Discontinuous gas exchange and water loss in the keratin beetle Omorgus radula: further evidence against the water conservation hypothesis?

Discontinuous gas exchange cycles are demonstrated in Omorgus radula (Erichson) (Coleoptera, Trogidae) for the first time, thus extending evidence for such cycles to another family of beetles. The closed, flutter and open phases of the cycle were clearly distinguishable in this species, and the duration of these phases was 221 ± 28, 1403 ± 148 and 755 ± 43 s (mean ± SE), respectively. No evidence for significant intraspecific mass scaling of VCO₂ or any of the components of the cycle was found. Although the prolonged F‐phase recorded here is unusual for many insects, it has previously been found in other scarabaeoid beetles, especially those from xeric environments. It has been suggested that such modulation of the discontinuous gas exchange cycle may result in a reduced VCO₂ and, consequently, reduced water loss. In O. radula VCO₂ (15.25 ± 1.49 μl/h) was considerably lower than that predicted from its body mass (0.207 ± 0.006 g). However, the small relative contribution of respiratory transpiration (6.5%) to total water loss indicated that reduced VCO₂ has little to do with water economy. Rather, it may be a consequence of generally low activity levels of these beetles. The low respiratory water loss, but distinct subterranean component in the adult life of O. radula, lend some credence to the hypothesis suggesting that regular use of subterranean habitats might have been responsible for the evolution of discontinuous gas exchange cycles. However, non‐adaptive hypotheses can still not be discounted.     

Breathe softly, beetle: continuous gas exchange, water loss and the role of the subelytral space in the tenebrionid beetle, Eleodes obscura

Flightless, diurnal tenebrionid beetles are commonly found in deserts. They possess a curious morphological adaptation, the subelytral cavity (an air space beneath the fused elytra) the function of which is not completely understood. In the tenebrionid beetle Eleodes obscura, we measured abdominal movements within the subelytral cavity, and the activity of the pygidial cleft (which seals or unseals the subelytral cavity), simultaneously with total CO2 release rate and water loss rate. First, we found that E. obscura has the lowest cuticular permeability measured in flow-through respirometry in an insect (0.90 microg H2O cm(-2) Torr(-1) h(-1)). Second, it does not exhibit a discontinuous gas exchange cycle. Third, we describe the temporal coupling between gas exchange, water loss, subelytral space volume, and the capacity of the subelytral space to exchange gases with its surroundings as indicated by pygidial cleft state. Fourth, we suggest possible mechanisms that may reduce respiratory water loss rates in E. obscura. Finally, we suggest that E. obscura cannot exchange respiratory gases discontinuously because of a morphological constraint (small tracheal or spiracular conductance). This "conductance constraint hypothesis" may help to explain the otherwise puzzling phylogenetic patterns of continuous vs. discontinuous gas exchange observed in tracheate arthropods.     

Respiratory physiology and water relations of three species of Pogonomyrmex harvester ants (Hymenoptera: Formicidae)

The respiratory physiology and water relations of three harvester ant species (Pogonomyrmex rugosus Emery, P. occidentalis[Cresson] and P. californicus[Buckley]) were examined at three temperatures (15, 25 and 35°C) using a flow-through respirometry system. As intact ants tended to be active during testing, we performed a parallel set of experiments on individuals rendered motionless by decapitation. Both intact and decapitated ants exhibited discontinuous ventilation. Decapitation caused metabolic rate (V˙CO₂) and burst frequency to decrease in all three species. Burst volume either remained constant or increased after removal of the head, though mass-specific V˙CO₂ was unaffected except in P. rugosus. Mass-specific V˙CO₂s of headless harvesters were comparable with published values derived from motionless specimens of other ant species. The mean Q₁₀ for intact ants of all three species was 2.37; for decapitated insects the mean was 2.32. Respiratory water constituted a small (< 5%) fraction of total loss, and we believe that discontinuous ventilation does not act to conserve water in these organisms, although it may serve other functions.     

Pump out the volume—The effect of tracheal and subelytral pressure pulses on convective gas exchange in a dung beetle, Circellium bacchus (Fabricus)

Many flightless beetles like the large apterous dung beetle Circellium bacchus, possess a subelytral cavity (SEC) providing an extra air space below the elytra which connects to the tracheal system (TS) via metathoracic and abdominal spiracles. By measuring subelytral and intratracheal pressure as well as body movements and gas exchange simultaneously in a flow-through setup, we investigated the contribution of convection on Circellium respiratory gas exchange.No constriction phase was observed. TS and SEC pressures were always around atmospheric values. During interburst phase open abdominal spiracles and a leaky SEC led to small CO₂-peaks on a continuous CO₂ baseline, driven by intermittent positive tracheal pressure peaks in anti-phase with small negative subelytral pressure peaks caused by dorso-ventral tergite action.Spiracle opening was accompanied by two types of body movements. Higher frequency telescoping body movements at the beginning of opening resulted in high amplitude SEC and TS pressure peaks. High frequency tergite movements caused subelytral pressure peaks and led to a saw tooth like CO₂ release pattern in a burst. We propose that during the burst open mesothoracic spiracles increase the compliance of the subelytral cavity allowing big volumes of tracheal air being pulled out by convection.     

