Gas Exchange Cycles Research Articles

Effects of Repeated Feed-Gas Exchange Processes on Degradation Processes of Pt-Based PEFC Cathode Catalysts Studied by In-situ XAFS, STEM-EDS and Electrochemical Methods

Start-up/shut-down procedures of polymer electrolyte fuel cells (PEFCs) result in temporal intermediate states of the anode Pt/C catalyst at the partial surface by exposing to both hydrogen and oxygen (air) in the feed-gas exchange processes of either air or H2 into the anode compartment. The inhomogeneous gas mixing in the anode leads to oxygen reduction reaction (ORR) at a part of the anode surfaces, which causes the corresponding cathode potential to spike up to about 1.6 V and hence carbon corrosion and Pt dissolution to deteriorate MEA performances. Recently, we reported the behavior of Pt/C cathode catalysts in MEAs during anode gas exchange cycles (AGEX )1,2 and cathode gas exchange cycles (CGEX)2 as characterized by in-situ time-resolved XAFS and STEM-EDS. Further investigations in this study have been done with an MEA Pt3Co/C cathode catalyst. The Pt3Co/C cathode catalyst gave different results from those for the Pt/C cathode catalyst under both AGEX and CGEX conditions. While both performance and ECSA of the Pt/C cathode decreased with increasing AGEX cycles, only the performance decreased with the Pt3Co/C cathode. In the degradation process by the CGEX cycles neither performance nor ECSA decreased with the Pt3Co/C, while only the ECSA decreased with the Pt/C. Pt LIII-edge white line peak intensities for the Pt3Co/C showed a larger decrease under the AGEX conditions, but less under the CGEX conditions, comparable to the results obtained for the Pt/C as shown in Figure 1. Figure1. Mass-activities (left axis) and averaged Pt LIII WL peak heights (right axis) for Pt/C (black) and Pt3Co/C (orange and green) during AGEX (left figure) and CGEX (right figure). The in-situ XAFS were measured at top (squares, open star for Pt3Co/C), middle (triangle), and bottom (circle) positions of the MEA. (1) G. Samjeské et al., ECS Transactions, 64, 113-119 (2014) (2) G. Samjeské et al., ChemElectroChem, DOI: 10.1002/celc.201500099 Figure 1

Respiration and metabolism of the resting European paper wasp (Polistes dominulus).

The European paper wasp, Polistes dominulus Christ, is an abundant wasp species in South and Central Europe which dispersed to the north in recent times. Polistes dominulus exhibits an energy-extensive mode of life, spending much time resting at the nest, which should be reflected in adaptations regarding gas exchange and standard metabolism. We analysed the resting metabolism (CO2 emission) of Polistes dominulus workers in the ambient temperature range an individual may be exposed to during a breeding season (Ta = 2.4–40.6 °C) via flow through respirometry. Behaviour and endothermic activity were assessed by infrared thermography. With rising Ta, CO2 release followed an exponential increase from 27 to 149 and 802 nl g−1 min−1 at Ta = 3, 20 and 35 °C, respectively. Measurements of the thermal regime at the nest showed that resting P. dominulus are most of the time in the lower range of their standard metabolic curve. A comparison with a “highly energetic” wasp like Vespula sp. revealed that Polistes dominulus not only optimises behaviour but also reduces metabolism to save energy. The CO2 emission patterns changed with ambient temperature, from discontinuous (≤25 °C) to cyclic (25–36 °C) and continuous gas exchange at higher temperatures. A pronounced break appeared in the data progression regarding cycle frequency and CO2 emission per gas exchange cycle between 15 and 10 °C. This striking change in gas exchange features indicates a physiological adaptation to special respiratory requirements at low temperatures.Electronic supplementary materialThe online version of this article (doi:10.1007/s00360-015-0915-7) contains supplementary material, which is available to authorised users.

Same-View Nano-XAFS/STEM-EDS Imagings of Pt Chemical Species in Pt/C Cathode Catalyst Layers of a Polymer Electrolyte Fuel Cell.

