Discontinuous gas exchange
Encyclopedia
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. Discontinuous gas exchange is traditionally defined in three phases, whose names reflect the behaviour of the spiracle
s: the closed phase, the flutter phase, and the open phase.
Until recently, insect respiration was believed to occur entirely by simple diffusion. It was believed that air entered the tracheae
through the spiracle
s, 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.
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
). 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
and are currently under debate.
s and are most commonly identified by their CO2
output, primarily observed using open flow respirometry
.
muscle
s 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. Independent of cycles of insect ventilation
which may be discontinuous, cellular respiration
on a whole animal
level continues at a constant rate. 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
. 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.
s open slightly and close in rapid succession. 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 spiracle
s are opened. However, the negative internal pressure
also prevents the liberation of CO2 from the haemolymph and its exportation through the tracheal system. 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
. 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
muscle
s and they are opened widely, initiating the open phase.
to the environment characterizes the open phase of discontinuous gas exchange cycles. During the open phase, spiracular muscle
s relax and the spiracle
s open completely. 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 spiracle
s. 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.
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. 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. 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. However, the effects of CO2 on both spiracles and the nervous system do not appear to be related to changes in pH
.
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. Environmental stimuli may affect one or more aspects of discontinuous cycling, such as cycle frequency
and the quantity of CO2 released at each burst. 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. At a species
-specific low temperature
discontinuous gas exchange cycles are known to cease entirely, as muscle function is lost and spiracle
s relax and open. The temperature at which muscular function is lost is known as the chill coma temperature.
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.
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). 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
.
is an adaptation
that allows terrestrial
insects to limit respiratory water loss to the environment. This hypothesis
is supported by studies that have demonstrated that respiratory water loss is substantially higher in insects forced to keep their spiracle
s open, than those of the same species who exhibit discontinuous gas exchange. In addition, laboratory selection experiments on Drosophila melanogaster
have shown that more variable gas exchange
patterns can emerge in population
s of insects artificially selected
for tolerance to dry conditions. 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. This has led to the belief that some other factor may have contributed to the evolution
of discontinuous gas exchange in insects.
s in 1995, doctors John Lighton and David Berrigan proposed the chthonic hypothesis. 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. 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. 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. 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.
to reduce the amount of O2
delivered to tissues under periods of low metabolic rate. 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. 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.
for the evolution of discontinuous gas exchange cycles. It was postulated that discontinuous gas exchange cycles and spiracle
s which close off the respiratory system
, may in part do so to prevent small arthropod
parasites
such as mite
s 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. The strolling arthropods hypothesis is supported by evidence that tracheal parasites can substantially limit O2 delivery to the flight muscle
s of active honeybees. As a result of large populations of tracheal
mite
s, honeybees are unable to reach metabolic rates in flight
muscle
necessary for flight
, and are grounded.
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
gas exchange
Gas exchange
Gas exchange is a process in biology where gases contained in an organism and atmosphere transfer or exchange. In human gas-exchange, gases contained in the blood of human bodies exchange with gases contained in the atmosphere. Human gas-exchange occurs in the lungs...
that have been documented primarily in insects; they occur when the insect is at rest. During DGC, oxygen (O2)
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
uptake and carbon dioxide (CO2)
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
release from the whole insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
follow a cyclical pattern characterized by periods of little to no release of CO2 to the external environment. Discontinuous gas exchange is traditionally defined in three phases, whose names reflect the behaviour of the spiracle
Spiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...
s: the closed phase, the flutter phase, and the open phase.
Until recently, insect respiration was believed to occur entirely by simple diffusion. It was believed that air entered the tracheae
Invertebrate trachea
The invertebrate trachea refers to the open respiratory system composed of spiracles, tracheae, and tracheoles that terrestrial arthropods have to transport metabolic gases to and from tissues....
through the spiracle
Spiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...
s, and diffused through the tracheal system to the tracheoles, whereupon O2
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
was delivered to the cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
. However, even at rest, insects show a wide variety of gas exchange
Gas exchange
Gas exchange is a process in biology where gases contained in an organism and atmosphere transfer or exchange. In human gas-exchange, gases contained in the blood of human bodies exchange with gases contained in the atmosphere. Human gas-exchange occurs in the lungs...
patterns, ranging from largely diffusive continuous ventilation, to cyclic respiration, of which discontinuous gas exchange cycles are the most striking.
