Buoyancy regulation in insects.

Multiple insect lineages have successfully reinvaded the aquatic environment, evolving to complete either part or all of their life cycle submerged in water. While these insects vary in their reliance on atmospheric oxygen, with many having the ability to extract dissolved oxygen directly from the water, all retain an internal air-filled respiratory system, their tracheal system, due to their terrestrial origins. However, carrying air within their tracheal system, and even augmenting this volume with additional air bubbles carried on their body, dramatically increases their buoyancy which can make it challenging to remain submerged. But by manipulating this air volume a few aquatic insects can deliberately alter or regulate their position in the water column. Unlike cephalopods and teleost fish that control the volume of gas within their hydrostatic organs by either using osmosis to pull liquid from a rigid chamber or secreting oxygen at high pressure to inflate a flexible chamber, insects have evolved hydrostatic control mechanisms that rely either on the temporary stabilization of a compressible air-bubble volume using O2 unloaded from hemoglobin, or the mechanical expansion and contraction of a gas-filled volume with rigid, gas-permeable walls. The ability to increase their buoyancy while submerged separates aquatic insects from the buoyancy compensation achieved by other air-breathing aquatic animals which also use air within their respiratory systems to offset their submerged weight. The mechanisms they have evolved to achieve this are unique and provide new insights into the function and evolution of mechanochemical systems.

Born suckers: the cibarial pump of Philaenus spumarius scales across ontogeny to ensure functional equivalence.

A select group of hemipterans within the suborder Auchenorrhyncha are the only animals that feed exclusively on xylem sap - a nutritionally poor liquid that exists under negative pressure within a plant's xylem vessels. To consume it, xylem-feeding bugs have evolved enlarged cibarial pumps capable of generating enormous negative pressures. A previous study examining the allometry of this feeding model suggested that small xylem feeders pay relatively higher energetic costs while feeding, favouring the evolution of larger-bodied species. However, this interspecific analysis only considered adult xylem-feeding insects and neglected the considerable intraspecific change in size that occurs across the insect's development. Here, we examine the changes in cibarial pump morphology and function that occur during the development of Philaenus spumarius, the common meadow spittlebug. We show that the cibarial pump scales largely as expected from isometry and that the maximum negative pressure is mass independent, indicating that size has no effect on the xylem-feeding capacity of juvenile spittlebugs. We conclude that a first instar nymph with a body mass 2% of the adult can still feed at the >1 MPa tension present in a plant's xylem vessels without a substantial energetic disadvantage.

Oxygen extraction efficiency of the tidally-ventilated rectal gills of dragonfly nymphs.

Dragonfly nymphs breathe water using tidal ventilation, a highly unusual strategy in water-breathing animals owing to the high viscosity, density and low oxygen (O2) concentration of water. This study examines how well these insects extract O2 from the surrounding water during progressive hypoxia. Nymphs were attached to a custom-designed respiro-spirometer to simultaneously measure tidal volume, ventilation frequency and metabolic rate. Oxygen extraction efficiencies (OEE) were calculated across four partial pressure of oxygen (pO2) treatments, from normoxia to severe hypoxia. While there was no significant change in tidal volume, ventilation frequency increased significantly from 9.4 ± 1.2 breaths per minute (BPM) at 21.3 kPa to 35.6 ± 2.9 BPM at 5.3 kPa. Metabolic rate increased significantly from 1.4 ± 0.3 µl O2 min-1 at 21.3 kPa to 2.1 ± 0.4 µl O2 min-1 at 16.0 kPa, but then returned to normoxic levels as O2 levels declined further. OEE of nymphs was 40.1 ± 6.1% at 21.3 kPa, and did not change significantly during hypoxia. Comparison to literature shows that nymphs maintain their OEE during hypoxia unlike other aquatic tidal-breathers and some unidirectional breathers. This result, and numerical models simulating experimental conditions, indicate that nymphs maintain these extraction efficiencies by increasing gill conductance and/or lowering internal pO2 to maintain a sufficient diffusion gradient across their respiratory surface.

Air sacs are a key adaptive trait of the insect respiratory system.