Bias, precision and accuracy in the estimation of cuticular and respiratory water loss: a case study from a highly variable cockroach, Perisphaeria sp

We compared the precision, bias and accuracy of two techniques that were recently proposed to estimate the contributions of cuticular and respiratory water loss to total water loss in insects. We performed measurements of VCO2 and VH2O in normoxia, hyperoxia and anoxia using flow through respirometry on single individuals of the highly variable cockroach Perisphaeria sp. to compare estimates of cuticular and respiratory water loss (CWL and RWL) obtained by the VH2O-VCO2 y-intercept method with those obtained by the hyperoxic switch method. Precision was determined by assessing the repeatability of values obtained whereas bias was assessed by comparing the methods' results to each other and to values for other species found in the literature. We found that CWL was highly repeatable by both methods (R0.88) and resulted in similar values to measures of CWL determined during the closed-phase of discontinuous gas exchange (DGE). Repeatability of RWL was much lower (R=0.40) and significant only in the case of the hyperoxic method. RWL derived from the hyperoxic method is higher (by 0.044 micromol min(-1)) than that obtained from the method traditionally used for measuring water loss during the closed-phase of DGE, suggesting that in the past RWL may have been underestimated. The very low cuticular permeability of this species (3.88 microg cm(-2) h(-1) Torr(-1)) is reasonable given the seasonally hot and dry habitat where it lives. We also tested the hygric hypothesis proposed to account for the evolution of discontinuous gas exchange cycles and found no effect of respiratory pattern on RWL, although the ratio of mean VH2O to VCO2 was higher for continuous patterns compared with discontinuous ones.     

Neural regulation of discontinuous gas exchange in Periplaneta americana

Patterns of gas exchange among terrestrial arthropods are highly variable from continuous to discontinuous with discretely partitioned phases. The underlying initiation and co-ordination of these patterns is relatively poorly understood. Here we present a novel method for the simultaneous measurement of central nervous system (CNS) activity of the metathoracic ganglion and VCO(2) in medium to large sized live terrestrial arthropods. Using Periplaneta americana at four oxygen levels (40%, 21%, 10% and 2% at 25 degrees C; n=6 per treatment), we present minimally invasive visualization of nervous output relative to typical resting discontinuous gas exchange (DGE) data for the first time. DGE was maintained when cockroaches were exposed to hyperoxia or moderate hypoxia, but was lost in severe hypoxia. CNS activity was manifested in three signal types: large CNS output coinciding with peak CO(2) production during a burst, moderate CNS output coinciding with CO(2) sawtoothing and fluttering, and minimal CNS activity during the closed phase of DGE in normoxia. Large and moderate CNS outputs were associated with observed abdominal pumping and congruent CO(2) peaks. At 10% oxygen, VCO(2) was significantly elevated during the inter-burst period in association with almost constant moderate CNS output between the periodic large CNS output. At 2% oxygen, DGE and large CNS output are lost to continuous CO(2) release and largely continuous moderate CNS output. As previously reported for this species, a central pattern generator for ventilation in the metathoracic ganglion is supported and we infer the presence of localized oxygen chemoreceptors based on clear CNS response to a change in oxygen tension.     

The role of the subelytral cavity in respiration in a tenebrionid beetle,Onymacris multistriata (Tenebrionidae: Adesmiini)

This study measured the respiratory patterns in the tenebrionid beetle, Onymacris multistriata, using flow-through respirometry to measure carbon dioxide emission from the mesothoracic spiracles separately and simultaneously with that from around the elytral case. 96% of the total CO(2) emitted was via the mesothoracic spiracles. These spiracles used a discontinuous gas exchange cycle similar to that measured from other tenebrionid beetles. Although the circadian rhythm of the beetles resulted in changes to the period durations and cycle frequencies in the discontinuous gas exchange cycles, the mesothoracic spiracle remained the major site for gas exchange. Thus the subelytral cavity plays a different role in respiration other than the elimination of CO(2) build-up. It is expected that other arid dwelling flightless beetles will also be shown to use the mesothoracic spiracle as the major route for CO(2) emission.     

