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Both show lower heart rates but similar metabolic rates compared to salticid spiders that are active hunters and have well developed prosomal tracheae , , There are four hypotheses for the evolution of spider tracheae 14 , , , Scorpions possess four pairs of book lungs on opisthosomal segments 1 to 4 , Each lung ends in a small, slit like spiracle; ultrastructure is the same as in spiders. The heart does not lie near spiracles but lies in the mesosoma, and arteries supply the organs and appendages.
In the first instars, lungs consist of a few lamellae but are nonfunctional. These stages are inactive, sitting on the back of the mother, and gas exchange takes place across the thin cuticle of the entire body. In later instars, after the first moult, lungs take over gas exchange Scorpions such as spiders have low metabolic rates and use anaerobic metabolism during activity , Scorpions have more numerous lungs and a 1.
Consequently, the recovery phase is shorter in scorpions than in spiders Continuous respiration is found in Dinothrombium magnificum giant red velvet mite when not burrowed. When burrowed underground for long periods, standard metabolic rate is low and respiration becomes discontinuous Solifugae sun spiders and wind spiders are fast runners living in tropics or subtropics, reaching up to 7 cm in length. They possess impressive large and sharp fangs chelicerae and highly developed tracheae with seven occludable spiracles. There is one pair of spiracles in the prosoma, two pairs in the opisthosoma, and one in the opisthosoma.
Air sacs attach to the branching tracheae, and tracheoles penetrate the epithelia of internal organs Breathing becomes continuous during activity with active ventilation accomplished by contractions of the prosoma , These animals do not possess hemocyanin in the hemolymph. Therefore, terminal diffusion should be the dominant mechanism but further studies are necessary to test this hypothesis. The oldest known Devonian fossil harvestmen already show a tracheal system , There are two spiracles on the ventral prosoma behind the fourth leg pair. They open and close indirectly via muscles attached to the tracheal wall and the atrium behind the spiracles.
Movement of the coxae leg also influences spiracular opening.
Spiracles are protected by densely packed hairs trichomes or a deep invagination , , Long-legged harvestmen possess additional spiracles in the tibia of the legs. From ventral spiracles, tracheae and tracheoles extend into the prosoma. Air sacs are lacking and only very small tracheae enter the opisthosoma.
Oxygen Homeostasis and Its Dynamics - stadunmuluc.tk
Except the central nervous system and muscles, most organs are not penetrated by tracheae. The estimated tracheal diffusing capacity D O 2 of Nemastoma lugubre 1. Harvestmen are omnivores that feed continuously. Their metabolic rates are higher than spiders but lower than insects 9 , Constant slow locomotion is supported by tracheal gas exchange without O 2 debt 11 , Spontaneous walking raises O 2 consumption up to threefold, while constant treadmill running raises the value to fivefold During activity spurts, respiration is continuous , Hemocyanin concentration is lower than in spiders It was hypothesized that gas exchange results from a mixture of terminal and lateral diffusion.
The large tracheae serve mainly convective transport, while small tracheae and tracheoles serve in gas exchange and may transport O 2 directly into cells or release it into hemolymph. As the opisthosoma possesses few tracheae, in this body region hemolymph takes over gas transport by lateral diffusion. The larger tracheae also permit limited gas exchange by lateral diffusion.
Insects were thought to have evolved from terrestrial myriapods , but recent molecular data place them close to crustaceans , , , arising from a common ancestor with branchiopods fresh-water crustaceans, e. Terrestrialization may have occurred more than once Tracheal placodes and leg primordia in Drosophila arise from a common cell pool. Homologues of tracheal inducer genes are specifically expressed in crustacean gills.
These shared features between crustacean gills and insect tracheae support a common origin While sluggish arthropods, for example, velvet worms onychophorans , breathe continuously at relatively high respiratory rates, highly active insects, for example, flies, mosquitos, gnats, and midges Diptera , breathe discontinuously and can reduce metabolic rates to near zero, thereby increasing the safety margins in their respiratory capacity Insect tracheae are metameric, tubelike invaginations of the body wall forming a system of air-filled tubes with an external epidermis and a cuticular lining stabilized by taenidial structures.
Spiracles openings permit one-way airflow occludable by valves that regulate air intake, CO 2 output and water loss in accordance with environmental conditions and metabolic demand From the spiracles large main tracheae arise, dividing into dorsal, visceral, and ventral branches and then into smaller tracheae down to the terminal tracheoles 0. In most insects, tracheae interconnect body segments and the left-right body halves. Volume variable air sacs are present and especially prominent in wasps, bees, ants Hymenoptera , and flies Diptera , but absent in cockroaches Blattodea.
In small or less-active insects, tracheal gas exchange occurs by simple diffusion.
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Large active insects e. Tracheae are formed by tracheal cells. The intima of all tracheae is made up of cuticle, which also lines the air sacs, and consists of a soft endocuticle and an epicuticle at least in the large tracheae. The exocuticle is restricted to the taenidia, which are hardened flexible threads, most often coiled on the tracheal wall like a corkscrew. Taenidia stabilize the tracheae and air sacs Tracheoles usually do not contain a teanidial coil, except in the locust , The entire system, with the exception of tracheoles, is shed during moulting ecdysis In wasps and bees, tracheoles penetrate muscle fibres to lie close to or in contact with mitochondria, resulting in negligible diffusion distance In the general insect bauplan , ten pairs of spiracles are situated in the pleura of meso- and metathorax and of the first eight abdominal segments, but in many groups the number of spiracles is reduced A precondition for terrestrialization is closable spiracles to i minimize the risk of desiccation, ii allow discontinuous respiration, and iii permit unidirectional airflow; the latter is mainly realized in large, flying insects, for example, grasshoppers.