We have made the first success in the same-view imagings of 2D nano-XAFS and TEM/STEM-EDS under a humid N2 atmosphere for Pt/C cathode catalyst layers in membrane electrode assemblies (MEAs) of polymer electrolyte fuel cells (PEFCs) with Nafion membrane to examine the degradation of Pt/C cathodes by anode gas exchange cycles (start-up/shut-down simulations of PEFC vehicles). The same-view imaging under the humid N2 atmosphere provided unprecedented spatial information on the distribution of Pt nanoparticles and oxidation states in the Pt/C cathode catalyst layer as well as Nafion ionomer-filled nanoholes of carbon support in the wet MEA, which evidence the origin of the formation of Pt oxidation species and isolated Pt nanoparticles in the nanohole areas of the cathode layer with different Pt/ionomer ratios, relevant to the degradation of PEFC catalysts.

Responses in metabolic rate to changes in temperature in diapausing Colorado potato beetle Leptinotarsa decemlineata from three European populations

AbstractMany insects survive adverse periods in seasonal environments by entering diapause, a deep resting stage, during which energy consumption is typically low and gas exchange is in the form of a discontinuous gas exchange cycle (DGC). Because insects in high‐latitude environments are severely time constrained during summer, an effective diapause termination with careful regulation of metabolic rate is important. The present study examines whether diapausing Colorado potato beetles Leptinotarsa decemlineata Say originating from three latitudinally different regions in Europe differ in their quantitative or qualitative gas exchange patterns in response to an increasing temperature. Overall production of gaseous CO2, as well as qualitative patterns relating to the DGC, are measured at a late stage of diapause at four different temperatures in increasing order from 13, 18, 23 to 28 °C. Overall CO2 production is found to be lower in the two northern populations (61°49′N and 55°75′N) compared with the southernmost population (45°48′N) but increases as a function of temperature in all populations in a similar way. However, in the northern populations, raising the temperature increases the amount of CO2 discharged during single DGC peaks, whereas the DGC frequency remains relatively unchanged. By contrast, in the southernmost population, the amount of CO2 discharged during individual DGC peaks remains relatively unchanged, whereas the DGC frequency increases as a function of temperature. The observed differences may relate to water retention benefits or, alternatively, energetic benefits relating to heightened gas exchange efficiency in hypoxic or hypercapnic environments. Overall, the results suggest that, although populations of L. decemlineata may have similar thermal sensitivities, they have different qualitative strategies to regulate metabolic re‐activation at diapause termination.

A computational model of insect discontinuous gas exchange: A two-sensor, control systems approach.

The insect gas exchange system is characterised by branching air-filled tubes (tracheae/tracheoles) and valve-like structures in their outer integument (spiracles) which allow for a periodic gas exchange pattern known as the discontinuous gas exchange cycle (DGC). The DGC facilitates the temporal decoupling of whole animal gas exchange from cellular respiration rates and may confer several physiological benefits, which are nevertheless highly controversial (primarily reduction of cellular oxidative damage and/or respiratory water saving). The intrinsic and extrinsic factors influencing DGCs are the focus of extensive ongoing research and little consensus has been reached on the evolutionary genesis or mechanistic costs and benefits of the pattern. Despite several hypotheses and much experimental and evolutionary biology research, a mechanistic physical model, which captures various key elements of the DGC pattern, is currently lacking. Here, we present a biologically realistic computational, two-sensor DGC model (pH/carbon dioxide and oxygen setpoints) for an Orthopteran gas exchange system, and show computationally for the first time that a control system of two interacting feedback loops is capable of generating a full DGC pattern with outputs which are physiologically realistic, quantitatively matching experimental results found in this taxonomic model elsewhere. A finite-element mathematical approach is employed and various trigger sets are considered. Parameter sensitivity analyses suggest that various aspects of insect DGC are adequately captured in this model. In particular, with physiologically relevant input parameters, the full DGC pattern is induced; and the phase durations, endotracheal carbon dioxide partial pressure ranges, and pH fluctuations which arise are physically realistic. The model results support the emergent property hypothesis for the existence of DGC, and indicate that asymmetric loading and off-loading (hysteresis) in one of the sensor feedback loops, which is a novel inclusion here, is a critical aspect of the insect spiracle-tracheal gas exchange system.

