Discontinuous gas exchange

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Discontinuous gas-exchange cycles (DGC), also called discontinuous ventilation or discontinuous ventilatory cycles, follow one of several patterns of arthropod gas exchange that have been documented primarily in insects; they occur when the insect is at rest. During DGC, oxygen (O2) uptake and carbon dioxide (CO2) release from the whole insect follow a cyclical pattern characterized by periods of little to no release of CO2 to the external environment. [1] Discontinuous gas exchange is traditionally defined in three phases, whose names reflect the behaviour of the spiracles: the closed phase, the flutter phase, and the open phase. [2]

Contents

Until recently, insect respiration was believed to occur entirely by simple diffusion. It was believed that air entered the tracheae through the spiracles, and diffused through the tracheal system to the tracheoles, whereupon O2 was delivered to the cells. However, even at rest, insects show a wide variety of gas exchange patterns, ranging from largely diffusive continuous ventilation, to cyclic respiration, of which discontinuous gas exchange cycles are the most striking. [3]

Discontinuous gas exchange cycles have been described in over 50 insect species, most of which are large beetles (order Coleoptera) or butterflies or moths (order Lepidoptera). [2] As the cycles have evolved more than once within the insects, discontinuous gas exchange cycles are likely adaptive, but the mechanisms and significance of their evolution are currently under debate. [2]

Phases

Discontinuous gas exchange cycles are characterized by a repeating pattern of three phases. These phases are named according to the behaviour of the spiracles and are most commonly identified by their CO2 output, primarily observed using open flow respirometry. [2]

Closed phase

During the closed phase of discontinuous gas exchange cycles, the spiracle muscles contract, causing the spiracles to shut tight. At the initiation of the closed phase, the partial pressure of both O2 and CO2 is close to that of the external environment, but closure of the spiracles drastically reduces the capacity for the exchange of gases with the external environment. [2] Independent of cycles of insect ventilation which may be discontinuous, cellular respiration on a whole animal level continues at a constant rate. [1] As O2 is consumed, its partial pressure decreases within the tracheal system. In contrast, as CO2 is produced by the cells, it is buffered in the haemolymph rather than being exported to the tracheal system. [1] This mismatch between O2 consumption and CO2 production within the tracheal system leads to a negative pressure inside the system relative to the external environment. Once the partial pressure of O2 in the tracheal system drops below a lower limit, activity in the nervous system causes the initiation of the flutter phase. [2]

Flutter phase

During the flutter phase of discontinuous gas exchange cycles, spiracles open slightly and close in rapid succession. [2] As a result of the negative pressure within the tracheal system, created during the closed phase, a small amount of air from the environment enters the respiratory system each time the spiracles are opened. However, the negative internal pressure also prevents the liberation of CO2 from the haemolymph and its exportation through the tracheal system. [1] As a result, during the flutter phase, additional O2 from the environment is acquired to satisfy cellular O2 demand, while little to no CO2 is released. The flutter phase may continue even after tracheal pressure is equal to that of the environment, and the acquisition of O2 may be assisted in some insects by active ventilatory movements such as contraction of the abdomen. [2] The flutter phase continues until CO2 production surpasses the buffering capacity of the haemolymph and begins to build up within the tracheal system. CO2 within the tracheal system has both a direct (acting on the muscle tissue) and indirect (through the nervous system) impact on the spiracle muscles and they are opened widely, initiating the open phase. [2]

Open phase

A rapid release of CO2 to the environment characterizes the open phase of discontinuous gas exchange cycles. During the open phase, spiracular muscles relax and the spiracles open completely. [2] The open phase may initiate a single, rapid release of CO2, or several spikes declining in amplitude with time as a result of the repeated opening and closing of the spiracles. During the open phase, a complete exchange of gases with the environment occurs entirely by diffusion in some species, but may be assisted by active ventilatory movements in others. [1]

Variability in discontinuous gas exchange cycles

The great variation in insect respiratory cycles can largely be explained by differences in spiracle function, body size and metabolic rate. Gas exchange may occur through a single open spiracle, or the coordination of several spiracles. [2] Spiracle function is controlled almost entirely by the nervous system. In most insects that demonstrate discontinuous gas exchange, spiracle movements and active ventilation are closely coordinated by the nervous system to generate unidirectional air flow within the tracheal system. [2] This coordination leads to the highly regulated bursting pattern of CO2 release. Building CO2 levels during the flutter phase may either directly affect spiracular opening, affect the nervous system while being pumped through the haemolymph, or both. [1] However, the effects of CO2 on both spiracles and the nervous system do not appear to be related to changes in pH. [1]

Variability in discontinuous gas exchange cycles is also dependent upon external stimuli such as temperature and the partial pressure of O2 and CO2 in the external environment. [1] Environmental stimuli may affect one or more aspects of discontinuous cycling, such as cycle frequency and the quantity of CO2 released at each burst. [1] Temperature can have massive effects on the metabolic rate of ectothermic animals, and changes in metabolic rate can create large differences in discontinuous gas exchange cycles. [1] At a species-specific low temperature discontinuous gas exchange cycles are known to cease entirely, as muscle function is lost and spiracles relax and open. The temperature at which muscular function is lost is known as the chill coma temperature. [4]

Discontinuous gas exchange cycles vary widely among different species of insects, and these differences have been used in the past to support or refute hypotheses of the evolution of respiratory cycling in insects.