Air sacs are a well-known aspect of insect tracheal systems, but have received little research attention. In this Commentary, we suggest that the study of the distribution and function of air sacs in tracheate arthropods can provide insights of broad significance. We provide preliminary phylogenetic evidence that the developmental pathways for creation of air sacs are broadly conserved throughout the arthropods, and that possession of air sacs is strongly associated with a few traits, including the capacity for powerful flight, large body or appendage size and buoyancy control. We also discuss how tracheal compression can serve as an additional mechanism for achieving advection in tracheal systems. Together, these patterns suggest that the possession of air sacs has both benefits and costs that remain poorly understood. New technologies for visualization and functional analysis of tracheal systems provide exciting approaches for investigations that will be of broad significance for understanding invertebrate evolution.

A pH-powered mechanochemical engine regulates the buoyancy of Chaoborus midge larvae.

The freshwater aquatic larvae of the Chaoborus midge are the world's only truly planktonic insects, regulating their buoyancy using two pairs of internal air-filled sacs, one in the thorax and the other in the seventh abdominal segment. In 1911, August Krogh demonstrated the larvae's ability to control their buoyancy by exposing them to an increase in hydrostatic pressure.1 However, how these insects control the volume of their air-sacs has remained a mystery. Gas is not secreted into the air-sacs, as the luminal gas composition is always the same as that dissolved in the surrounding water.1,2 Instead, the air-sac wall was thought to play some role.3-6 Here we reveal that bands of resilin in the air-sac's wall are responsible for the changes in volume. These bands expand and contract in response to changes in pH generated by an endothelium that envelops the air-sac. Vacuolar type H+ V-ATPase (VHA) in the endothelium acidifies and shrinks the air-sac, while alkalinization and expansion are regulated by the cyclic adenosine monophosphate signal transduction pathway. Thus, Chaoborus air-sacs function as mechanochemical engines, transforming pH changes into mechanical work against hydrostatic pressure. As the resilin bands interlaminate with bands of cuticle, changes in resilin volume are constrained to a single direction along the air-sac's longitudinal axis. This makes the air-sac functionally equivalent to a cross-striated pH muscle and demonstrates a unique biological role for resilin as an active structural element.

Discontinuous gas exchange in Madagascar hissing cockroaches is not a consequence of hysteresis around a fixed PCO2 threshold.

It has been hypothesised that insects display discontinuous gas-exchange cycles (DGCs) as a result of hysteresis in their ventilatory control, where CO2-sensitive respiratory chemoreceptors respond to changes in haemolymph PCO2 only after some delay. If correct, DGCs would be a manifestation of an unstable feedback loop between chemoreceptors and ventilation, causing PCO2 to oscillate around some fixed threshold value: PCO2 above this ventilatory threshold would stimulate excessive hyperventilation, driving PCO2 below the threshold and causing a subsequent apnoea. This hypothesis was tested by implanting micro-optodes into the haemocoel of Madagascar hissing cockroaches and measuring haemolymph PO2 and PCO2 simultaneously during continuous and discontinuous gas exchange. The mean haemolymph PCO2 of 1.9 kPa measured during continuous gas exchange was assumed to represent the threshold level stimulating ventilation, and this was compared with PCO2 levels recorded during DGCs elicited by decapitation. Cockroaches were also exposed to hypoxic (PO2 10 kPa) and hypercapnic (PCO2 2 kPa) gas mixtures to manipulate haemolymph PO2 and PCO2. Decapitated cockroaches maintained DGCs even when their haemolymph PCO2 was forced above or below the putative ∼2 kPa ventilation threshold, demonstrating that the characteristic oscillation between apnoea and gas exchange is not driven by a lag between changing haemolymph PCO2 and a PCO2 chemoreceptor with a fixed ventilatory threshold. However, it was observed that the gas exchange periods within the DGC were altered to enhance O2 uptake and CO2 release during hypoxia and hypercapnia exposure. This indicates that while respiratory chemoreceptors do modulate ventilatory activity in response to haemolymph gas levels, their role in initiating or terminating the gas exchange periods within the DGC remains unclear.

Economies of scale shape energetics of solitary and group-living spiders and their webs.

