Flutter Phase Research Articles

Spiracular fluttering decouples oxygen uptake and water loss: a stochastic PDE model of respiratory water loss in insects.

In insect respiration, oxygen from the air diffuses through a branching system of air-filled tubes to the cells of the body and carbon dioxide produced in cellular respiration diffuses out. The tracheal system has a very large surface area, so water loss is a potential threat and the question of how insects regulate oxygen uptake and water loss has been an important issue in insect physiology for the past century. The tracheal system starts at spiracles on the surface of the body that insects can open and close, and three phases are observed experimentally, open or closed for relatively long periods of time and opening and closing rapidly, which is called fluttering. In previous work we have shown that during this flutter phase, no matter how small the percentage of time that the spiracles are open, the insect can absorb almost as much oxygen as if the spiracle were always open, if the insect flutters fast enough. This left open the question of water loss during the flutter phase, which is the question addressed in this paper. We formulate a stochastic diffusion-convection model for the concentration of water vapor in the tracheae. Mathematical analysis of the model yields an explicit formula for water loss as a function of six non-dimensional parameters and we use experimental data from various insects to show that, for parameters in the physiological ranges, water loss during the flutter phase is approximately proportional to the percentage of time open. This means that the insect can solve the oxygen uptake versus water loss problem by choosing to have their spiracles open a small percentage of time during the flutter phase and fluttering rapidly.

Spiracular fluttering increases oxygen uptake.

Many insects show discontinuous respiration with three phases, open, closed, and fluttering, in which the spiracles open and close rapidly. The relative durations of the three phases and the rate of fluttering during the flutter phase vary for individual insects depending on developmental stage and activity, vary between insects of the same species, and vary even more between different species. We studied how the rate of oxygen uptake during the flutter phase depends on the rate of fluttering. Using a mathematical model of oxygen diffusion in the insect tracheal system, we derive a formula for oxygen uptake during the flutter phase and how it depends on the length of the tracheal system, percentage of time open during the flutter phase, and the flutter rate. Surprisingly, our results show that an insect can have its spiracles closed a high percentage of time during the flutter phase and yet receive almost as much oxygen as if the spiracles were always open, provided the spiracles open and close rapidly. We investigate the respiratory gain due to fluttering for four specific insects. Our formula shows that respiratory gain increases with body size and with increased rate of fluttering. Therefore, insects can regulate their rate of oxygen uptake by varying the rate of fluttering while keeping the spiracles closed during a large fraction of the time during the flutter phase. We also use a mathematical model to show that water loss is approximately proportional to the percentage of time the spiracles are open. Thus, insects can achieve both high oxygen intake and low water loss by keeping the spiracles closed most of the time and fluttering while open, thereby decoupling the challenge of preventing water loss from the challenge of obtaining adequate oxygen uptake.

DGC not necessarily saving water

Equipped with a delicate network of tubules that deliver oxygen directly to every tissue in the body, insects open and close the valves (spiracles) at the ends of the tubules (tracheae) to admit oxygen to the body and release carbon dioxide in a series of well-established patterns matched to their activity. One such pattern, known as the discontinuous gas exchange cycle (DGC), occurs when some insects are inactive. The spiracles are initially closed for extended periods as the insect consumes oxygen from the air in the closed tracheae. However, once the oxygen level has fallen sufficiently, the spiracles begin to open and close rapidly (flutter phase), to draw in more air. The carbon dioxide levels in the body then continue rising until they are high enough to trigger the final phase of the cycle, when the spiracles open for an extended period as the insect releases the accumulated carbon dioxide and takes in more oxygen ready to begin the cycle again. Yet, how this particular gas exchange pattern evolved is a mystery. Several competing theories had been proposed, but Eran Gefen says, ‘None of the existing hypotheses… is backed by unequivocal support’. Explaining that one theory suggests that DGC evolved to protect insects from dehydration, Shu-Ping Huang, Stav Talal, Amir Ayali and Gefen decided to test whether grasshoppers from three dramatically different environments saved water when using DGC.‘We wanted to know whether xeric grasshoppers [that live in dry conditions] “do DGC better”’, says Gefen, so Huang and Talal headed out to three locations in Israel – ranging from the desiccated Negev desert to a hill overlooking the Sea of Galilee (where the rainfall is higher), to the wettest location, Mount Hermon on the Syrian border – to collect grasshoppers endemic to each environment. Back in the lab, Huang began the laborious process of measuring the insects’ gas exchange patterns and water losses over periods of several hours before scrutinising the carbon dioxide traces to identify examples of DGC and continuous breathing for analysis.Noticing that 60% of the desert dwelling Tmethis pulchripennis grasshoppers used DGC while only 19% of the grasshoppers from the most humid environment – Ocneropsis lividipes – used DGC, Huang then compared the insects’ respiratory water loss rates when they were breathing discontinuously and continuously. If the grasshoppers were breathing discontinuously to protect themselves from desiccation, they would all reduce their respiratory water losses when they used DGC. However, the team was impressed to see that even though the desert species (T. pulchripennis) reduced its respiratory water losses, neither of the species that were adapted to more humid conditions (Ocneropsis bethlemita and O. lividipes) did. ‘DGC in the two Ocneropsis species did not result in any measureable water savings,’ says Gefen, adding ‘Our results suggest that DGC has not necessarily evolved as a water-saving mechanism’.Instead, Gefen suggests that the ability to reduce water loss through DGC in some, but not all, species may be related to differences in tracheal dimensions. He explains that although T. pulchripennis may have developed a large tracheal system to conserve water during DGC, there is another possible explanation: that T. pulchripennis – which is the only insect in the study that can fly – evolved a larger tracheal system to deliver sufficient oxygen for the high metabolic demands of flight. ‘It would be extremely interesting to tease apart the two in the future’, says Gefen.

