Tracheal volume in the pupa of the Saturniid moth Hyalophora cecropia determined with inert gases

A micro-CT approach for determination of insect respiratory volume

Variation in the morphology of the insect tracheal system can strongly affect respiratory physiology, with implications for everything from pest control to evolution of insect body size. However, the small size of most insects has made measuring the morphology of their tracheal systems difficult. Historical approaches including light microscopy and scanning and transmission electron microscopy (SEM, TEM) are technically difficult, labor intensive, and can introduce preparation artifacts. More recently, synchrotron X-ray microtomography (SR-μCT) has allowed for detailed analysis of tracheal morphology of diverse insects. However, linear accelerators required for SR-μCT are not readily available, making the approach unavailable for most labs. Recent advancements in microcomputed tomography (μCT) have made possible fine resolution of internal morphology of very small insects. However, μCT has never been used to quantify insect tracheal system dimensions. We measured respiratory volume of a grasshopper (Schistocerca americana) by analysis of high resolution μCT scans. Volume estimates from μCT closely matched volume estimates by water displacement as well as literature estimates for this species. The μCT approach may thus provide a widely available, cost-effective, and straightforward approach to characterizing the internal morphology of insect respiratory systems.

Respiration patterns of resting wasps (Vespula sp.)

We investigated the respiration patterns of wasps (Vespula sp.) in their viable temperature range (2.9-42.4°C) by measuring CO2 production and locomotor and endothermic activity. Wasps showed cycles of an interburst-burst type at low ambient temperatures (Ta<5°C) or typical discontinuous gas exchange patterns with closed, flutter and open phases. At high Ta of >31°C, CO2 emission became cyclic. With rising Ta they enhanced CO2-emission primarily by an exponential increase in respiration frequency, from 2.6 mHz at 4.7°C to 74 mHz at 39.7°C. In the same range of Ta CO2 release per cycle decreased from 38.9 to 26.4 μl g(-1)cycle(-1). A comparison of wasps with other insects showed that they are among the insects with a low respiratory frequency at a given resting metabolic rate (RMR), and a relatively flat increase of respiratory frequency with RMR. CO2 emission was always accompanied by abdominal respiration movements in all open phases and in 71.4% of the flutter phases, often accompanied by body movements. Results suggest that resting wasps gain their highly efficient gas exchange to a considerable extent via the length and type of respiration movements.

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 P(O(2)) and P(CO(2)) 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 P(O(2)) and P(CO(2)) 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 P(CO(2)) the minimum P(O(2)) resulting in open spiracles was linearly dependent upon P(CO(2)). Above a threshold of 1-1.5 kPa CO(2) the spiracles were open irrespective of P(O(2)). 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.

Issues of convection in insect respiration: insights from synchrotron X-ray imaging and beyond

While it has long been known that in small animals, such as insects, sufficient gas transport could be provided by diffusion, it is now recognized that animals generate and control convective flows to improve oxygen delivery across a range of body sizes and taxa. However, size-based methodological limitations have constrained our understanding of the mechanisms that underlie the production of these convective flows. Recently, new techniques have enabled the elucidation of the anatomical structures and physiological processes that contribute to creating and maintaining bulk flow in small animals. In particular, synchrotron X-ray imaging provides unprecedented spatial and temporal resolution of internal functional morphology and is changing the way we understand gas exchange in insects. This symposium highlights recent efforts towards understanding the relationship between form, function, and control in the insect respiratory system.

Cavitation in the embryonic tracheal system of Manduca sexta

Insect tracheae form during embryonic development and initially contain liquid, which impedes transport of oxygen and carbon dioxide. Only later do tracheae fill with gas and come to support high rates of gas flux. This liquid-to-gas transition is poorly understood. Using eggs of the sphingid moth Manduca sexta, we show that longitudinal tracheae in embryos fill with gas in less than 5 s, without invasion of external air, by a process of cavitation. Cavitation requires that tracheal liquids be under tension, and we propose two complementary processes for generating it. One likely, classical mechanism is tracheolar fluid absorption, first proposed by Wigglesworth. Our data support this mechanism in Manduca: after cavitation, liquids are progressively drawn out of finer tracheal branches. The second, previously unknown, mechanism is evaporative water loss across the eggshell, which leads both to declining egg volume and to a larger negative pressure potential of water. The pressure potential helps to drive rapid expansion of small bubbles nucleated near spiracles. Once bubbles are large enough to have displaced liquid across the diameter of a trachea, negative capillary pressure reinforces subsequent expansion of the bubble. Together with predictions from modern cavitation theory, our observations substantiate Wigglesworth's contention that gas filling is promoted by increasing hydrophobicity associated with tanning of the spiracles and major tracheal branches.