A spiracle-closing apparatus allows insects to survive nearly all environments, from desert to high mountains, and with special adaptations secondarily in water. Aquatic species may have an open or a closed tracheal system. In the latter species, air enters skin via regions that are highly penetrated with thin tracheoles, then reach larger tracheae and finally return to tracheoles to supply the tissue with O 2. Tracheal adaptations minimize respiratory water loss, which comprises only a small proportion of total water loss except in true xeric species that show greater respiratory water loss than species from more moderate habitats Muscles attached to the cuticular spiracular valve control spiracular opening-closing.
In most insects, closing is active while opening is achieved passively via elasticity of the valves or elastic filaments attached to the valves. Hairs or bristles, sieve plates, deep invagination, elytra, or wings that cover the opening offer additional protection. In aquatic insects with open tracheal systems, glands may deliver a fine lipid film to seal the spiracle. The closing mechanism may be external or internal In external closing apparatus, spiracular valves are thickened cuticular lips attached to muscles and form part of the body wall.
Oxygen Homeostasis and Its Dynamics
Behind the spiracle, an atrium is often lined by thick cuticle and bristles or hairs. This type of closing apparatus occurs in more basal insects, for example, cockroaches or grasshoppers, and the metathorax of butterflies Lepidoptera. In internal closing apparatus, cuticular valve is situated behind the atrium at the beginning of the main trachea.
In most insects, for example, butterflies, bees, and beetles, there is one stationary valve and one moveable valve. Muscle and ligament attach at the moveable valve 45 , , Tracheal systems vary in size and complexity. Small or nonflying insects with low metabolism have simple tracheae characterized by small volumes, few tracheoles, and low diffusing capacities.
Species with high metabolic rates, for example, flying or fast running insects, possess highly complex tracheae with large volumes, inflatable air sacs, penetrating tracheoles, and high diffusing capacities. In adult grasshoppers Schistocerca americana , tracheal volume scales with body mass 1. Diffusing capacity of the jumping legs in adults is about fourfold that in the second instars due to a larger surface-to-volume ratio, thinner tracheal walls, more intracellular tracheae, tracheoles, and mitochondria in the legs of adults. Therefore, both tracheal volume and diffusing capacity of the entire tracheae increase with growth Large insects, for example, adult grasshoppers, have smaller morphological safety margins for O 2 diffusion ; under hypoxia they compensate by increasing abdominal pumping frequency and tidal volume leading to fourfold increase in convective gas exchange In adult species body mass 0.
However, during hypoxia ventilation scales directly with mass, suggesting convection as the major mechanism for enhancing gas exchange and body size does not affect the safety margin for O 2 delivery During development tracheoles migrate to hypoxic regions, first described in the blood-sucking kissing bug Rhodnius prolixus , Drosophila tracheae originate from lateral respiratory placodes thickened epithelium consisting approximately 80 cells per body segment.
These cells proliferate and invaginate to form successively branching tubules that terminate at individual organs. Specific cell types migrate to predetermined positions and form a network of tracheae, including i branch cells that transport gas from spiracles, ii terminal cells for gas exchange, and iii fusion cells for interconnections among tracheae. Branching of major tracheae is developmentally hardwired whereas branching of tracheoles is variable depending on O 2 demands, that is, local hypoxic signals.
Members of the fibroblast growth factor FGF and receptor FGFR family are among more than 30 genes known to regulate branching morphogenesis. FGF expression is prominently regulated by hypoxia and targets several developmental steps such as local fusion abilities, tube size and shape, and substrate outgrowth preferences 5 , , , Tracheal branching in Drosophila larvae increases in hypoxia and decreases in hyperoxia , Similar results are seen in mealworm Tenebrio molitor larvae for the diameters of secondary and tertiary tracheae , Insect tracheal systems, adapted for survival in air or water, enable rapid recovery from hypoxia in a manner matched to respiratory and metabolic needs.
Insect flight muscles have the highest aerobic scopes a factor of several hundred 89 , that is, anaerobic metabolism is seldom needed except during hopping Tracheae transport O 2 directly to organs without a blood or hemolymph system. Oxygen diffuses rapidly from tracheal endings to the mitochondria of end cells because of a high diffusivity of O 2 in air compared to fluids and because most insects actively ventilate their tracheal system.
Gas phase diffusion is an important process in insect respiration, first discussed by August Krogh - Convection further aids gas exchange , , , , , , , , In small insects or those with low metabolic rate, diffusion is sufficient to meet metabolic demands whereas in large or in metabolically active insects ventilation is necessary.
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Owing to a larger surface area, O 2 diffusion occurs mainly in terminal tracheoles in species with simple stick insect or complex flies or bees tracheal systems. Thus, tracheal O 2 supply reflects the O 2 needs of the organism In moth and butterfly Lepidoptera pupae, tracheal spiracles provide the main diffusional resistance , Gas exchange may also occur across the walls of proximal tracheae, for example, in the caterpillar Calpodes ethlius.
In segment 11 near the eighth last pair of abdominal spiracles, thin walled tracheal tufts are surrounded by many hemocytes and act as gas-exchange units by transferring O 2 onto hemocytes Another interesting phenomenon is fluid-filled terminal tracheoles found in resting flight muscles of some species of Orthoptera, Diptera, and Coleoptera, thought to be protective against hyperoxic toxicity. During activity the fluid disappears as tissue osmotic pressure increases, allowing air to enter terminal tracheoles and tissue diffusion to increase , Energy costs of walking or running depend on the length and number of legs and the load an animal carries.
Crickets, ants, flies, and cockroaches quickly reach steady state O 2 consumption, which can be sustained aerobically for a long time. Steady-state O 2 uptake increases linearly with running speed and minimum cost of transport is relatively high compared to vertebrates.
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