Allometric scaling of discontinuous gas exchange patterns in the locust Locusta migratoria throughout ontogeny

The discontinuous gas exchange cycle (DGC) is a three-phase breathing pattern displayed by many insects at rest. The pattern consists of an extended breath-hold period (closed phase), followed by a sequence of rapid gas exchange pulses (flutter phase), and then a period in which respiratory gases move freely between insect and environment (open phase). This study measured CO(2) emission in resting locusts Locusta migratoria throughout ontogeny, in normoxia (21 kPa P(O2)), hypoxia (7 kPa P(O2)) and hyperoxia (40 kPa P(O2)), to determine whether body mass and ambient O(2) affect DGC phase duration. In normoxia, mean CO(2) production rate scales with body mass (M(b); g) according to the allometric power equation , closed phase duration (C; min) scales with body mass according to the equation C=8.0M(b)(0.38±0.29), closed+flutter period (C+F; min) scales with body mass according to the equation C+F=26.6M (0.20±0.25)(b) and open phase duration (O; min) scales with body mass according to the equation O=13.3M(b) (0.23±0.18). Hypoxia results in a shorter C phase and longer O phase across all life stages, whereas hyperoxia elicits shorter C, C+F and O phases across all life stages. The tendency for larger locusts to exhibit longer C and C+F phases might arise if the positive allometric scaling of locust tracheal volume prolongs the time taken to reach the minimum O(2) and maximum CO(2) set-points that determine the duration of these respective periods, whereas an increasingly protracted O phase could reflect the additional time required for larger locusts to expel CO(2) through a relatively longer tracheal pathway. Observed changes in phase duration under hypoxia possibly serve to maximise O(2) uptake from the environment, whereas the response of the DGC to hyperoxia is difficult to explain, but could be affected by elevated levels of reactive oxygen species.

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 1–1.5kPa 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.

Stability of a deploying/extruding beam in dense fluid

The equations of motion of a flexible slender cantilevered beam with uniform circular cross-section, extending axially in a horizontal plane at a known rate while immersed in an incompressible fluid are derived. An “axial added mass coefficient” is implemented in these equations in order to better approximate the mass of fluid which stays attached to the oscillating beam while moving in the axial direction. Realistic initial conditions are given to the system and numerical solutions are obtained. The dynamical behaviour of the system is observed for cases of constant extension rate and for a trapezoidal deployment rate profile. In the case of low constant extension rates, the system displays a phase of oscillation with increasing amplitude and decreasing frequency until the motion is strongly damped and later becomes statically unstable. For faster deployment rates, the beam has a short flutter phase at the beginning of the deployment, followed by a brief phase of damped oscillation until it exhibits static divergence. For fast enough deployment rates, the system is unstable from the beginning and never stabilizes. The effect the axial added mass coefficient has on the system is studied and it is found that it plays two roles in the stability of the system. The trapezoidal deployment rate profile is studied because it is deemed more representative of real-life applications. For long deployment times, the system behaves in a very similar manner to one with low constant extension rate, except that it does not become statically unstable. For shorter deployment times, the maximum amplitude of the tip displacement is usually attained after the beam has stopped extruding.