Metabolic scaling, whereby larger individuals use less energy per unit mass than smaller ones, may apply to the combined metabolic rate of group-living organisms as group size increases. Spiders that form groups in high disturbance environments can serve to test the hypothesis that economies of scale benefit social groups. Using solitary and group-living spiders, we tested the hypothesis that spiders exhibit negative allometry between body or colony mass and the standing mass of their webs and whether, and how, such a relationship may contribute to group-living benefits in a cooperative spider. Given the diverse architecture of spider webs-orb, tangle and sheet-and-tangle, and associated differences in silk content, we first assessed how standing web mass scales with spider mass as a function of web architecture and whether investment in silk differs among web types. As group-living spiders are predominantly found in clades that build the presumably costlier sheet-and-tangle webs, we then asked whether cost-sharing through cooperative web maintenance contributes to a positive energy budget in a social species. We found that larger spiders had a relatively smaller investment in silk per unit mass than smaller ones, but more complex sheet-and-tangle webs contained orders of magnitude more silk than simpler orb or tangle ones. In the group-living species, standing web mass per unit spider mass continued to decline as colony size increased with a similar slope as for unitary spiders. When web maintenance activities were considered, colonies also experienced reduced mass-specific energy expenditure with increasing colony size. Activity savings contributed to a net positive energy balance for medium and large colonies after inputs from the cooperative capture of large prey were accounted for. Economies of scale have been previously demonstrated in animal societies characterized by reproductive and worker castes, but not in relatively egalitarian societies as those of social spiders. Our findings illustrate the universality of scaling laws and how economies of scale may transcend hunting strategies and levels of organization.

From chemoreception to regulation: filling the gaps in understanding how insects control gas exchange.

Insects coordinate the opening and closing of spiracles with convective ventilatory movements to produce considerable intraspecific and interspecific variation in gas exchange patterns. But fundamental questions remain regarding how these movements are coordinated and modulated by central and peripheral respiratory chemoreceptors, and where these chemoreceptors are located and how they function. Recent findings have revealed regions of the CNS that generate coordinated respiratory motor activity, while peripheral neurons sensitive to respiratory gases have been identified in Drosophila. Importantly, plasticity in structure and function of neural elements of respiratory control indicate the need for caution when generalizing the mechanistic basis for breathing in insects, and an adaptive explanation for breathing pattern variability.

The cibarial pump of the xylem-feeding froghopper Philaenus spumarius produces negative pressures exceeding 1 MPa.

The xylem sap of vascular plants is an unlikely source of nutrition, being both nutrient poor and held under tensions (negative pressures) that can exceed 1 MPa. But some insects feed on xylem sap exclusively, extracting copious quantities using a muscular cibarial pump. However, neither the strength of the insect's suction, nor the direct energetic cost of xylem ingestion, have ever been quantified. Philaenus spumarius froghoppers were used to address these gaps in our knowledge. Micro-CT scans of its cibarium and measurements of cibarial muscle sarcomere length revealed that P. spumarius can generate a maximum tension of 1.3 ± 0.2 MPa within its cibarium. The energetic cost of xylem extraction was quantified using respirometry to measure the metabolic rate (MR) of P. spumarius while they fed on hydroponically grown legumes, while xylem sap excretion rate and cibarial pumping frequency were simultaneously recorded. Increasing the plants' xylem tensions up to 1.1 MPa by exposing their roots to polyethylene glycol did not reduce the insects' rate of xylem excretion, but significantly increased both MR and pumping frequency. We conclude that P. spumarius can gain energy feeding on xylem sap containing previously reported energy densities and at xylem tensions up to their maximum suction capacity.

How insects transition from water to air: Respiratory insights from dragonflies.

The transition of animal life from water onto land is associated with well-documented changes in respiratory physiology and blood chemistry, including a dramatic increase in blood pCO2 and bicarbonate, and changes in ventilatory control. However, these changes have primarily been documented among ancestrally aquatic animal lineages that have evolved to breathe air. In contrast, the physiological consequences of air-breathing animals secondarily adopting aquatic gas exchange are not well explored. Insects are arguably the most successful air-breathing animals, but they have also re-evolved the ability to breathe water multiple times. The juvenile life stages of many insect lineages possess tracheal gills for aquatic gas exchange, but all shift back to breathing air in their adult form. This makes these amphibiotic insects an instructive contrast to most other animal groups, being not only an ancestrally air-breathing group of animals that have re-adapted to life in water, but also a group that undergoes an ontogenetic shift from water back to air across their life cycle. This graphical review summarizes the current knowledge on how blood acid-base balance and ventilatory control change in the dragonfly during its water-to-air transition, and highlights some of the remaining gaps to be filled.

A novel acidification mechanism for greatly enhanced oxygen supply to the fish retina.