Discontinuous carbon dioxide release in the German cockroach, Blattella germanica (Dictyoptera: Blattellidae), and its effect on respiratory transpiration

The discontinuous gas exchange cycle (DGC) was described in the German cockroach, Blattella germanica (L.) (Dictyoptera: Blattellidae) for the first time. Also, the effect of the DGC on water loss was investigated. The CO(2) emission pattern in both insecticide resistant and susceptible B. germanica varied with temperature. At 10, 15, and 20 degrees C the pattern was discontinuous. Cycle frequency increased at 25 and 30 degrees C, and at 35 degrees C the pattern became cyclic. In most DGCs, there was no clear distinction between the closed and flutter phases in both strains thus data for these phases were combined and analyzed as the interburst phase. The probability that B. germanica would breath discontinuously varied with temperature. Most cockroaches (62.8%) displayed DGCs at 10 degrees C, therefore measurement of metabolic rate and water loss was carried out at this temperature. Using repeated measures of analysis of variance, the interburst and burst V(.)(CO(2))(ml h(-1)) were not significantly different between the two strains. The variability in CO(2) emission during the interburst and burst phases over time was not significantly different from cycle to cycle or between strains. Overall metabolic rate during the entire recording was not significantly different between both strains. There was a significant difference in the duration of the interburst and burst phases between the strains. The susceptible strain had significantly longer interburst and burst phase durations during a complete DGC than the resistant strain. The interburst and burst phase durations were 5.01+/-0.19 and 6.21+/-0.13 min, respectively, for the resistant strain, whereas the durations were 7.16+/-0.37 and 6.73+/-0.17 min, respectively, for the susceptible strain. This resulted in a DGC of significantly longer duration (13.89+/-0.44 min) in the susceptible strain compared with the resistant strain (11.23+/-0.26 min). The duration of the interburst phase was significantly different from the open phase duration in the resistant strain such that during a single DGC lasting approximately 11.23 min, 43.5% consisted of the interburst phase while the burst phase made up 56.5% of the cycle. The cuticular permeability at 10 degrees C and 0% RH was 2.26 microg cm(-2) h(-1) mmHg(-1) for the resistant strain and 3.42 microg cm(-2) h(-1) mmHg(-1) for the susceptible strain. In both strains, cuticular transpiration accounted for approximately 95% of total water loss. The significantly longer duration of the interburst phase of the susceptible strain was not important in reducing water loss.     

热带柚木(Tectona grandis L. f.)人工林中不同邻体干扰强度下林木CO2气体交换特征

研究以海南岛尖峰岭21a生的珍贵树种柚木人工林为对象,以邻体干扰指数为干扰强度评价指标,用Li-6400(Li-cor, Inc., USA)便携式光合作用测定系统研究了邻体干扰强度对柚木个体CO2气体交换的日动态和季节变化特性,光合-光响应特性及光合-CO2响应特性的影响.结果表明:邻体干扰并未改变个体叶片光合作用日进程,但对日净光合速率(Pn)的影响差异较大,表现为弱、中、强和极强度干扰下日Pn时段均值比为2.5 : 2.3 : 1.7 : 1.0,弱干扰下日最大净光合速率为极强干扰下的2.8倍;随着干扰强度的增加,柚木叶片的蒸腾速率、气孔导度等特征参数也出现不同程度的下降.邻体干扰对光合-光响应和光合-CO2响应特征参数的影响明显,弱干扰下柚木叶Asat、Qsat 、αA、CE、Vcmax和Jmax分别是极强干扰下的2.7、1.3、1.4、2.7、1.9和2.8倍,Qcomp和Rd则受邻体干扰的影响不明显.干扰强度对光合光响应和光合CO2响应特征的影响随光环境和CO2浓度的改变而有所差异,光环境的改变对弱干扰下柚木个体的影响明显要强于强度以上干扰的个体,而CO2浓度的改变对弱干扰个体的影响则明显要弱于强度干扰以上个体的影响.因此,在林分结构相对单一、经营管理措施基本一致的人工林内,邻体干扰是个体竞争的主要影响因素之一,干扰强度的不同将影响种内个体光合碳同化能力出现差别,导致个体生长差异.可以基于邻体干扰指数评价柚木人工林内种内竞争程度的大小,并以此为依据合理调整柚木人工林的林分结构,达到对环境资源的最合理利用.     

