08-29-2006, 02:09 AM
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#29 (permalink) |
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Vendor
Join Date: Sep 2005
Location: Broken Arrow Oklahoma
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here's something I copid from the internet about rocket science or something...
The solid-core has the downside that it can only be run at temperatures below the melting point of the materials used in the reactor core. Since the efficiency of a rocket engine is strongly related to the temperature of the working fluid, the solid-core design needs to be constructed of materials that remain strong at as high a temperature as possible. Even the most advanced materials melt at temperatures below that which the fuel can actually create, meaning that much of the potential energy of the reactions is lost. Generally the solid-core design is expected to deliver specific impulses (Isp) on the order of 800 to 900 seconds, about twice of LH2-LOX designs such as the SSME. The weight of a complete nuclear reactor is so great that solid-core engines would be hard-pressed to achieve a thrust-to-weight ratio of 1:1, which would be needed to overcome the gravity of the Earth on launch. Nevertheless the overall weight of the engine and fuel for a given amount of total impulse is lower. This means that solid-core engines are only really useful for upper-stage uses where the vehicle is already in orbit, or close to it, and the required thrust is lower. To be a useful launch engine, the system would have to be either much lighter, or provide even higher specific impulse. Both would, of course, be even better. One way to increase the temperature, and thus the specific impulse, is to isolate the fuel elements so they no longer have to be rigid. This is the basis of the particle-bed reactor, also known as the fluidized-bed, dust-bed, or rotating-bed design. In this design the fuel is placed in a number of (typically spherical) elements which "float" inside the hydrogen working fluid. Spinning the entire engine forces the fuel elements out to walls that are being cooled by the hydrogen. This design increases the specific impulse to about 1000 seconds (9.8 kN·s/kg), allowing for thrust-to-weight ratios just over 1:1, although at the cost of increased complexity. Such a design could share design elements with a pebble-bed reactor, several of which are currently generating electricity. [edit] Liquid Core Dramatically greater improvements can be had by mixing the nuclear fuel into the working fluid, and allowing the reaction to take place in the liquid mixture itself. This is the basis of the so-called liquid-core engine, which can operate at higher temperatures beyond the melting point of the fuel. In this case the maximum temperature is whatever the container wall (typically a neutron reflector of some sort) can handle, while actively cooled by the hydrogen. It is expected that the liquid-core design can deliver performance on the order of 1300 to 1500 seconds (12.8–14.8 kN·s/kg). These engines are difficult to build however; the reaction time of the nuclear fuel is much higher than the heating time of the working fluid, meaning that some system must be used to trap the fuel inside the engine while still allowing the working fluid to easily exit through the nozzle. Most liquid-phase engines have focussed on rotating the fuel/fluid mixture at very high speeds, forcing the fuel to the outside due to centrifugal force (uranium is heavier than hydrogen). In many ways the design mirrors the particle-bed design, although operating at even higher temperatures. An alternative liquid-core design, the nuclear salt-water rocket has been proposed by Robert Zubrin. In this design, the working fluid is water, which serves as neutron moderator as well. The nuclear fuel is not retained, drastically simplifying the design. However, by its very design, the rocket would discharge massive quantities of extremely radioactive waste and could only be safely operated well outside the earth's atmosphere and perhaps even entirely outside earth's magnetosphere. [edit] Gas Core The final classification is the gas-core engine. This is a modification to the liquid-core design which uses rapid circulation of the fluid to create a toroidal pocket of gaseous uranium fuel in the middle of the reactor, surrounded by hydrogen. In this case the fuel does not touch the reactor wall at all, so temperatures could reach several tens of thousands of degrees, which would allow specific impulses of 3000 to 5000 seconds (30 to 50 kN·s/kg). In this basic design, the "open cycle", the losses of nuclear fuel would be difficult to control, which has led to studies of the "closed cycle" or nuclear lightbulb engine, where the gasseous nuclear fuel is contained in a super-high-temperature quartz container, over which the hydrogen flows. The closed cycle engine actually has much more in common with the solid-core design, but this time limited by the critical temperature of quartz instead of the fuel stack. Although less efficient than the open-cycle design, the closed-cycle design is expected to deliver a rather respectable specific impulse of about 1500-2000 seconds (15–20 kN·s/kg). [edit] Practical testing Although engineering studies of all of these designs were made, only the solid-core engine was ever built. Development of such engines started under the