Previously, we showed that the evolution of high acuity vision in fishes was directly associated with their unique pH-sensitive hemoglobins that allow O2 to be delivered to the retina at PO2s more than ten-fold that of arterial blood (Damsgaard et al., 2019). Here, we show strong evidence that vacuolar-type H+-ATPase and plasma-accessible carbonic anhydrase in the vascular structure supplying the retina act together to acidify the red blood cell leading to O2 secretion. In vivo data indicate that this pathway primarily affects the oxygenation of the inner retina involved in signal processing and transduction, and that the evolution of this pathway was tightly associated with the morphological expansion of the inner retina. We conclude that this mechanism for retinal oxygenation played a vital role in the adaptive evolution of vision in teleost fishes.

The bimodal gas exchange strategies of dragonfly nymphs across development.

Dragonfly nymphs are aquatic and breathe water using a rectal gill. However, it has long been known that the nymphs of many species appear to possess the ability to breathe air, either during their final instar when they leave the water prior to metamorphosis, or during periods of aquatic hypoxia. The aerial gas exchange associated with these activities has not been quantified. This study used flow-through respirometry to measure the rate of aerial CO2 release (V̇CO2) from dragonfly nymphs as a proxy for their aerial gas exchange, both across development and in response to progressive aquatic hypoxia. It examined a total of four species from two families (Libellulidae and Aeshnidae). In both families, the late-final instar nymphs developed functional mesothoracic spiracles, allowing them to breathe air by positioning their head and thorax above the water's surface. While breathing air in this position, the nymphs could also ventilate their submerged rectal gill. Thus, during bimodal gas exchange in normoxic water, it was calculated that aeshnid nymphs expelled 39% of their respiratory CO2 into the air through their spiracles, while libellulid nymphs expelled 56% into the air. Decreasing the aquatic PO2 to 2.5 kPa and then below 1 kPa increased the proportion of respiratory CO2 expelled into the air from 69% to 100%, respectively. Thus, bimodally breathing late-final nymphs can vary how they partition gas exchange between their spiracles and their gill depending on aquatic PO2. Aeshnid nymphs of all developmental stages were also found to use their rectal gill as an air-breathing organ; pre-final nymphs performing 'surface skimming' while late final nymphs aspirated air bubbles directly into their gill's branchial basket. Mass-specific rates of aerial V̇CO2 also increased as the nymphs approached metamorphosis. These findings indicate that aeshnid nymphs are capable of accessing aerial O2 across development using their rectal gill as an air breathing organ, while the aquatic nymphs of both aeshnid and libellulid dragonflies undergo a progressive shift towards using the atmosphere for respiration as they approach metamorphosis.

Quantifying the acid-base status of dragonflies across their transition from breathing water to breathing air.

Amphibiotic dragonflies show a significant increase in hemolymph total CO2 (TCO2) as they transition from breathing water to breathing air. This study examined the hemolymph acid-base status of dragonflies from two families (Aeshnidae and Libellulidae) as they transition from water to air. CO2 solubility (αCO2 ) and the apparent carbonic acid dissociation constant (pKapp) were determined in vitro, and pH/bicarbonate concentration ([HCO3-]) plots were produced by equilibrating hemolymph samples with PCO2  between 0.5 and 5 kPa in custom-built rotating microtonometers. Hemolymph αCO2  varied little between families and across development (mean 0.355±0.005 mmol l-1 kPa-1) while pKapp was between 6.23 and 6.27, similar to values determined for grasshopper hemolymph. However, the non-HCO3- buffer capacity for dragonfly hemolymph was uniformly low relative to that of other insects (3.6-5.4 mmol l-1 pH-1). While aeshnid dragonflies maintained this level as bimodally breathing late-final instars and air-breathing adults, the buffer capacity of bimodally breathing late-final instar Libellula nymphs increased substantially to 9.9 mmol l-1 pH-1 Using the pH/[HCO3-] plots and in vivo measurements of TCO2 and PCO2  from early-final instar nymphs, it was calculated that the in vivo hemolymph pH was 7.8 for an aeshnid nymph and 7.9 for a libellulid nymph. The pH/[HCO3-] plots show that the changes in acid-base status experienced by dragonflies across their development are more moderate than those seen in vertebrate amphibians. Whether these differences are due to dragonflies being secondarily aquatic, or arise from intrinsic differences between insect and vertebrate gas exchange and acid-base regulatory mechanisms, remains an open question.

Studies on gas exchange in the meadow spittlebug, Philaenus spumarius: the metabolic cost of feeding on, and living in, xylem sap.