Evolution of discontinuous gas exchange cycles

Despite being well described, the mechanisms responsible for the evolution of discontinuous gas exchange cycles are largely unknown. Discontinuous gas exchange cycles have long been thought to be an adaptation to conserve water when living in a terrestrial environment (the hygric hypothesis). [2] However, recent studies question the hygric hypothesis, and several alternative hypotheses have been proposed. For discontinuous gas exchange cycles to be considered adaptive, the origin and subsequent persistence of the trait must be demonstrated to be a result of natural selection. [2]

Hygric hypothesis

The hygric hypothesis was first proposed in 1953, making it the earliest posed hypothesis for the evolution of discontinuous gas exchange. [1] The hygric hypothesis proposes that the discontinuous release of CO2 is an adaptation that allows terrestrial insects to limit respiratory water loss to the environment. [5] This hypothesis is supported by studies that have demonstrated that respiratory water loss is substantially higher in insects forced to keep their spiracles open, than those of the same species who exhibit discontinuous gas exchange. [5] In addition, laboratory selection experiments on Drosophila melanogaster have shown that more variable gas exchange patterns can emerge in populations of insects artificially selected for tolerance to dry conditions. [6] However, water loss during discontinuous gas exchange is only limited during the flutter phase if gas exchange during the flutter phase is convective (or assisted by muscular contraction). From a water conservation perspective, if ventilation during the flutter phase occurs entirely by simple diffusion, there is no benefit to having a flutter phase. [1] This has led to the belief that some other factor may have contributed to the evolution of discontinuous gas exchange in insects.

Chthonic and chthonic-hygric hypotheses

Following work on harvester ants in 1995, doctors John Lighton and David Berrigan proposed the chthonic hypothesis. [7] It was observed that many insects that demonstrate discontinuous gas exchange cycles are exposed to hypoxia (low O2 levels) and hypercapnia (high CO2 levels) by spending at least part of their life cycle in enclosed spaces underground. [1] Lighton and Berrigan hypothesized that discontinuous gas exchange cycles may be an adaptation to maximize partial pressure gradients between an insect’s respiratory system and the environment in which it lives. [7] Alternatively, insects could obtain enough O2 by opening their spiracles for extended periods of time. However, unless their environment is very humid, water will be lost from the respiratory system to the environment. [2] Discontinuous gas exchange cycles, therefore, may limit water loss while facilitating O2 consumption and CO2 removal in such environments. Many researchers describe this theory as the chthonic-hygric hypothesis and consider it to support the hygric hypothesis. However, others emphasize the importance of maximizing partial pressure gradients alone and consider the chthonic hypothesis to be distinct from the hygric hypothesis. [2]

Oxidative damage hypothesis

The oxidative damage hypothesis states that discontinuous gas exchange cycles are an adaptation to reduce the amount of O2 delivered to tissues under periods of low metabolic rate. [1] During the open phase, O2 partial pressure in the tracheal system reaches levels near that of the external environment. However, over time during the closed phase the partial pressure of O2 drops, limiting the overall exposure of tissues to O2 over time. [2] This would lead to the expectation of prolonged flutter periods in insects that may be particularly sensitive to high levels of O2 within the body. Strangely however, termites that carry a highly oxygen-sensitive symbiotic bacteria demonstrate continuous, diffusive ventilation. [8]

Strolling arthropods hypothesis

The strolling arthropods hypothesis was a very early hypothesis for the evolution of discontinuous gas exchange cycles. [2] It was postulated that discontinuous gas exchange cycles and spiracles which close off the respiratory system, may in part do so to prevent small arthropod parasites such as mites and particulate matter such as dust from entering the respiratory system. This hypothesis was largely dismissed in the 1970s, but has recently gained additional attention. [2] The strolling arthropods hypothesis is supported by evidence that tracheal parasites can substantially limit O2 delivery to the flight muscles of active honeybees. As a result of large populations of tracheal mites, honeybees are unable to reach metabolic rates in flight muscle necessary for flight, and are grounded. [9]

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Respiratory system Biological system in animals and plants for gas exchange

The respiratory system is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In land animals the respiratory surface is internalized as linings of the lungs. Gas exchange in the lungs occurs in millions of small air sacs; in mammals and reptiles these are called alveoli, and in birds they are known as atria. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood. These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the trachea, which branches in the middle of the chest into the two main bronchi. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the bronchioles. In birds the bronchioles are termed parabronchi. It is the bronchioles, or parabronchi that generally open into the microscopic alveoli in mammals and atria in birds. Air has to be pumped from the environment into the alveoli or atria by the process of breathing which involves the muscles of respiration.