Discontinuous gas exchange patterns of beet armyworm pupae, Spodoptera exigua (Lepidoptera: Noctuidae): effects of Bacillus thuringiensis Cry1C toxin, pupal age and temperature

Abstract. Changes in the discontinuous gas exchange cycle of pupal beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), exposed or not to Cry1C Bacillus thuringiensis toxin, are examined against developmental age (1–7 days) and at different temperatures (10–25 °C) using flow through respirometry. Both exposed and nonexposed pupae exhibit discontinuous gas exchange, but only at 10 °C; the frequency of cyclic release of CO2 increases with increasing temperatures. The three phases of the discontinuous gas exchange cycle are distinct for both treatment groups. However, the duration of each phase is significantly greater for pupae exposed previously to toxin. The closed phase is 40 ± 14% longer, the flutter phase 23 ± 19% longer, and the open phase is 28 ± 12% longer when pupae were exposed to toxin. Respiratory water loss is 4.5 ± 1.3% for toxin exposed pupae and 2.1 ± 2.4% for unexposed pupae. Furthermore, the exposed pupae have significantly greater cuticular permeability (26.01 ± 1.9 µg cm−2 h−1 mmHg−1) than the nonexposed pupae (9.64 ± 0.9 µg cm−2 h−1 mmHg−1). However, in both strains, cuticular transpiration (>93%) far exceeds respiratory transpiration. Overall, total water loss is significantly greater in pupae whose larvae are exposed to toxin compared with pupae from nontreated larvae. Toxin exposed pupae have a mean cycle duration of 60 ± 2.5 min whereas that of nonexposed pupae is 42 ± 1.8 min.(ml g−1 h−1) of the open phase is greater earlier in pupal life followed by a minimum at mid‐pupal stage and an increase at late‐pupal development in both treatment groups. Combining all 7 days, closed, flutter and open phase (ml g−1 h−1), pupae exposed to toxin produce significantly more CO2 during each phase. On average, toxin exposed pupae produce 52 ± 12, 43 ± 10 and 15 ± 37% more CO2 than the untreated pupae during the closed, flutter and open phases, respectively. Therefore, the present study reinforces the need to use insects of similar developmental age in studies of insect respiration patterns and energy metabolism.

The role of the subelytral spiracles in respiration in the flightless dung beetle Circellium bacchus.

The role of the subelytral cavity in flightless beetle species as an adaptation to water saving in arid habitats is still in dispute. We found that relatively little CO(2) was released from the subelytral cavity of a large apterous beetle Circellium bacchus during simultaneous measurements of CO(2) emission from the anterior mesothoracic spiracles and posterior body, which included the subelytral spiracles. However, when we sampled air directly from inside the subelytral cavity, we discovered that this pattern was reversed. A discontinuous gas exchange cycle (DGC) was recorded from the posterior body half, revealing a flutter phase that had been absent from the anterior mesothoracic DGC. The anterior mesothoracic and posterior subelytral spiracles act in synchrony to maintain high CO(2) and water vapour levels inside the subelytral cavity. In addition, the O(2) concentration of the air within the subelytral cavity is lower than the air around the elytral case, irrespective of the time of sampling. These findings lead us to conclude that the subelytral spiracles work in a coordinated fashion with the anterior spiracles to create a DGC, which allows us to extend the hypothesis of the function of the subelytral cavity as a respiratory water-saving device.

Mass, Temperature and Metabolic Effects on Discontinuous Gas Exchange Cycles in Eucalyptus-Boring Beetles (Coleoptera: Cerambycidae)

Ventilatory accommodation of changing metabolic rates is a relatively little-studied aspect of the discontinuous gas exchange cycles (DGCs) that occur in a wide variety of terrestrial arthropods. We used correlation analysis of resting metabolic rate (RMR, measured as the rate of CO(2) emission; V(CO2)) and several components of the DGC to examine accommodation to both temperature-induced changes and individual variation in RMR in two wood-boring beetles (Phorocantha recurva and P. semipunctata; Coleoptera: Cerambycidae). At low to moderate ambient temperatures (T(a); 10-20 degrees C), Phorocantha spp. displayed a characteristic DGC with relatively brief but pronounced open (O) phase bursts of CO(2) emission separated by longer periods of low V(CO2), the flutter (F) phase. However, the V(CO2) never fell to zero, and we could not reliably differentiate a typical closed (C) phase from the F phase. Accordingly, we pooled the C and F phases for analysis as the C+F phase. At higher T(a) (30 degrees C), the duration of the combined C+F phase was greatly reduced. There were no differences between the two species or between males and females in either RMR or characteristics of the DGC. We found large variation in the major DGC components (cycle frequency, durations and emission volumes of the O and C+F phases); much of this variation was significantly repeatable. Accommodation of temperature-induced RMR changes was almost entirely due to changes in frequency (primarily in the C+F phase), as has been found in several other discontinuously ventilating arthropods. Frequency changes also contributed to accommodation at constant T(a), but modulation of emission volumes (during both O and C+F phases) played a larger role in this case. The DGC is often viewed as a water conservation mechanism, on the basis that respiratory evaporation is minimal during the C and F phases. This hypothesis assumes that the F phase is primarily convective (because of a reduction in tracheal P(O2) and total intratracheal pressure during the C phase). To test this, we measured the DGC in beetles subjected to varying degrees of hypoxia in addition to normoxia. As predicted for a largely diffusive F phase, we found an increase in the volume of CO(2) emitted during the C+F phase in hypoxic conditions (10.4 % oxygen). This finding, together with a reduced tendency to utilize a DGC at high T(a) (when water stress is greatest) and a natural history in which water availability is probably not limiting for any life stage, suggests that a reduction of respiratory evaporation may not have been critical in the evolution of the DGC of Phorocantha spp. Instead, selection may have favored discontinuous ventilation because it facilitates gas exchange in the hypercapnic and hypoxic environments commonly encountered by animals (such as Phorocantha spp.) that live in confined spaces.