Spittlebugs (superfamily Cercopoidea) live within a mass of frothy, spittle-like foam that is produced as a by-product of their xylem-feeding habits. The wet spittle represents a unique respiratory environment for an insect, potentially acting either as a reserve of trapped oxygen (O2) or as a significant barrier to O2 diffusion from the surrounding atmosphere. Feeding on xylem sap under tension is also assumed to be energetically expensive, potentially placing further constraints on their gas exchange. To understand the respiratory strategies used by spittlebugs, this study measured the PO2  within the spittle of the meadow spittlebug, Philaenus spumarius, as well as the non-feeding metabolic rate (RMR) and respiratory quotient (RQ) of both nymphs and adults. The metabolic rate of nymphs feeding on xylem was also measured. In separate experiments, the ability of a nymph to obtain O2 from bubbles while submerged in foam was determined using a glass microscope slide coated in an O2-sensitive fluorophore. We determined that P. spumarius breathes atmospheric O2 by extending the tip of its abdomen outside of its spittle, rather than respiring the O2 trapped in air bubbles within the foam. However, spittlebugs can temporarily use these air bubbles to breathe when forcibly submerged. V̇O2  and V̇CO2  did not differ statistically within life stages, giving a RQ of 0.92 for nymphs and 0.95 for adults. Feeding on xylem was found to increase the nymphs' V̇CO2  by only 20% above their RMR. From this cost of feeding, cibarial pump pressures were estimated to be between -0.05 and -0.26 MPa.

Metabolic fuel use after feeding in the zebrafish (Danio rerio): a respirometric analysis.

We used respirometric theory and a new respirometry apparatus to assess, for the first time, the sequential oxidation of the major metabolic fuels during the post-prandial period (10 h) in adult zebrafish fed with commercial pellets (51% protein, 2.12% ration). Compared with a fasted group, fed fish presented peak increases of oxygen consumption (78%), and carbon dioxide (80%) and nitrogen excretion rates (338%) at 7-8 h, and rates remained elevated at 10 h. The respiratory quotient increased slightly (0.89 to 0.97) whereas the nitrogen quotient increased greatly (0.072 to 0.140), representing peak amino acid/protein usage (52%) at this time. After 48-h fasting, endogenous carbohydrate and lipid were the major fuels, but in the first few hours after feeding, carbohydrate oxidation increased greatly, fueling the first part of the post-prandial specific dynamic action, whereas increased protein/amino acid usage predominated from 6 h onwards. Excess dietary protein/amino acids were preferentially metabolized for energy production.

Radical nephrectomy and intracaval thrombectomy for advanced renal cancer with extensive inferior vena cava involvement utilising cardiopulmonary bypass and hypothermic circulatory arrest: Is it worthwhile?

OBJECTIVE: To report our long-term outcomes of surgical treatment of renal tumours with inferior vena cava (IVC) tumour thrombus above the hepatic veins, utilising cardiopulmonary bypass (CBP) and hypothermic circulatory arrest (HCA), as surgical resection remains the only effective treatment for renal cancers with extensive IVC tumour thrombus. PATIENTS AND METHODS: We retrospectively reviewed 48 consecutive patients (median age 58 years) who underwent surgical treatment for non-metastatic renal cancer with IVC tumour thrombus extending above the hepatic veins. Perioperative, histological, disease-free (DFS) and overall survival (OS) data were recorded. RESULTS: Tumour thrombus was level III in 23 patients and level IV in 25 patients. The median (range) CBP and HCA times were 162 (120-300) min and 35 (9-64) min, respectively. Three patients underwent synchronous cardiac surgical procedures. There were three (6.3%) perioperative deaths. American Society of Anesthesiologists grade and perioperative blood transfusion requirement were significant factors associated with perioperative death (P < 0.05). Despite extensive preoperative screening for metastases the median (range) DFS was only 10.2 (1.2-224.4) months. The median (range) OS was 23 (0-224.4) months. Cox regression analysis revealed that perinephric fat invasion conferred a significantly poorer DFS (P = 0.005). CONCLUSIONS: Radical surgery for patients with extensive IVC tumour thrombus has acceptable operative morbidity and mortality. It provides symptom palliation and the possibility of long-term survival. Improvements in preoperative detection of occult metastasis may improve case selection and newer adjuvant therapies may improve survival in this high-risk group.

Changes in hemolymph total CO2 content during the water-to-air respiratory transition of amphibiotic dragonflies.