aegis of the Atomic Energy Commission in 1956 as Project Rover, with work on a suitable reactor starting at LANL. Two basic designs came from this project, Kiwi and NRX. Kiwi was the first to be fired, starting in July 1959 with Kiwi 1. The reactor was not intended for flight, hence the naming of the rocket after a flightless bird. This was unlike later tests because the engine design could not really be used, the core was simply a stack of uncoated uranium oxide plates onto which the hydrogen was dumped. Nevertheless it generated 70 MW and produced an exhaust of 2683 K. Two additional tests of the basic concept, A' and A3, added coatings to the plates to test fuel rod concepts. The Kiwi B series fully developed the fuel system, which consisted of the uranium fuel in the form of tiny uranium dioxide (UO2) spheres embedded in a low-boron graphite matrix, and then coated with niobium carbide. Nineteen holes ran the length of the bundles, and through these holes the liquid hydrogen flowed for cooling. A final change introduced during the Kiwi program changed the fuel to uranium carbide, which was run for the last time in 1964. Using information developed from the Kiwi series, the Phoebus series developed much larger reactors. The first 1A test in June 1965 ran for over 10 minutes at 1090 MW, with an exhaust temperature of 2370 K. The B run in February 1967 improved this to 1500 MW for 30 minutes. The final 2A test in June 1968 ran for over 12 minutes at 4,000 MW, the most powerful nuclear reactor ever built. For contrast, the largest hydroelectric plant in the world, Itaipu, produces 12,600 MW, 25% of all the power used in Brazil. A smaller version of Kiwi, the Peewee was also built. It was fired several times at 500 MW in order to test coatings made of zirconium carbide (instead of niobium carbide) but also increased the power density of the system. An unrelated water-cooled system known as NF-1 (for Nuclear Furnace) was used for future materials testing. While Kiwi was being run, NASA joined the effort with their NERVA program (Nuclear Engine for Rocket Vehicle Applications). Unlike the AEC work, which was intended to study the reactor design itself, NERVA was aiming to produce a real engine that could be deployed on space missions. A 75,000 lbf (334 kN) thrust baseline design was considered for some time as the upper stages for the Saturn V, in place of the J-2s that were actually flown. The design that eventually developed, known as NRX for short, started testing in September 1964. The final engine in this series was the EX, which was the first designed to be fired in a downward position (like a "real" rocket engine) and was fired twenty-eight times in March 1968. The series all generated 1100 MW, and many of the tests concluded only when the test-stand ran out of hydrogen fuel. EX produced the baseline 75,000 lbf (334 kN) thrust that NERVA required. A KIWI engine being destructively tested All of these designs also shared a number of problems that were never completely cured. The engines were also quite easy to break, and on many firings the vibrations inside the reactors cracked the fuel bundles and caused the reactors to break apart. This problem was largely solved by the end of the program, and related work at Argonne National Laboratory looked promising. However, while the graphite construction was indeed able to be heated to high temperatures, it likewise eroded quite heavily due to the hydrogen. The coatings never wholly solved this problem, and significant "losses" of fuel occurred on most firings. This problem did not look like it would be solved any time soon. The NERVA/Rover project was eventually cancelled in 1972 with the general wind-down of NASA in the post-Apollo era. Without a manned mission to Mars, the need for a nuclear thermal rocket was unclear. To a lesser extent it was becoming clear that there could be intense public outcry against any attempt to use a nuclear engine. Although the Kiwi/Phoebus/NERVA designs were the only to be tested in any substantial program, a number of other solid-core engines were also studied to some degree. The Small Nuclear Rocket Engine, or SNRE, was designed at the Los Alamos National Laboratory (LANL) for upper stage use, both on unmanned launchers as well as the Space Shuttle. It featured a split-nozzle that could be rotated to the side, allowing it to take up less room in the Shuttle cargo bay. The design provided 73 kN of thrust and operated at a specific impulse of 875 seconds (8.58 kN·s/kg), and it was planned to increase this to 975 with fairly basic upgrades. This allowed it to achieve a mass fraction of about 0.74, comparing with 0.86 for the SSME, one of the best conventional engines. A related design that saw some work, but never made it to the prototype stage, was Dumbo. Dumbo was similar to Kiwi/NERVA in concept, but used more advanced construction techniques to lower the weight of the reactor. The Dumbo reactor consisted of several large tubes (more like barrels) which were in turn constructed of stacked plates of corrugated material. The corrugations were lined up so that the resulting stack had channels running from the inside to the outside. Some of these channels were filled with uranium fuel, others with a moderator, and some were left open as a gas channel. Hydrogen was pumped into the middle of the tube, and would be heated by the fuel as it travelled through