Gas exchange pattern transitions in the workers of the harvester termite

The evolutionary genesis and the current adaptive significance of the use of the discontinuous gas exchange cycle (DGC) for respiration by insects is the subject of intense debate. Years of research have resulted in several leading hypotheses, one of which is the emergent-property hypothesis. This hypothesis states that DGC is an emergent property or consequence of interactions between the O2 and CO2 set points that regulate spiracular function, i.e. opening and closing. Workers of the harvester termite, Hodotermes mossambicus were selected as a model to test this hypothesis. The respiratory patterns of major workers, investigated using flow-through respirometry, were obtained at 100% relative humidity (RH) under varying temperature to evaluate the assumptions of the emergent-property hypothesis. Metabolic rate, measured as VCO2 increased significantly after 15°C. As VCO2 increased in response to increasing temperature and activity, the gas exchange pattern displayed by workers transitioned to a continuous gas exchange. A true DGC, defined as showing all three phases and a coefficient of variation value close to 2, was not expressed under the experimental conditions. The conclusion drawn from this study of termite workers is that changes in respiratory patterns are most likely an emergent property of the insects’ nervous and respiratory system.

The effects of temperature on the gas exchange cycle in Agathemera crassa

Insects exhibit three patterns of gas exchange: continuous (CoGE), cyclic (CGE) and discontinuous (DGE). In this work, we present the first record of a DGE in Phasmatodea and its transition to CGE and to CoGE through a thermal gradient. The rate of CO2 production (VCO2) at 10, 20 and 30°C was examined in adults of Agathemera crassa, a high-Andean phasmid of central Chile. Carbon dioxide release was recorded during 24h with L:D cycle of 12:12h in order to record both rest and activity periods. At rest, A. crassa showed three patterns of gas exchange, highlighting the use of DGE preferably at 10°C. As the temperature increased, the CoGE pattern was more frequent being the only pattern observed in all individuals at 30°C. During activity, patterns changed to CoGE with a significant increase in VCO2. Our results support the idea that gas exchange patterns in insects are not distinct but correspond to a continuum of responses addressed by metabolic demand and where DGE can be expressed only under an absolute state of rest. Our results support the idea that the presence of the DGE may be underestimated in other insect taxa because they may have been measured under conditions where this pattern not necessarily can be expressed.

Math-based spark ignition engine modelling including emission prediction for control applications

A complete spark ignition (SI) engine model is a multi-domain model including fluid dynamics, thermodynamics, combustion, electrical, and mechanical sub-models. The complexity of these models depends on the type of analysis used for model development, which may vary from highly detailed computational fluid dynamics (CFD) analysis (multi-dimensional model) to simpler data-based analysis in which the data is obtained from experiments (zero-dimensional model). The main objective of our research is to develop a math-based SI engine model for control application and real time simulation. The model must be accurate enough to capture the combustion characteristics (e.g., combustion temperature) and predict emission gases, while being fast enough for real time simulation purposes. In this paper, a physics-based model of an SI engine is presented which consists of different sub-models including: throttle body and manifold model, four-stroke quasi-dimensional thermodynamic model of gas exchange and power cycles, two-zone combustion and flame propagation model, emission gases model based on the chemical kinetics equations, and mechanical torque model. Moreover, part of the simulation results is validated against the GT-Power simulation results. The math-based model is created in the MapleSim environment. The symbolic nature of MapleSim significantly shortens the simulation time and also enables parametric sensitivity analysis.