Discontinuous gas exchange cycles have been described in over 50 insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
, most of which are large beetles (order Coleoptera) or butterflies or moths (order Lepidoptera
Lepidoptera
Lepidoptera is a large order of insects that includes moths and butterflies . It is one of the most widespread and widely recognizable insect orders in the world, encompassing moths and the three superfamilies of butterflies, skipper butterflies, and moth-butterflies...
). As the cycles have evolved more than once
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
within the insects, discontinuous gas exchange cycles are likely adaptive
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
, but the mechanisms and significance of their evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
and are currently under debate.
Phases
Discontinuous gas exchange cycles are characterized by a repeating pattern of three phases. These phases are named according to the behaviour of the spiracleSpiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...
s and are most commonly identified by their CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
output, primarily observed using open flow respirometry
Respirometry
Respirometry is a general term that encompass a number of techniques for obtaining estimates of the rates of metabolism of vertebrates, invertebrates, plants, tissues, cells, or microorganisms via an indirect measure of heat production ....
.
Closed phase
During the closed phase of discontinuous gas exchange cycles, the spiracleSpiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...
muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s contract, causing the spiracles to shut tight. At the initiation of the closed phase, the partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
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. Independent of cycles of insect ventilation
Ventilation (physiology)
In respiratory physiology, ventilation is the rate at which gas enters or leaves the lung. It is categorized under the following definitions:-Sample values:...
which may be discontinuous, cellular respiration
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...
on a whole animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
level continues at a constant rate. As O2 is consumed, its partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
decreases within the tracheal system. In contrast, as CO2 is produced by the cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
, it is buffered
Buffer solution
A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. It has the property that the pH of the solution changes very little when a small amount of strong acid or base is added to it. Buffer solutions are used as a...
in the haemolymph rather than being exported to the tracheal system
Invertebrate trachea
The invertebrate trachea refers to the open respiratory system composed of spiracles, tracheae, and tracheoles that terrestrial arthropods have to transport metabolic gases to and from tissues....
. This mismatch between O2 consumption and CO2 production within the tracheal system
Invertebrate trachea
The invertebrate trachea refers to the open respiratory system composed of spiracles, tracheae, and tracheoles that terrestrial arthropods have to transport metabolic gases to and from tissues....
leads to a negative pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
inside the system relative to the external environment. Once the partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of O2 in the tracheal system
Invertebrate trachea
The invertebrate trachea refers to the open respiratory system composed of spiracles, tracheae, and tracheoles that terrestrial arthropods have to transport metabolic gases to and from tissues....
drops below a lower limit, activity in the nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
causes the initiation of the flutter phase.
Flutter phase
During the flutter phase of discontinuous gas exchange cycles, spiracleSpiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...
s open slightly and close in rapid succession. As a result of the negative pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
within the tracheal system, created during the closed phase, a small amount of air from the environment enters the respiratory system
Respiratory system
The respiratory system is the anatomical system of an organism that introduces respiratory gases to the interior and performs gas exchange. In humans and other mammals, the anatomical features of the respiratory system include airways, lungs, and the respiratory muscles...
each time the spiracle
Spiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...
s are opened. However, the negative internal pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
also prevents the liberation of CO2 from the haemolymph and its exportation through the tracheal system. As a result, during the flutter phase, additional O2 from the environment is acquired to satisfy cellular
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
O2 demand, while little to no CO2 is released. The flutter phase may continue even after tracheal pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
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
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
. The flutter phase continues until CO2 production surpasses the buffering capacity
Buffer solution
A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. It has the property that the pH of the solution changes very little when a small amount of strong acid or base is added to it. Buffer solutions are used as a...
of the haemolymph and begins to build up within the tracheal system
Invertebrate trachea
The invertebrate trachea refers to the open respiratory system composed of spiracles, tracheae, and tracheoles that terrestrial arthropods have to transport metabolic gases to and from tissues....
. CO2 within the tracheal system
Invertebrate trachea
The invertebrate trachea refers to the open respiratory system composed of spiracles, tracheae, and tracheoles that terrestrial arthropods have to transport metabolic gases to and from tissues....
has both a direct (acting on the muscle tissue
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
) and indirect (through the nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
) impact on the spiracle
Spiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...
muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s and they are opened widely, initiating the open phase.