Discontinuous gas exchange in dung beetles: patterns and ecological implications

This study correlates a distinctive pattern of external gas exchange, referred to as the discontinuous gas exchange cycle (DGC), observed in the laboratory, with habitat associations of five species of telecoprid dung beetles. The beetles were chosen from a variety of habitats that would be expected to present different amounts of water stress. All five species exhibited DGC. Sisyphus fasciculatus has been recorded only in woodland areas, and does not have strict spiracular control during its DGC. Anachalcos convexus and Scarabaeus rusticus are associated with open mesic habitats. Both species exhibit a distinct DGC, previously found in some other insect species, but intermediate within this study group. Sc. flavicornis and Circellium bacchus are typically found in arid regions, and have the most unusual form of DGC, with spiracular fluttering during the burst phase. These results support the hypothesis that spiracular fluttering reduces respiratory water loss. From this study we conclude that the DGC is an ancestral adaptation, most probably as a result of anoxic environments in underground burrows, but that spiracular control is enhanced to reduce respiratory water loss in beetle species that live in arid habitats. Received 4 August 1999 / Accepted: 7 October 1999     

Effects of flow rate and temperature on cyclic gas exchange in tsetse flies (Diptera, Glossinidae)

Air flow rates may confound the investigation and classification of insect gas exchange patterns. Here we report the effects of flow rates (50, 100, 200, 400 ml min⁻¹) on gas exchange patterns in wild-caught Glossina morsitans morsitans from Zambia. At rest, G. m. morsitans generally showed continuous or cyclic gas exchange (CGE) but no evidence of discontinuous gas exchange (DGE). Flow rates had little influence on the ability to detect CGE in tsetse, at least in the present experimental setup and under these laboratory conditions. Importantly, faster flow rates resulted in similar gas exchange patterns to those identified at lower flower rates suggesting that G. m. morsitans did not show DGE which had been incorrectly identified as CGE at lower flow rates. While CGE cycle frequency was significantly different among the four flow rates (p < 0.05), the direction of effects was inconsistent. Indeed, inter-individual variation in CGE cycle frequency exceeded flow rate treatment variation. Using a laboratory colony of closely related, similar-sized G. morsitans centralis we subsequently investigated the effects of temperature, gender and feeding status on CGE pattern variation since these factors can influence insect metabolic rates. At 100 ml min⁻¹ CGE was typical of G. m. centralis at rest, although it was significantly more common in females than in males (57% vs. 43% of 14 individuals tested per gender). In either sex, temperature (20, 24, 28 and 32 °C) had little influence on the number of individuals showing CGE. However, increases in metabolic rate with temperature were modulated largely by increases in burst volume and cycle frequency. This is unusual among insects showing CGE or DGE patterns because increases in metabolic rate are usually modulated by increases in frequency, but either no change or a decline in burst volume.     

Spiracle activity in moth pupae—The role of oxygen and carbon dioxide revisited

After decades of intensive research, the actual mechanism behind discontinuous gas exchange in insects has not been fully understood. One open question concerns the actual way (closed, flutter, and open) of how spiracles respond to tracheal gas concentrations. As the results of a classic paper [Burkett, B.N., Schneiderman, H.A., 1974. Roles of oxygen and carbon dioxide in the control of spiracular function in cecropia pupae. Biological Bulletin 147, 274-293] allow ambiguous interpretation, we thus reexamined the behavior of the spiracles in response to fixed, controlled endotracheal gas concentrations.The tracheal system of diapausing pupae of Attacus atlas (Saturniidae, Lepidoptera) was flushed with gas mixtures varying in PO₂ and PCO₂ while the behavior of the spiracles was monitored using changes in the pressure signal. This novel pressure based technique proved to be superior to classic visual observation of single spiracles. A two-dimensional map of the spiracle behavior in response to endotracheal PO₂ and PCO₂ was established. Typically, it contained two distinct regions only, corresponding to “closed” and “open” spiracles. A separate “flutter” region was missing. Because fluttering is commonly observed in moth pupae, we suggest that the intermittent spiracle opening during a flutter phase is an effect of non-steady-state conditions within the tracheal system. For low PCO₂ the minimum PO₂ resulting in open spiracles was linearly dependent upon PCO₂. Above a threshold of 1-1.5 kPa CO₂ the spiracles were open irrespective of PO₂. We propose a hypothetical spiracular control model, which is simple and explains the time course of endotracheal partial pressures during all phases of discontinuous gas exchange.     