Discontinuous gas-exchange cycle characteristics are differentially affected by hydration state and energy metabolism in gregarious and solitarious desert locusts

The termination of discontinuous gas exchange cycles (DGCs) in severely dehydrated insects casts doubt on the generality of the hygric hypothesis, which posits that DGCs evolved as a water conservation mechanism. We followed DGC characteristics in the two density-dependent phases of the desert locust Schistocerca gregaria throughout exposure to an experimental treatment of combined dehydration and starvation stress, and subsequent rehydration. We hypothesized that, under stressful conditions, the more stress-resistant gregarious locusts would maintain DGCs longer than solitary locusts. However, we found no phase-specific variations in body water content, water loss rates (total and respiratory) or timing of stress-induced abolishment of DGCs. Likewise, locusts of both phases re-employed DGCs after ingesting comparable volumes of water when rehydrated. Despite comparable water management performances, the effect of exposure to stressful experimental conditions on DGC characteristics varied significantly between gregarious and solitary locusts. Interburst duration, which is affected by the ability to buffer CO2, was significantly reduced in dehydrated solitary locusts compared with gregarious locusts. Moreover, despite similar rehydration levels, only gregarious locusts recovered their initial CO2 accumulation capacity, indicating that cycle characteristics are affected by factors other than haemolymph volume. Haemolymph protein measurements and calculated respiratory exchange ratios suggest that catabolism of haemolymph proteins may contribute to a reduced haemolymph buffering capacity, and thus a compromised ability for CO2 accumulation, in solitary locusts. Nevertheless, DGC was lost at similar hydration states in the two phases, suggesting that DGCs are terminated as a result of inadequate oxygen supply to the tissues.

Oxygen-induced plasticity in tracheal morphology and discontinuous gas exchange cycles in cockroaches Nauphoeta cinerea.

The function and mechanism underlying discontinuous gas exchange in terrestrial arthropods continues to be debated. Three adaptive hypotheses have been proposed to explain the evolutionary origin or maintenance of discontinuous gas exchange cycles (DGCs), which may have evolved to reduce respiratory water loss, facilitate gas exchange in high CO2 and low O2 micro-environments, or to ameliorate potential damage as a result of oversupply of O2. None of these hypotheses have unequivocal support, and several non-adaptive hypotheses have also been proposed. In the present study, we reared cockroaches Nauphoeta cinerea in selected levels of O2 throughout development, and examined how this affected growth rate, tracheal morphology and patterns of gas exchange. O2 level in the rearing environment caused significant changes in tracheal morphology and the exhibition of DGCs, but the direction of these effects was inconsistent with all three adaptive hypotheses: water loss was not associated with DGC length, cockroaches grew fastest in hyperoxia, and DGCs exhibited by cockroaches reared in normoxia were shorter than those exhibited by cockroaches reared in hypoxia or hyperoxia.

The length of discontinuous gas exchange cycles in lepidopteran pupae may serve as a mechanism for natural selection

AbstractGas exchange is studied in diapausing pupae of Mamestra brassicae L., whose larvae are reared under identical conditions. The release of CO2 gas is recorded with infrared gaseous analyzers. Oxygen convective uptake into the tracheae and oxygen consumption rates are recorded by means of a constant‐volume coulometric respirometer. Outputs from both of these respirometry systems are combined with infrared actographs. All 3‐month‐old pupae of M. brassicae display a pattern of discontinuous gas exchange (DGE) cycles of CO2 gas release by bursts, although the lengths of these cycles varies between individuals. Some pupae exhibit long DGE cycles of at least 20 h in duration, with negligible CO2 gas release during interburst periods, and there is presumed to be a convective gas exchange at this time. As a result of a partial vacuum inside the tracheae, a large oxygen convective uptake always occurs at the start of the spiracular opening phase. Other pupae have short DGE cycles of less than 3 h in duration, with elevated CO2 gas release during the interburst period, when gas exchange is predominantly diffusive. The spiracular open phase in these pupae consists of frequent separate convective bursts of CO2 gas release, with the opening–closing rhythms of the spiracles, which are considered as O phase fluttering. The pupae with long DGE cycles exhibit extremely low metabolic rates and very low total water loss rates, whereas those with short DGE cycles have higher metabolic and total water loss rates. The pupae with long DGE cycles live approximately twice as long as those with short cycles; thus, the present study demonstrates that long DGE cycles confer a fitness benefit on pupae as a result of a lower metabolic rate associated with water economy, conferring on them a longer life.