Discontinuous gas exchange cycles in aphodius fossor (Scarabaeidae): a test of hypotheses concerning origins and mechanisms.

It has been suggested that discontinuous gas exchange cycles (DGCs) in insects evolved in response to hypoxic and/or hypercapnic environments, and that the flutter phase of the DGC is predominantly diffusive. We tested these hypotheses by examining the rate of CO(2) release at a declining series of oxygen partial pressures in adults of Aphodius fossor, an inhabitant of moist to wet dung pats that can have low oxygen and high carbon dioxide concentrations. This mesic species showed a pronounced DGC, but progressively abandoned it when exposed to declining oxygen concentrations. That is, in response to a reduction in oxygen concentration, the closed phase declined in length and eventually disappeared, the flutter phase declined in length, while the flutter phase rate of CO(2) release increased, the open phase peak rate of CO(2) release declined and DGC frequency increased. Nonetheless, overall CO(2) release rate remained virtually unchanged. Thus, it appears that the flutter phase in A. fossor has a significant convective component. In addition, in response to declining oxygen concentrations, the spiracles are opened for prolonged periods to increase gas exchange. This behaviour is unlikely to incur a significant water loss penalty in the moist habitats in which A. fossor lives. Given the switch to continuous ventilation by A. fossor under hypoxic conditions, we conclude that current adaptive explanations for the DGC are inadequate. Rather, the DGC may be the default state of a system regulated by two interacting feedback loops when demands are absent.

Discontinuous ventilation and energetics of locomotion in the desert‐dwelling female mutillid wasp, Dasymutilla gloriosa

Abstract. Data on the discontinuous ventilation cycle and cost of pedestrian locomotion in female Dasymutilla gloriosa (Sauss.), a desert‐dwelling mutillid, are described and compared with equivalent data from other Hymenoptera. The discontinuous ventilation cycle was intermediate between that found in xeric and mesic hymenopterans, with the open phase being about 20% of the cycle. No noticeable flutter phase was observed. Thus D. gloriosa does not attempt to reduce respiratory water loss to the same extent as found in other desert dwelling Hymenoptera. The minimum cost of transport was significantly higher than that obtained for several ant species, indicating that ants are probably more efficient runners than any other Hymenoptera.

Ventilation in Cataglyphis bicolor: Regulation of carbon dioxide release from the thoracic and abdominal spiracles

Using several different experimental approaches, we show that the discontinuous ventilation cycle (DVC) in the arid-adapted ant Cataglyphis bicolor is of the passive suction ventilation (PSV) type. Gas flow is tidal, without significant active ventilation. Direct observations of spiracular activity and recordings of endotracheal pressure with simultaneous gas exchange measurements are fully consistent with this ventilation regime. Separate simultaneous recordings of the thoracic and abdominal spiracles show that they have distinct functions and capabilities. The thoracic spiracles act as high-capacity gateways to the tracheal system and are responsible for some 80–90% of overall gas exchange when the ant is active. At rest, a single open mesothoracic spiracle can meet an ant's oxygen uptake and carbon dioxide release requirements. The abdominal spiracles combined have less than half the diffusive capacity of a single thoracic spiracle (although they can meet the gas exchange requirements of an ant at rest) and their contribution to overall gas exchange is minor. However, they are important during the Flutter (F) phase of the DVC, especially in its early stages, before the thoracic spiracles take over (the thoracic and abdominal spiracles exhibit inverse modulation during the F phase).