Intraspecific variation in tracheal volume in the American locust,Schistocerca americana, measured by a new inert gas method

The volume of a tracheal system influences breath-holding capacity and provides an index of an insect's investment in its respiratory system. Here,we describe a new, generally applicable method to measure tracheal volume that enables repeatable determinations on live animals. Animals are isolated in a closed chamber of a known volume and equilibrated with a helium:oxygen gas mixture. The chamber is then rapidly flushed with a nitrogen:oxygen gas mixture to eliminate the helium surrounding the animal, and sealed. After a period of time sufficient to allow equilibration of helium between tracheal system and chamber air, a gas sample is taken from the chamber, and tracheal volumes are calculated from the helium content of the sample, using a gas chromatograph. We show that relative investment in the tracheal system increases with age/size in the grasshopper; tracheal volume scales with mass to the power 1.3. This increased proportional investment in the tracheal system provides a mechanistic basis for the enhanced respiratory capacity of older grasshoppers. Tracheal volumes decrease strongly as grasshoppers grow within an instar stage, explaining reduced safety margins for oxygen delivery. Finally, tracheal volumes are smaller in gravid females than males, probably due to compression of air sacs by eggs.

The role of the mesothoracic spiracles in respiration in flighted and flightless dung beetles

The relative role of the mesothoracic and abdominal spiracles in respiration was examined using flow-through respirometry in four dung beetle species from different habitats. Two species of flightless beetles, Scarabaeus (Pachysoma) gariepinus and Scarabaeus (Pachysoma) striatum, from the arid western region of southern Africa and a large flighted species, Pachylomerus femoralis, from a more mesic habitat were compared with Circellium bacchus, a flightless beetle from a low rainfall eastern area. All species showed a form of the discontinuous gas exchange pattern at rest. The mesic flighted species used a closed, flutter, open, cycle (CFO) while those species from more arid habitats used a closed, ventilation, cycle (CV) or a closed, burst cycle (CB). The relative importance of the mesothoracic spiracles in CO(2) emission varied between the species, even between those from the same genus and habitat. C. bacchus and P. femoralis represent extremes of CO(2) emission from the mesothoracic spiracles; from almost total to almost none, respectively. Overall, mesothoracic CO(2) emission and convection were more pronounced in the dry habitat species, supporting the hypothesis that both strategies aid in the reduction of water loss.

Metabolism of the sub-Antarctic caterpillar Pringleophaga marioni during cooling, freezing and thawing

Although general models of the processes involved in insect survival of freezing exist, there have been few studies directly investigating physiological processes during cooling, freezing and thawing, without which these models remain hypothetical. Here, we use open-flow respirometry to investigate the metabolism of the freeze-tolerant sub-Antarctic caterpillar Pringleophaga marioni Viette (Lepidoptera: Tineidae) during cooling, freezing and thawing and to compare animals exposed to non-lethal (-5.8 degrees C) and lethal (-6.0 degrees C, after which caterpillars are moribund for several days, and -18 degrees C, after which caterpillars are completely unresponsive) freezing stress. We found a large decrease in metabolic rate (that is not associated with freezing) at -0.6+/-0.1 degrees C and calculated a Q10 of 2.14 x 10(3) at this breakpoint. This breakpoint is coincident with the critical thermal minimum (CTmin) and is hypothesised to be a metabolic manifestation of the latter, possibly a failure of the Na+/K(+)-ATPase pump. This provides a plausible link between processes at the cellular level and observations of the action of the CTmin at tissue and whole-organism levels. Caterpillars froze at -4.6+/-0.1 degrees C and had detectable metabolism when frozen. Post-thaw, metabolic rates were lower than pre-freezing measurements. Post-thaw metabolic rates did not differ between temperatures that did and did not kill the caterpillars, which suggests that mortality may be a result of a breakdown in processes at the organismal, rather than cellular, level of organisation.

Stereological determination of tracheal volume and diffusing capacity of the tracheal walls in the stick insect Carausius morosus (Phasmatodea, Lonchodidae)

First instars of Carausius morosus provide a good model for morphometric evaluation of the diffusing capacity between the tracheal system and hemolymph: air sacs are lacking, tracheoles do not penetrate the organs and muscles, and entire animals can be evaluated electron microscopically without subsampling. The tracheal volume makes up 1.3% of the volume of the whole insect excluding appendages. We calculated the lateral diffusing capacity for oxygen and carbon dioxide for five classes of tracheae according to their diameters, from 0.2 microm to 35 microm. The harmonic mean thickness of the tracheal epithelium is lowest in smallest tracheae and increases with increasing tracheal diameter. Although the smallest tracheae make up 70% (O2) and 60% (CO2) of the total diffusing capacity, the proximal four classes may also be significant in diffusion of oxygen and particularly of carbon dioxide. The suppression of the development of respiratory pigments in the evolution of terrestrial insects may have increased the relative importance of small tracheal elements for local oxygen consumption.