MEA Pt/C Cathode Catalyst Degradation during Repeated Anode Gas Exchange Cycles: In-Situ XAFS, STEM-EDS and Electrochemical Analysis

The effects of repeated start-up/shut-down cycles on degradation of Pt/C cathode catalysts in polymer electrolyte fuel cells (PEFC) were investigated by repeated anode gas exchange (AGEX) cycles. The degradation of MEA Pt/C catalysts during the AGEX cycles was examined at three different areas of the MEAs by in-situ XAFS and STEM-EDS. The steady decreases of the electrochemical surface area and ORR performance by the AGEX processes were observed. In contrast to the aspects averaged over the whole MEA areas, in-situ XAFS can provide the position-dependent structural and electronic information on Pt nanoparticles at different positions in the MEA during the AGEX cycles. The white line peak intensity in in-situ XANES spectra at Pt LIII-edge decreased in the order for the MEA regions, top > bottom ≥ middle. STEM images showed that Pt particles dropped off from the carbon supporting particles in the cathode catalyst layer and large cracks and voids formed in the top area of the MEA. These aspects of the in-situ XANES spectra and STEM images are correlated with each other and the MEA degradation.

MEA Pt/C Cathode Catalyst Degradation during Repeated Anode Gas Exchange Cycles: In-Situ XAFS, STEM-EDS and Electrochemical Analysis

The start-up and shut-down processes in polymer electrolyte fuel cell (PEFC) operations involve anode gas (fuel) mixing with air, which may diffuse into the Pt/C anode from either the anode gas outlet or the Pt/C cathode through the Nafion membrane. This causes a transient, short-lived anode-condition, in which a part of the anode catalyst layer is exposed to hydrogen while another is exposed to oxygen (air). The resulting mixed gas-condition leads to oxygen reduction reaction (ORR) at the oxygen-covered area of the anode, while hydrogen oxidation reaction takes place at the hydrogen-covered area of the anode, by the so called reverse current decay mechanism./1/ The electrolyte potential (potential of the Nafion layer) is drawn to negative values in the air-covered regions to facilitate the ORR on the anode, thus causing the potential difference between the Nafion layer and cathode catalyst layer on the corresponding areas of the cathode to increase up to 1.6 V. At the high potentials not only dissolution of Pt from the cathode catalyst but also oxidative corrosion of the carbon support take place.We have measured the Pt/C cathode catalyst in PEFC during repeated fast anode-gas exchanges (AGEX) cycles by means of in-situ time-resolved XAFS, to study the structural and electronic transformations of Pt nanoparticles at the cathode caused by the AGEX cycles. The in situ XAFS and time-resolved XAFS techniques together with STEM-EDS provided new insights into the origin and mechanism for the Pt/C cathode catalyst degradation by the AGEX cycles. /1/ Carl A. Reiser et al.; Electrochem. Solid-State Lett.; 2005 (6) A273

Temperature-dependent variation in gas exchange patterns and spiracular control in Rhodnius prolixus.

Insects display an array of respiratory behaviors, including the use of discontinuous gas exchange. This pattern is characterized by periods of spiracular closure, micro-openings (flutter), and complete openings during which the majority of gas exchange takes place. A current model of insect spiracular control suggests that spiracles are controlled by two interacting feedback loops, which produce the discontinuous pattern. The flutter period is thought to be initiated by a critically low partial pressure of oxygen, while the open period is initiated by a critically high CO2 threshold. The goal of our study was to test this control model under conditions of feeding-induced or temperature-induced changes in metabolic rate. We manipulated the metabolic rate of the insect Rhodnius prolixus using two discrete mechanisms: (1) feeding the insects a bloodmeal or (2) exposing them to a range of temperatures (18-38°C). Examining the variation in the gas exchange patterns produced by insects in each of these treatments allowed us to determine whether spiracular control is sensitive to metabolic rate and/or temperature. We found that increases in temperature caused significant decreases in open phase burst volumes and premature abandonment of discontinuous gas exchange cycles. These effects were not observed in fed individuals maintained at a single temperature despite their higher metabolic rates. Our results indicate that some part of the spiracular control mechanism is temperature sensitive, suggesting a possible role for pH in CO2 sensing.

