature compensation, if achieved cell-intrinsically, compromises the neuronal energy budget. In other words, is temperature robustness metabolically expensive? To address these questions, we varied the temperature dependence of ionic conductances in a conductance-based neuron model. Based on the spike frequency vs. input current (f-I) relation, we estimated the ability of the model neurons to keep a robust firing rate despite changing temperature. Moreover, we computed the average energetic cost per action potential [2]. Using a database modeling approach [3], we performed a systematic sensitivity analysis for firing-rate changes and energetic cost as a function of the temperature dependence of conductance parameters (i.e. Q10 values of transition rates and peak conductances). Our analysis shows that the key parameters determining the robustness of spike generation relate to the temperature-dependence of the models potassium conductances. In contrast, energy consumption is governed by the temperature dependence of the sodium conductance. Consequently, a neuron can achieve temperature-compensation of its firing rate without compromising the energy budget. To constrain hypothesis II, we used the experimentally observed f-I curves in an objective function and inferred the corresponding transduction process for each spike generation in our sensitivity analysis. Our results predict that thermosensitive Transient Receptor Potential (TRP) channels have a role in mechanosensory transduction at the grasshopper tympanum, and therefore motivate further experiments.

Temperature differentially affects subsequent layers of auditory neurons in the locust.

Temperature influences basic properties of nerve cells such as spike rate, conduction velocity, and spike amplitude. This is relevant for ectothermic animals whose body temperature changes with ambient temperature. Here, we investigate the effect of temperature on signal processing in the grasshopper acoustic communication system. For these insects, the decoding of temporal characteristics of conspecific calls is crucial for mate recognition and may be impaired by temperature differences between sender and receiver. The peripheral auditory system is located within the metathoracic ganglion, where the first steps of song pattern recognition and analysis of sound direction are accomplished. Receptor neurons, local interneurons, and ascending neurons constitute these first three processing stages, forming a hierarchically organized feed-forward network. Previous studies revealed an improvement of temporal resolution at higher temperatures due to a higher precision of spike timing. However, neurons of the three processing stages were not equally affected by variation in temperature. In the present study, responses of locust auditory receptors, local interneurons and ascending neurons to short acoustic broad-band noise stimuli of various intensities were recorded intracellularly at a set of behaviorally relevant temperatures. Based on these data, temperature coefficients (Q10) were determined for the intensity-response curves of all neurons. Our results confirm an influence of temperature on spike count, shape, and duration, as well as first spike latencies. However, the overall response pattern and the shape of the intensity-response curve varied less than expected. In particular, receptor neurons and ascending neurons exhibited lower Q10 values than local interneurons. We conclude that distinct mechanisms of temperature compensation are present at subsequent processing stages. To understand these phenomena, we reproduced the observed electrophysiological responses at each processing stage using conductance-based neuronal models. We then investigated how the observed temperature compensation can arise from specific cell-intrinsic mechanisms or, alternatively, from the network structure of the metathoracic auditory pathway.