Discontinuous Ventilation in a Non-Insect, The Tick Amblyomma Marmoreum (Acari, Ixodidae): Characterization and Metabolic Modulation

ABSTRACT We examined and quantified the discontinuous ventilation cycle (DVC) characteristics of unfed nymphs and adults, as well as engorged nymphal and engorged diapausing and non-diapausing female adult life-stages, of the African tortoise tick Amblyomma marmoreum (Koch). All engorged stages ventilated continuously, with little evidence of active spiracular control. Unfed nymphs and adults ventilated discontinuously; at low activity and standard metabolic rate (SMR) levels, mean DVC duration was approximately 0.4h in nymphs (mean mass 0.7mg) and 2.8h in female adults (mean mass 70mg). SMR, measured as rate of CO2 production (; 0.064 μl mg−1 h−1 and 0.019 μl mg−1 h−1, respectively), was almost tenfold lower than that estimated for spiders of equivalent mass. In adults, the DVC was modulated to accommodate changing chiefly by changes in DVC frequency. Modulation of other DVC characteristics was bimodal; at low (below the ‘SMR threshold’), burst volumes were large and not correlated with but the rate of CO2 emission during the burst was modulated by Above the SMR threshold, burst volumes were small and tightly correlated with . No fluttering-spiracle phase could be detected, but CO2 bursts were triggered at low volumes above the SMR threshold, suggesting that hypoxia in addition to hypercapnia may initiate the termination of DVCs in the burst phase, rather than initiating the flutter phase as in insects. To explain this bimodal modulation of the DVC by (and hence ) above and below the SMR threshold, we hypothesize that, below the SMR threshold, unfed ixodid ticks – with their very low SMR and large surface area/volume ratio – may obtain significant amounts of O2 by transcuticular or other putative non-spiracular avenues of O2 uptake (larval ticks obtain all their O2 in this way).

Ventilation and respiratory metabolism in the thermophilic desert ant, Cataglyphis bicolor (Hymenoptera, Formicidae)

The discontinuous ventilation cycle of the Saharan desert ant Cataglyphis bicolor was studied over the range 15–40°C, corresponding to a >2-fold increase in the rate of CO2 output and hence metabolic rate (Q10=2.1). Over this range, metabolic rate modulated only ventilation frequency; the volume of CO2 emitted per ventilation remained constant. The closed-spiracle phase accounted for a small, constant proportion (ca. 14%) of total CO2 output. In the flutter phase, the rate of CO2 output increased at a greater than exponential rate from 29% of total CO2 output at 15°C to 52% at 40°C. CO2 output rate in the ventilation phase increased, and its duration decreased, exponentially with temperature. Relative to total duration of discontinuous ventilation cycle, the length of each phase was constant over the entire range of metabolic rates measured. These data are the first thorough characterization of the effect of changing metabolic rate on all phases of the discontinuous ventilation cycle of an adult insect. Clearly, C. bicolor maximizes ventilation-phase emission volumes and enhances the contribution of the flutter phase to total CO2 release relative to other ants for which comparable data are available, and does so in ways that may reduce respiratory water loss rates.

Simultaneous measurement of oxygen uptake and carbon dioxide emission during discontinuous ventilation in the tok-tok beetle, Psammodes striatus

Rates of oxygen uptake and carbon dioxide emission during cycles of discontinuous ventilation by the tok-tok beetle, Psammodes striatus, were measured with computerized flowthrough respirometry. Mean long-term rates of oxygen uptake (LV̇O 2) and carbon dioxide emission (LV̇CO 2) were 0.160 ml g −1 h −1 and 0.130 ml g −1 h −1, respectively (mean of > = 10 discontinuous ventilation cycles in each of 8 beetles, mean mass 2.80 g). RQ over the entire ventilation cycle was 0.83. During the closed-spiracle (closed) phase of the cycle, oxygen and carbon dioxide exchange declined to very low levels. During the fluttering-spiracle (flutter) phase, short-term V̇O 2 (SV̇O 2) increased to ∼ LV̇O 2, thus meeting tissue oxygen requirements, while short-term V̇CO 2 (SV̇CO 2) was only 13% of LV̇CO 2. The flutter phase respiratory exchange ratio was 0.15. During the open-spiracle phase, this ratio increased to 1.27, carbon dioxide emission increased to 228% of LV̇CO 2, and oxygen uptake peaked rapidly and then settled to a second plateau marginally above LV̇O 2. Abdominal pulsations invariably occurred in the open phase, and cost 2% of total oxygen uptake. The flutter phase accounted for 33.9% and the open phase for 63.9% of total oxygen uptake; in terms of carbon dioxide emission, the flutter and open phases accounted for 5.7 and 93.1%, respectively.