Open phase
A rapid release of CO2Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
to the environment characterizes the open phase of discontinuous gas exchange cycles. During the open phase, spiracular muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s relax and the spiracle
Spiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...
s open completely. The open phase may initiate a single, rapid release of CO2, or several spikes declining in amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
with time as a result of the repeated opening and closing of the spiracle
Spiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...
s. During the open phase, a complete exchange of gases with the environment occurs entirely by diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
in some species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
, but may be assisted by active ventilatory movements in others.
Variability in discontinuous gas exchange cycles
The great variation in insectInsect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
respiratory cycles can largely be explained by differences in spiracle
Spiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...
function, body size and metabolic rate. Gas exchange
Gas exchange
Gas exchange is a process in biology where gases contained in an organism and atmosphere transfer or exchange. In human gas-exchange, gases contained in the blood of human bodies exchange with gases contained in the atmosphere. Human gas-exchange occurs in the lungs...
may occur through a single open spiracle, or the coordination of several spiracles. Spiracle function is controlled almost entirely by the nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
. 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. 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. However, the effects of CO2 on both spiracles and the nervous system do not appear to be related to changes in pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
.
Variability in discontinuous gas exchange cycles is also dependent upon external stimuli
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....
such as temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
and the partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of O2
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
in the external environment. Environmental stimuli may affect one or more aspects of discontinuous cycling, such as cycle frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
and the quantity of CO2 released at each burst. Temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
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. At a species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
-specific low temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
discontinuous gas exchange cycles are known to cease entirely, as muscle function is lost and spiracle
Spiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...
s relax and open. The temperature at which muscular function is lost is known as the chill coma temperature.
Discontinuous gas exchange cycles vary widely among different species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of insects, and these differences have been used in the past to support or refute hypotheses
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
of the evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
of respiratory cycling in insects.
Evolution of discontinuous gas exchange cycles
Despite being well described, the mechanisms responsible for the evolutionEvolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
of discontinuous gas exchange cycles are largely unknown. Discontinuous gas exchange cycles have long been thought to be an adaptation
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
to conserve water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
when living in a terrestrial
Terrestrial animal
Terrestrial animals are animals that live predominantly or entirely on land , as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats...
environment (the hygric hypothesis). However, recent studies question the hygric hypothesis, and several alternative hypotheses have been proposed. For discontinuous gas exchange cycles to be considered adaptive
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
, the origin and subsequent persistence of the trait
Trait (biology)
A trait is a distinct variant of a phenotypic character of an organism that may be inherited, environmentally determined or be a combination of the two...
must be demonstrated to be a result of natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
.
Hygric hypothesis
The hygric hypothesis was first proposed in 1953, making it the earliest posed hypothesis for the evolution of discontinuous gas exchange. The hygric hypothesis proposes that the discontinuous release of CO2Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
is an adaptation
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
that allows terrestrial
Terrestrial animal
Terrestrial animals are animals that live predominantly or entirely on land , as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats...
insects to limit respiratory water loss to the environment. This hypothesis
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
is supported by studies that have demonstrated that respiratory water loss is substantially higher in insects forced to keep their spiracle
Spiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...
s open, than those of the same species who exhibit discontinuous gas exchange. In addition, laboratory selection experiments on Drosophila melanogaster
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...
have shown that more variable gas exchange
Gas exchange
Gas exchange is a process in biology where gases contained in an organism and atmosphere transfer or exchange. In human gas-exchange, gases contained in the blood of human bodies exchange with gases contained in the atmosphere. Human gas-exchange occurs in the lungs...
patterns can emerge in population
Population
A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...
s of insects artificially selected
Artificial selection
Artificial selection describes intentional breeding for certain traits, or combination of traits. The term was utilized by Charles Darwin in contrast to natural selection, in which the differential reproduction of organisms with certain traits is attributed to improved survival or reproductive...
for tolerance to dry conditions. However, water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
loss during discontinuous gas exchange is only limited during the flutter phase if gas exchange during the flutter phase is convective
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
(or assisted by muscular contraction
Muscle contraction
Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same...
). From a water conservation perspective, if ventilation
Ventilation (physiology)
In respiratory physiology, ventilation is the rate at which gas enters or leaves the lung. It is categorized under the following definitions:-Sample values:...
during the flutter phase occurs entirely by simple diffusion, there is no benefit to having a flutter phase. This has led to the belief that some other factor may have contributed to the evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
of discontinuous gas exchange in insects.