Dragonflies (Odonata, Anisoptera) are amphibiotic; the nymph is aquatic and breathes water using a rectal gill before metamorphosing into the winged adult, which breathes air through spiracles. While the evolutionary and developmental transition from water breathing to air breathing is known to be associated with a dramatic rise in internal CO2 levels, the changes in blood-gas composition experienced by amphibiotic insects, which represent an ancestral air-to-water transition, are unknown. This study measured total CO2 (TCO2) in hemolymph collected from aquatic nymphs and air-breathing adults of Anax junius, Aeshna multicolor (Aeshnidae), Libellula quadrimaculata and Libellulaforensis (Libellulidae). Hemolymph PCO2  was also measured in vivo in both aeshnid nymphs and marbled crayfish (Procambarus fallax. f. virginalis) using a novel fiber-optic CO2 sensor. The hemolymph TCO2 of the pre- and early-final instar nymphs was found to be significantly lower than that of the air-breathing adults. However, the TCO2 of the late-final instar aeshnid nymphs was not significantly different from that of the air-breathing adults, despite the late-final nymphs still breathing water. TCO2 and PCO2  were also significantly higher in the hemolymph of early-final aeshnid nymphs compared with values for the water-breathing crayfish. Thus, while dragonfly nymphs show an increase in internal CO2 as they transition from water to air, from an evolutionary standpoint, the nymph's ability to breathe water is associated with a comparatively minor decrease in hemolymph TCO2 relative to that of the air-breathing adult.

The transition from water to air in aeshnid dragonflies is associated with a change in ventilatory responses to hypoxia and hypercapnia.

Dragonflies are amphibiotic, spending most of their lives as aquatic nymphs before metamorphosing into terrestrial, winged imagoes. Both the nymph and the adult use rhythmic abdominal pumping movements to ventilate their gas exchange systems: the nymph tidally ventilates its rectal gill with water, while the imago pumps air into its tracheal system through its abdominal spiracles. The transition from water to air is known to be associated with changes in both respiratory chemosensitivity and ventilatory control in vertebrates and crustaceans, but the changes experienced by amphibiotic insects have been poorly explored. In this study, dragonfly nymphs (Anax junius) and imagoes (Anax junius and Aeshna multicolor) were exposed to hypoxia and hypercapnia while their abdominal ventilation frequency and amplitude was recorded. Water-breathing nymphs showed a significant increase in abdominal pumping frequency when breathing hypoxic water (<10 kPa O2), but no strong response to CO2, even in severe hypercapnia (up to 10 kPa CO2). In contrast, both species of air-breathing imago increased their abdominal pumping amplitude when exposed to either hypoxia or hypercapnia, but did not show any significant increase in frequency. These results demonstrate that aquatic dragonfly nymphs possess a respiratory sensitivity that is more like other water breathing animals, being sensitive to hypoxia but not hypercapnia, while their air-breathing adult form responds to both respiratory challenges, like other terrestrial insects. Shifting from ventilating a rectal gill with water to ventilating a tracheal system with air is also associated with a change in how abdominal ventilation is controlled; nymphs regulate gas exchange by varying frequency while imagoes respond by varying amplitude.

The mechanisms underlying the production of discontinuous gas exchange cycles in insects.

This review examines the control of gas exchange in insects, specifically examining what mechanisms could explain the emergence of discontinuous gas exchange cycles (DGCs). DGCs are gas exchange patterns consisting of alternating breath-hold periods and bouts of gas exchange. While all insects are capable of displaying a continuous pattern of gas exchange, this episodic pattern is known to occur within only some groups of insects and then only sporadically or during certain phases of their life cycle. Investigations into DGCs have tended to emphasise the role of chemosensory thresholds in triggering spiracle opening as critical for producing these gas exchange patterns. However, a chemosensory basis for episodic breathing also requires an as-of-yet unidentified hysteresis between internal respiratory stimuli, chemoreceptors, and the spiracles. What has been less appreciated is the role that the insect's central nervous system (CNS) might play in generating episodic patterns of ventilation. The active ventilation displayed by many insects during DGCs suggests that this pattern could be the product of directed control by the CNS rather than arising passively as a result of self-sustaining oscillations in internal oxygen and carbon dioxide levels. This paper attempts to summarise what is currently known about insect gas exchange regulation, examining the location and control of ventilatory pattern generators in the CNS, the influence of chemoreceptor feedback in the form of O2 and CO2/pH fluctuations in the haemolymph, and the role of state-dependent changes in CNS activity on ventilatory control. This information is placed in the context of what is currently known regarding the production of discontinuous gas exchange patterns.