Periodic heartbeat reversals cause cardiogenic inspiration and expiration with coupled spiracle leakage in resting blowfliesCalliphora vicinaR.-D.

Respiration in insects is thought to be independent of the circulatory system because insects typically lack respiratory pigments and because oxygen transport occurs in the gaseous phase through a ramified tracheal system by diffusion and convection directly to the tissues. In the blowfly, as in other insects with periodic heartbeat reversal, the haemolymph is periodically shifted between the anterior body and abdomen, exerting alternating pressure changes on the compliant tracheae in the thorax and in the abdomen. Simultaneous pressure and O2 optode measurements show that, during negative pressure periods, the tracheal partial pressure of oxygen (PO2) increases by 0.5 kPa. In the quiescent fly, tracheal PO2 is rather high (17.5-18.9 kPa), although the thoracic spiracles remain constricted. Microscopic video recordings and reflectance measurements revealed that the dorsal soft edges of the valve lips of the second spiracle leave a very small leak, which is passively widened during backward pulses of the heart. Thus, negative pressure, combined with increased leakage of the spiracle Sp2 valve enable inspiration in the thorax. The positive pressure periods are correlated with a new type of convective CO2 micro-bursts as shown in flow-through measurements. The bulk of the CO2 is, however, released after longer interbursts in macro-bursts with actively opening valves reminiscent of the open phase in a cyclic gas exchange. When the valves open, the PO2 in the thoracic air sacs unexpectedly drops by a mean of 2.75±1.09 kPa, suggesting a displacement of O2 by the transient accumulation of CO2 in the tracheal system before its release.

Oxygen diffusion limitation triggers ventilatory movements during spiracle closure when insects breathe discontinuously.

During discontinuous gas exchange cycles in insects, spiracular opening follows a typical prolonged period of spiracle closure. Gas exchange with the environment occurs mostly during the period of full spiracular opening. In this study we tested the hypothesis that recently reported ventilatory movements during the spiracle closure period serve to mix the tracheal system gaseous contents, and support diffusive exchanges with the tissues. Using heliox (21% O2, 79% He), we found that by increasing oxygen diffusivity in the gas phase, ventilatory movements of Schistocerca gregaria were significantly delayed compared with normoxic conditions. Exposure to hyperoxic conditions (40% O2, 60% N2) resulted in a similar delay in forced ventilation. Together, these results indicate that limits to oxygen diffusion to the tissues during spiracle closure trigger ventilatory movements, which in turn support tissue demands. These findings contribute to our understanding of the mechanistic basis of respiratory gas exchange between insect tissues and the environment.

Discontinuous gas exchange exhibition is a heritable trait in speckled cockroaches Nauphoeta cinerea

The regulation of insect respiratory gas exchange has long been an area of interest. In particular, the reason why insects from at least five orders exhibit patterns of gas exchange that include regular periods of spiracular closure has been the source of much controversy. Three adaptive hypotheses propose that these discontinuous gas-exchange cycles (DGCs) evolved to either limit water loss across respiratory surfaces, facilitate gas exchange in underground environments or to limit oxidative damage. It is possible that DGCs evolved independently multiple times and for different reasons, but for DGCs to be a plausible target for natural selection, they must be heritable and confer a fitness benefit. In a previous study of cockroaches Nauphoeta cinerea, we demonstrated that DGCs are repeatable and extend survival under food and water restriction. Here, we show for the first time that DGCs are heritable, suggesting that they are a plausible target for natural selection.