Trachea Cartilaginous tube that connects the pharynx and larynx to the lungs

The trachea, also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air-breathing animals with lungs. The trachea extends from the larynx and branches into the two primary bronchi. At the top of the trachea the cricoid cartilage attaches it to the larynx. The trachea is formed by a number of horseshoe-shaped rings, joined together vertically by overlying ligaments, and by the trachealis muscle at their ends. The epiglottis closes the opening to the larynx during swallowing.

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Hypercapnia Abnormally high tissue carbon dioxide levels

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Insect physiology includes the physiology and biochemistry of insect organ systems.

Opiliones anatomy

Opiliones are an order of arachnids and share many common characteristics with other arachnids. However, several differences separate harvestmen from other arachnid orders such as spiders. The bodies of opiliones are divided into two tagmata : the abdomen (opisthosoma) and the cephalothorax (prosoma). Unlike spiders, the juncture between the abdomen and cephalothorax is often poorly defined. Harvestmen have chelicerae, pedipalps and four pairs of legs. Most harvestmen have two eyes, although there are eyeless species.

Breathing Process of moving air into and out of the lungs

Breathing is the process of moving air out and in the lungs to facilitate gas exchange with the internal environment, mostly to flush out carbon dioxide and bring in oxygen.

Insect morphology Description of the physical form of insects

Insect morphology is the study and description of the physical form of insects. The terminology used to describe insects is similar to that used for other arthropods due to their shared evolutionary history. Three physical features separate insects from other arthropods: they have a body divided into three regions, have three pairs of legs, and mouthparts located outside of the head capsule. It is this position of the mouthparts which divides them from their closest relatives, the non-insect hexapods, which includes Protura, Diplura, and Collembola.

Respiratory system of insects

An insect's respiratory system is the system with which it introduces respiratory gases to its interior and performs gas exchange.

Spiracle (arthropods)

A spiracle or stigma is the opening in the exoskeletons of insects and some spiders to allow air to enter the trachea. In the respiratory system of insects, the tracheal tubes primarily deliver oxygen directly into the animals' tissues. The spiracles can be opened and closed in an efficient manner to reduce water loss. This is done by contracting closer muscles surrounding the spiracle. In order to open, the muscle relaxes. The closer muscle is controlled by the central nervous system, but can also react to localized chemical stimuli. Several aquatic insects have similar or alternative closing methods to prevent water from entering the trachea. The timing and duration of spiracle closures can affect the respiratory rates of the organism. Spiracles may also be surrounded by hairs to minimize bulk air movement around the opening, and thus minimize water loss.

References

  1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Chown, S.L.; S.W. Nicholson (2004). Insect Physiological Ecology. New York: Oxford University Press. ISBN   0-19-851549-9.
  2. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Chown SL, Gibbs AG, Hetz SK, Klok CJ, Lighton JR, Marias E (2006). "Discontinuous gas exchange in insects: a clarification of hypotheses and approaches". Physiological and Biochemical Zoology. 79 (2): 333–343. doi:10.1086/499992. PMID   16555192.
  3. ↑ Nation, J.L. (2002). Insect Physiology and Biochemistry. Boca Raton: C.R.C. Press. ISBN   0-8493-1181-0.
  4. ↑ Lighton JR, Lovegrove BG (1990). "A temperature-induced switch from diffusive to convective ventilation in the honeybee" (PDF). Journal of Experimental Biology. 154: 509–516.
  5. 1 2 Chown SL (2002). "Respiratory water loss in insects". Comparative Biochemistry and Physiology A. 133: 791–804. doi:10.1016/s1095-6433(02)00200-3. PMID   12443935.
  6. ↑ Williams AE, Rose MR, Bradley TJ (1997). "CO2 release patterns in Drosophila melanogaster: the effect of selection for desiccation resistance". Journal of Experimental Biology. 200 (Pt 3): 615–624. PMID   9057311.
  7. 1 2 Lighton JR, Berrigan D (1995). "Questioning paradigms: caste-specific ventiation in harvester ants, Messor pergandei and M. julianus (Hymenoptera: Formicidae)". Journal of Experimental Biology. 198 (Pt 2): 521–530. PMID   9318205.
  8. ↑ Lighton JRB (2007). "Respiratory Biology: why insects evolved discontinuous gas exchange". Current Biology. 17 (16): R645–647. doi: 10.1016/j.cub.2007.06.007 . PMID   17714655.
  9. ↑ Harrison JF, Camazine S, Marden JH, Kirkton SD, Rozo A, Yang X (1995). "Mite not make it home: tracheal mites reduce the safety margin for oxygen delivery of flying honeybees". Journal of Experimental Biology. 204 (Pt 4): 805–13. PMID   11171363.
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