Chthonic and chthonic-hygric hypotheses
Following work on harvester antMessor (ant genus)
Messor is a myrmicine genus of ants with more than 100 species, all of which are harvester ants; the generic name comes from the Roman god of crops and harvest, Messor. The subterranean colonies tend to be found in open fields and near roadsides, openings are directly to the surface...
s in 1995, doctors John Lighton and David Berrigan proposed the chthonic hypothesis. It was observed that many insects that demonstrate discontinuous gas exchange cycles are exposed to hypoxia
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
(low O2 levels) and hypercapnia
Hypercapnia
Hypercapnia or hypercapnea , also known as hypercarbia, is a condition where there is too much carbon dioxide in the blood...
(high CO2 levels) by spending at least part of their life cycle
Biological life cycle
A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...
in enclosed spaces underground. Lighton and Berrigan hypothesized that discontinuous gas exchange cycles may be an adaptation
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
to maximize partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
gradients between an insect’s respiratory system and the environment in which it lives. Alternatively, insects could obtain enough O2 by opening their spiracles for extended periods of time. However, unless their environment is very humid
Humidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
, water will be lost from the respiratory system to the environment. 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
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
gradients alone and consider the chthonic hypothesis to be distinct from the hygric hypothesis.
Oxidative damage hypothesis
The oxidative damage hypothesis states that discontinuous gas exchange cycles are an adaptationAdaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
to reduce the amount of O2
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
delivered to tissues under periods of low metabolic rate. During the open phase, O2 partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
in the tracheal system
Invertebrate trachea
The invertebrate trachea refers to the open respiratory system composed of spiracles, tracheae, and tracheoles that terrestrial arthropods have to transport metabolic gases to and from tissues....
reaches levels near that of the external environment. However, over time during the closed phase the partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of O2 drops, limiting the overall exposure of tissues to O2 over time. 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
Symbiotic bacteria
Symbiotic bacteria are bacteria living in symbiosis with another organism or each other. For example, Zoamastogopera, found in the stomach of termites, enable them to digest cellulose....
demonstrate continuous, diffusive
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
ventilation.
Strolling arthropods hypothesis
The strolling arthropods hypothesis was a very early hypothesisHypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
for the evolution of discontinuous gas exchange cycles. It was postulated that discontinuous gas exchange cycles and spiracle
Spiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...
s which close off the respiratory system
Respiratory system of insects
The respiratory system of insects is the means by which insects obtain oxygen from the air.-The Respiratory System:Air enters the respiratory systems of most insects through a series of external openings called spiracles. These external openings, which act as muscular valves in some insects, lead...
, may in part do so to prevent small arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
parasites
Parasitism
Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...
such as mite
Mite
Mites, along with ticks, are small arthropods belonging to the subclass Acari and the class Arachnida. The scientific discipline devoted to the study of ticks and mites is called acarology.-Diversity and systematics:...
s and particulate matter such as dust
Dust
Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...
from entering the respiratory system. This hypothesis
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
was largely dismissed in the 1970s, but has recently gained additional attention. The strolling arthropods hypothesis is supported by evidence that tracheal parasites can substantially limit O2 delivery to the flight muscle
Insect flight
Insects are the only group of invertebrates known to have evolved flight. Insects possess some remarkable flight characteristics and abilities, still far superior to attempts by humans to replicate their capabilities. Even our understanding of the aerodynamics of flexible, flapping wings and how...
s of active honeybees. As a result of large populations of tracheal
Invertebrate trachea
The invertebrate trachea refers to the open respiratory system composed of spiracles, tracheae, and tracheoles that terrestrial arthropods have to transport metabolic gases to and from tissues....
mite
Mite
Mites, along with ticks, are small arthropods belonging to the subclass Acari and the class Arachnida. The scientific discipline devoted to the study of ticks and mites is called acarology.-Diversity and systematics:...
s, honeybees are unable to reach metabolic rates in flight
Insect flight
Insects are the only group of invertebrates known to have evolved flight. Insects possess some remarkable flight characteristics and abilities, still far superior to attempts by humans to replicate their capabilities. Even our understanding of the aerodynamics of flexible, flapping wings and how...
muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
necessary for flight
Insect flight
Insects are the only group of invertebrates known to have evolved flight. Insects possess some remarkable flight characteristics and abilities, still far superior to attempts by humans to replicate their capabilities. Even our understanding of the aerodynamics of flexible, flapping wings and how...
, and are grounded.