E-Book, Englisch, Band 49, 186 Seiten, Format (B × H): 152 mm x 229 mm
Jurenka Advances in Insect Physiology
1. Auflage 2015
ISBN: 978-0-12-802679-3
Verlag: Academic Press
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)
E-Book, Englisch, Band 49, 186 Seiten, Format (B × H): 152 mm x 229 mm
Reihe: Advances in Insect Physiology
ISBN: 978-0-12-802679-3
Verlag: Academic Press
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)
Advances in Insect Physiology provides readers with the latest interdisciplinary reviews on the topic. It is an essential reference source for invertebrate physiologists, neurobiologists, entomologists, zoologists, and insect chemists.
- Contains important, comprehensive, and in-depth reviews on insect physiology
- Provides an essential reference source for invertebrate physiologists and neurobiologists, entomologists, zoologists, and insect biochemists.
- First published in 1963, this serial is ranked second in the highly competitive ISI category of entomology
Zielgruppe
Entomologists, zoologists, insect biochemists, insect physiologists
Fachgebiete
Weitere Infos & Material
- Evolution of the Mechanisms Underlying Insect Respiratory Gas Exchange
Philip G. D. Matthews and John S. Terblanche
- Physiological and Molecular Mechanisms of Nutrition in Honey Bees
Ying Wang and Hongmei Li-Byarlay
- Antennal Mechanosensors and their Evolutionary Antecedents
Anand Krishnan and Sanjay P. Sane
- Comb Architecture of the Eusocial Bees Arises from Simple Rules Used During Cell Building
Benjamin P. Oldroyd and Stephen C. Pratt
- The Prxamide Neuropeptide Signaling System: Conserved in Animals
Russell Jurenka
Chapter One Evolution of the Mechanisms Underlying Insect Respiratory Gas Exchange
Philip G.D. Matthews*; John S. Terblanche† * Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada
† Department of Conservation Ecology and Entomology, Stellenbosch University, Stellenbosch, South Africa Abstract
Many factors influence gas exchange patterns in insects and are generally treated in isolation from one another. Here, we provide a review of the current state of knowledge on the physics of gas exchange, insect respiratory chemoreceptors, the diversity and the methods typically used in the characterisation of respiratory pattern types, briefly covering some of the new tools and techniques that are being incorporated into this field. We then discuss the functional significance of insect gas exchange pattern variation, and possible evolutionary explanations of discontinuous gas exchange as a derived control mechanism for effecting physiological change in the context of (a) adaptive hypotheses, (b) non-adaptive hypotheses and (c) mathematical modelling of gas exchange. The lack of consensus in the literature for all proposed adaptive or mechanistic hypotheses suggests that multiple factors influence which gas exchange pattern is displayed by any particular insect during a given experiment. Thus, while the primary function of a breathing pattern is to meet an animal's gas exchange requirements, it is an interacting hierarchy of constraints that most likely determines how this demand may be met. We conclude the review with a brief discussion of future directions for the field. Keywords Discontinuous gas exchange cycles Periodic ventilation Convection Diffusion Respiration Chemoreceptors Model Metabolic downregulation 1 Introduction
Insects were among the very first terrestrial organisms on Earth, with current phylogenomic evidence indicating that they first arose in the early Ordovician period, over 479 million years ago (Misof et al., 2014). Their enormous success on land and subsequent colonisation and expansion into all terrestrial habitats, with the exception of the polar regions, were made possible by the evolution of a suite of adaptations to cope with the many challenges associated with life in air. Not least among these was the evolution of an air-filled respiratory system—the tracheal system (Fig. 1). The tracheal system is a network of air-filled tubes that develops from invaginations of the cuticular exoskeleton. These tubes subdivide and proliferate throughout the insect's body, providing a continuous air-filled lumen for the rapid movement of oxygen (O2) and carbon dioxide (CO2) between the insect's cells and the atmosphere. It is made up of collapsible air sacs, as well as tracheal tubes that range in size from the large tracheal trunks that run the length of the insect's body and communicate with the atmosphere through pores (spiracles) in the insect's cuticle, all the way down to the terminal branches of the blind-ending microscopic (> 1 µm diameter) tracheoles that pervade the insect's tissues. Stereological studies on locusts have shown that while trachea and air sacs comprise the majority of the intratracheal volume (> 50%), they are not the primary gas exchange surface. Rather, they are the conduits that carry respiratory gases from the atmosphere to the tracheoles. While the tracheoles comprise only 13% of the intratracheal volume, their high surface area-to-volume ratio allows them to provide more than 90% of the tracheal system's lateral diffusing capacity (Snelling et al., 2011). Figure 1 X-ray 3D CT scan of a silkworm (Bombyx mori) larval tracheal system at a