Partitioning of transpiratory water loss of the desert scorpion, Hadrurus arizonensis (Iuridae)

Terrestrial arthropods lose body water to the environment mainly through transpiration. The aim of this study was to determine the fraction of respiratory losses from total transpiratory water loss in scorpions, as relatively high respiratory losses would indicate a fitness benefit from regulation of gas-exchange rate under stressful desiccating conditions. We measured metabolic rates and water-loss rates of Hadrurus arizonensis (Iuridae) at a range of ecologically-relevant temperatures. Calculation of respiratory water losses was based on increased metabolic and water-loss rates during nocturnal activity (assuming no change in cuticular resistance at a given constant experimental temperature). Respiratory losses accounted for 9.0 ± 1.7% of total transpiratory losses at 25 °C, doubled to 17.9 ± 1.8% at 30 °C and increased to 31.0 ± 2.0% at 35 °C (n = 5, 15 and 15, respectively). Furthermore, the relative importance of respiratory transpiration is likely to be higher at temperatures above 35 °C, which have been recorded even within the burrows of H. arizonensis. Measurements of cuticular lipid melting points do not provide evidence for increased cuticular resistance to water loss at higher temperatures. However, the relatively high fraction of respiratory water losses reported here for H. arizonensis supports the notion of respiratory regulation as an evolved mechanism for conserving scorpion body water stores under stressful conditions.     

Use of urease inhibitors to reduce ammonia loss following application of urea to flooded rice fields

Ammonia (NH₃) volatilization is an important mechanism for nitrogen (N) loss from flooded rice fields following the application of urea into the floodwater. One method of reducing losses is to use a urease inhibitor that retards the hydrolysis of urea by soil urease and allows the urea to diffuse deeper into the soil. The two chemicals that have shown most promise are phenylphosphorodiamidate [PPD] and N(n-butyl)thiophosphorictriamide [NBPT], but they seldom work effectively. PPD decomposes rapidly when the pH departs from neutrality, and NBPT must be converted to the oxygen analogue for it to be effective. Our field studies in Thailand show that the activity of PPD can be prolonged, and NH₃ loss markedly reduced, by controlling the floodwater pH with the algicide terbutryn. A mixture of NBPT and PPD in the presence of terbutryn was even more effective than PPD alone. It appears that during the time when the PPD was effective, NBPT was being converted to the oxygen analogue. The combined urease inhibitor-algicide treatment reduced NH₃ loss from 10 to 0.4 kg N ha^-1.     

Photosynthesis and related physiological processes in two mangrove species,Rhizophora mucronata and Ceriops tagal,at Gazi Bay,Kenya

Measurements were carried out of the gas exchange properties (namely, photosynthesis, stomatal conductance and transpiration rates), water use efficiency and water relations of two mangrove species, Rhizophora mucronata and Ceriops tagal at Gazi Bay, Kenya. Rhizophora mucronata had significantly higher photosynthetic rates than C. tagal. Internal CO₂ concentrations were higher during the wet season than the dry season in both species. Gas exchange properties were correlated positively with photon flux density in both species. Leaf water potentials were highest during the morning and lowest at midday and were also highest in the lower canopy leaves in both species. The two mangrove species had conservative water use. Management potential for the East African mangroves based on the results of this study is suggested.     

Bimodal respiration, water balance and acid-base regulation in a high-shore crab, Cyclograpsus lavauxi H. Milne Edwards

Cyclograpsus lavauxi H. Milne Edwards, 1853 occurs under boulders near the littoral fringe, where it is wetted only briefly, if at all, in each tide. Mean water content of sea-water equilibrated crabs was 62.3 % of body weight. On warm, windy days crabs on the shore lose > 17.5% of body water. Water loss for crabs of 1.6 g mean body weight was 0.29 % water content · h ⁻¹ · mm Hg saturation deficit ⁻¹, which is similar to that of species occupying positions lower on the shore. C. lavauxi is, however, able to tolerate comparatively greater losses of body water (up to 36%) compared with species from lower shore levels.

As the gills dry, the lamellae separate into regular clumps which may help to maintain gas exchange. Resting oxygen consumption (V_o2) after 3–5 h settlement was similar in water and air. During enforced activity, V_o2 increased by factors of 5.3 in water and 2.6 in air at a standardized body weight of 1.5 g, indicating an appreciable aerobic scope in both media. Loss of up to 16% of body water did not depress resting aerial V_o2 or aerobic scope.

In resting crabs there was no change in haemolymph pH after 24 h of either immersion or emersion, but haemolymph [Ca²⁺], [HCO⁻₃] + [CO²⁻₃], and calculated P_co2 all rose in air relative to aquatic values. These results suggest that cuticular or other endogenous CaCO₃ is mobilized to compensate the respiratory acidosis in air. The implications of such physiological properties for aquatic and aerial activity on the shore are discussed.