Scanning and transmission electron microscopic study of the tracheal air sac system in a grasshopper Chrotogonus senegalensis (Kraus)--Orthoptera: Acrididae: Pyrgomorphinae

The morphology of the trachea-air sac system in a species of grasshopper Chrotogonus senegalensis has been studied by using scanning and transmission electron microscopes. Capacious air sacs were formed as dilatations along the primary tracheal trunks. Narrower secondary trachea arose either directly from the primary trachea that bypassed the air sacs or from the air sacs themselves. At or close to the organ or tissue supplied with air, the secondary trachea gave rise to the notably smaller tertiary trachea that penetrated the tissue, giving rise terminally to the extremely small tracheoles that indent some cells. The trachea and the air sacs were basically made up of an inner cuticular lining, helical taenidial rings, and an overlying epithelial cell cover. The air sacs may be important in efficient ventilation of the respiratory system. The supply of air directly to the tissue cells was viewed as an exemplary efficient design when compared to that prevailing in the nontracheate air-breathing animals, where the vascular system is interposed between the respiratory organ and the target tissue cells. A similarity in the general morphological design of the insect and avian respiratory systems has been observed, mainly in respect to the presence of the air sacs and that of the respiratory shunts. This, together with the reported functional features like the unidirectional mode of ventilation, has been interpreted as a classic case of structural and functional convergent evolution leading to the evolution of similar and comparably efficient respiratory systems capable of providing the large amount of oxygen demanded by flight.

A morphometric study of the tracheal system of Peripatus acacioi Marcus and Marcus (Onychophora)

Volumes and surfaces of the tracheal system in Peripatus acacioi Marcus and Marcus (Onychophora) were evaluated using stereological techniques. The onychophorans were divided into three portions, arbitrarily denominated lead, body and tail segments. This procedure was used to provide information on the spatial distribution of tracheal tubes along the body. The mean weight specific tracheal volume for a 212 mg animal (average body weight) was 7.27 microliters g-1. Size appears to affect tracheal volume; the surface density of the tracheal system was greater in both lead and tail segments. There appears to be a positive correlation between tracheal surface density and tissue metabolic activity.

Oxygen supply and limiting oxygen pressures in an insect larva

Larvae of the moth, Carpocapsa saltitans, demonstrate a diurnal activity pattern of rhythmic twitching which, under conditions of controlled light and temperature, is characterized by a predictable frequency and regularity. The twitching activity is shown to be sensitive to the partial pressure of environmental oxygen, and it ceases altogether at a particular PO2 called 'critical'. Use is made of the 'critical' PO2 in normobaric and hypobaric conditions to deduce the roles of diffusion and convection in the larval oxygen transport mechanisms; and also as a value for the total decrement of PO2 from ambient air to mitochondria, in order to evaluate predicted values based on calculations of resistance to oxygen flow. For this latter study 'porosity' of the larva and the seed pod in which it is normally housed was inferred from measured rates of water vapor loss, and oxygen uptake rates of the larvae were measured by the manometric technique of Warburg. Applying these data to a model system the conclusion was reached that almost the total resistance to oxygen flow is at the spiracle.

Buffering and CO2 dissociation of body fluids in the pupa of the silkworm moth, Hyalophora cecropia

To assess the extent of CO2 storage and the changes in the acid-base status that occur during intermittent CO2 excretion in insects, total CO2 content and pH were measured in whole body homogenates (= tissue homogenates) of Hyalophora cecropia pupae at various levels of Pco2 at 20 degrees C. The CO2 dissociation curve, i.e. plot of total CO2 content in tissue homogenates against Pco2 was nearly linear in the Pco2 range from 15 to 50 Torr, the mean slope being 0.138 mM . Torr-1. This value, which constitutes the effective CO2 solubility, was nearly three times the physical solubility in the tissue homogenate which averaged 0.053 mM . Torr-1. Plots of bicarbonate concentration in whole body tissue water against pH yielded an average buffer value of 75 mmol . pH-1 per kg tissue water. The high buffer value results in a small pH change, about 0.04 units, when Pco2 varies between 20 and 45 Torr in the respiratory cycle. The absolute value of mean tissue pH at Pco2 = 30 Torr predicted from the buffer line, 6.57, agrees well with direct measurement in hemolymph samples.