resolution of 0.0189 mm. (Note scale bar shows 3.5 mm.) Figure kindly provided by Leigh Boardman with technical assistance from Anton Du Plessis, Central Analytical Facilities at Stellenbosch University. Like most animal life, insects produce the majority of the ATP energy they require through the glycolytic pathway of aerobic respiration (Raven and Johnson, 2002). This process occurs primarily in the mitochondria, which require a constant supply of O2 to be delivered from the surrounding environment in order for electrons to be passed down the electron transport chain embedded in the mitochondrial inner membrane and ultimately for the phosphorylation of ADP to ATP to occur. Just as O2 must be delivered continuously to the respiring mitochondria, so too must the CO2 produced as a by-product of the Krebs cycle be removed continuously from the respiring tissues and released into the environment. The tracheal system fulfils both of these tasks by providing a bidirectional conduit for both O2 and CO2 to move between the atmosphere and the mitochondria located within the insect's cells. The rate at which O2 must be supplied to, and CO2 removed from, respiring tissues is determined by their metabolic demand. This, in turn, varies with exogenous and endogenous factors, including activity (i.e. movement, growth, digestion, etc.) and body temperature. 2 The Physics of Gas Exchange
The tracheal system is a ‘direct-delivery’ respiratory system, allowing the movement of respiratory gases between the atmosphere and cells without requiring an intermediate circulatory system. Early work on insect gas exchange proposed that small insects may have sufficiently low metabolic rates (MRs) and have small diffusion pathways, such that diffusion alone may be sufficient to meet all gas flux needs (Harrison et al., 2012). While this may in fact be the case for some small insects (Lighton, 1988) or insect eggs (Woods and Hill, 2004), larger insects likely rely on a combination of convection and diffusion to meet gas flux requirements: diffusion across the tracheal wall and within the blind-ending tracheoles, and a mixture of diffusion and convection in the tracheae and air sacs (Kestler, 1985). The diffusion of O2 and CO2 along the length of the blind-ending tracheoles, and across the walls of the tracheoles to the respiring tissues, occurs due to the random molecular movement of each gas species from a region of high partial pressure to low partial pressure, down a partial pressure gradient. Each gas species in a mixture of gases exerts its own partial pressure—the pressure that it would exert if it alone occupied the same volume of the total gas mixture at the same temperature. Described by the Fick equation, the rate at which diffusion occurs is proportional to the conductance of the diffusion pathway and the magnitude of the partial pressure gradient across it: ?x=KxAX?Px, (1) where Vx is the diffusion rate of gas x, Kx is Krogh's coefficient for the diffusivity of gas x through the material being considered (air or tracheal wall), A is the area of the diffusion pathway, X is the length of the diffusion pathway and ?Px is the partial pressure difference of gas x across the diffusion pathway. The product of Kx and A/X is the conductance of the gas exchange pathway (Gx), and in the case of the tracheoles, can be considered to be a fixed property of the tracheal morphology. There is some indication that X can be varied dynamically over the medium term by filling the tips of the tracheoles with fluid (Wigglesworth, 1935), or developmentally over the long term by increasing the total number of branches during development (Jarecki et al., 1999). But given that the conductance of the tracheoles is largely fixed at any given moment, the rate of gas exchange ?O2/V?CO2 can only be varied in response to changing gas-exchange demands by changing ?Px, the partial pressure gradient driving diffusion along their length. The maximum partial pressure gradient available to drive the diffusion of O2 to the cells is fixed by the atmospheric PO2 (the O2 source) and occurs when the PO2 in the mitochondria (the O2 sink) is effectively zero. However, the diffusion distance between the atmosphere and the tips of the tracheoles can be reduced by convectively flushing air through the tracheae. This bulk flow of air through the tracheae reduces the distance that gases must diffuse between the atmosphere and tracheoles as well as increasing the PO2 in the air sacs and tracheae to near atmospheric levels (Matthews et al., 2012). This increases the partial pressure gradient along the length of the tracheole while decreasing the length of the diffusion-only pathway from the atmosphere to the tip of the tracheole. The convective movement of air within the tracheal system is achieved by active muscular movements of the body that can be coordinated with the opening and closing of the spiracles. Oscillations in haemolymph pressure within the insect brought about by the contraction and relaxation of the body wall muscles serves to collapse the flexible walls of the air sacs as well as sections of the larger trachea (Socha et al., 2008; Westneat et al., 2003). When this activity occurs in synchrony with the opening and closing of the spiracles, insects can...