Respiration and Ventilation Responses to Acute Hypoxic Stress in the Guyana Orange Spotted Cockroach

Insect ventilation follows characteristic patterns, from the discontinuous gas exchange cycle (DGC), with periods of open and closed spiracles (bursts and interbursts), to continuous gas exchange. Resting insects are relatively resistant to changes in ambient pO2 with respiration rates maintained by changes in ventilation pattern during hypoxic exposure. We measured CO2 release (Vco2) of the Guyana Orange Spotted Cockroach (Blaptica dubia) to test the hypothesis that B. dubia increases ventilation and transitions from DGC to continuous gas exchange in response to acute decreases in pO2 from 21kPa O2 to 3 kPa O2 at 25°C and 35°C. Between 21 and 6 kPa O2, animals had a significantly higher Vco2 at 35°C than at 25°C. Cockroaches increased total Vco2 at the lowest O2 level at 25°C, but not at 35°C. Mean burst and interburst Vco2 during severe hypoxia (3kPa pO2) at 35°C was higher when compared to values at 21kPa O2. Mean cycle rate during a burst increased as pO2 decreased at both temperatures. Interburst period increased significantly under severe hypoxia at 35°C. Overall, animals at the higher temperature had higher Vco2, and as O2 decreased the spiracles opened more during interbursts; hyperventilation begins during moderate hypoxia (6 kPa O2), but bursts become less frequent and ventilation becomes more continuous in severe hypoxia. Supported by NSF IOS 1146758 (EMD).

Neural Control of Gas Exchange Patterns in Insects: Locust Density-Dependent Phases as a Test Case

The adaptive significance of discontinuous gas exchange cycles (DGC) in insects is contentious. Based on observations of DGC occurrence in insects of typically large brain size and often socially-complex life history, and spontaneous DGC in decapitated insects, the neural hypothesis for the evolution of DGC was recently proposed. It posits that DGC is a non-adaptive consequence of adaptive down-regulation of brain activity at rest, reverting ventilatory control to pattern-generating circuits in the thoracic ganglia. In line with the predictions of this new hypothesis, we expected a higher likelihood of DGC in the gregarious phase of the desert locust (Schistocerca gregaria, Orthoptera), which is characterized by a larger brain size and increased sensory sensitivity compared with the solitary phase. Furthermore, surgical severing of the neural connections between head and thoracic ganglia was expected to increase DGC prevalence in both phases, and to eliminate phase-dependent variation in gas exchange patterns. Using flow-through respirometry, we measured metabolic rates and gas exchange patterns in locusts at 30°C. In contrast to the predictions of the neural hypothesis, we found no phase-dependent differences in DGC expression. Likewise, surgically severing the descending regulation of thoracic ventilatory control did not increase DGC prevalence in either phase. Moreover, connective-cut solitary locusts abandoned DGC altogether, and employed a typical continuous gas exchange pattern despite maintaining metabolic rate levels of controls. These results are not consistent with the predictions of the neural hypothesis for the evolution of DGC in insects, and instead suggest neural plasticity of ventilatory control.

Gas exchange patterns in Platynus assimilis (Coleoptera: Carabidae): Respiratory failure induced by a pyrethroid

Discontinuous gas exchange (DGE) is the main (23 individuals) breathing mode in resting adult Platynus assimilis. Few of the beetles tested (13 individuals) displayed a pattern of cyclic gas exchange or CGE. The burst of CO2 release in DGE and CGE was always accompanied by abdominal pumping (active ventilation or V). Seven individuals displayed a pattern of continuous respira- tion, characterized by regular abdominal pumping. Resting metabolic rate (RMR) in continuously breathing beetles was higher than in those using DGE and CGE. After treatment with sub-lethal doses of alpha-cypermethrin DGE ceased. Treated beetles were char- acterized by continuous pumping and almost regular periods of activity. RMR increased significantly after treatment with a pyre- throid.