the channels as it worked its way to the outside. The resulting system was lighter than a conventional design for any particular amount of fuel. The project developed some initial reactor designs and appeared to be feasible. More recently an advanced engine design was studied under Project Timberwind, under the aegis of the Strategic Defence Initiative ("Star Wars"), which was later expanded into a larger design in the Space Thermal Nuclear Propulsion (STNP) program. Advances in high-temperature metals, computer modelling and nuclear engineering in general resulted in dramatically improved performance. Whereas the NERVA engine was projected to weigh about 6,803 kg, while the final STNP offered just over 1/3rd the thrust from an engine of only 1,650 kg, while further improving the Isp to 930 to 1000 seconds. [edit] Nuclear vs Chemical Directly comparing the performance of a nuclear engine and a chemical one is not easy; the design of any rocket is a study in compromises and different ideas of what constitutes "better". In the outline below we will consider the NERVA-derived engine that was considered by NASA in the 1960s, comparing it with the S-IVB stage from the Saturn it was intended to replace. For any given thrust, the amount of power that needs to be generated is defined by P = T * Ve / 2, where T is the thrust, and Ve is the exhaust velocity. Ve can be calculated from the specific impulse, Isp, where Ve = Isp * g, Using the J-2 on the S-IVB as a baseline design, we have P = 414 s * (1014 kN * 9.81) / 2 = 2,060 MW. This is about the amount of power generated in a large nuclear reactor. However, as outlined above, even the simple solid-core design provided a large increase in Isp to about 850 seconds. Using the formula above, we can calculate the amount of power that needs to be generated, at least given extremely efficient heat transfer: P = 1014kN * (850 * 9.81) / 2 = 4,227 MW. Note that it is the Isp improvement that demands the higher energy. Given inefficiencies in the heat transfer, the actual NERVA designs were planned to produce about 5 GW, which would make them the largest nuclear reactors in the world. The fuel flow for any given thrust level can be found from m = T / Ve. For the J-2, this is m = 1014 kN / (414 * 9.81), or about 250 kg/s. For the NERVA replacement considered above, this would be 121 kg/s. Remember that the mass of hydrogen is much lower than the hydrogen/oxygen mix in the J-2, where only about 1/6th of the mass is hydrogen. Since liquid hydrogen has a density of about 70 kg/m³, this represents a flow of about 1,725 litres per second, about three times that of the J-2. This requires additional plumbing but is by no means a serious problem; the famed F-1 had flow rates on the order of 25,000 l/s. Finally, one must consider the design of the stage as a whole. The S-IVB carried just over 300,000 litres of fuel, 229,000 litres of liquid hydrogen (38,000 lb), and 72,700 litres of liquid oxygen (191,000 lb). The S-IVB uses a common bulkhead between the tanks, so removing it to produce a single larger tank would increase the total load only slighly, for argument's sake, perhaps 2,000 litres. Assuming this for the moment, this means the new hydrogen-only nuclear stage would carry about 231,000 litres in total (231 m³), or about 16,500 kg (36,350 lb). At 1,725 litres per second, this is a burn time of only 135 seconds, compared to about 500 in the original S-IBV (although some of this is at a lower power setting). The total impuse generated over time, the so-called delta-V, can be found from the rocket equation, which is based on the starting and ending masses of the stage: Where m0 is the initial mass with fuel, m1 the final mass without it, and Ve is as above. The total empty mass of the J-2 powered S-IVB was 13,311 kg, of which about 1,600 kg was the J-2 engine. Removing the inter-tank bulkhead to improve hydrogen storage would likely lighten this somewhat, perhaps to 10,500 kg for the tankage alone. The baseline NERVA designs were about 15,000 lb, or 6,803 kg, making the total unfueled mass (m1) of a "drop-in" S-IVB replacement around 17,300 kg. The lighter weight of the fuel more than makes up for the increase in engine weight; whereas the fueled mass (m0) of the original S-IVB was 119,900 kg, for the nuclear-powered version this drops to only 33,800 kg. Following the forumla above, this means the J-2 powered version generates a of (414sec * 9.81) ln (119,900 / 13,311), or 8,925 m/s. The nuclear-powered version assumed above would be (850 * 9.81) ln (33,800 / 17,300), or 5,585 m/s. This drop in overall performance is due largely to the much higher "burnout" weight of the engine, and to smaller burn time due to the less-dense fuel. As a drop-in replacement, then, the nuclear engine does not seem to offer any advantages. However, this simple examination hides several important facts. For one, the new stage weighs considerably less than the older one, which means that the lower stages below it will leave the new upper stage at a higher velocity. This alone will make up for much of the difference in performance. Additionally, this ignores the effects of the payload above the stage on the rocket equation, although its effect is not pronounced as it basically increases the (m1) equally in both cases. More importantly, the comparison assumes that the stage would otherwise remain the same design overall. This is a bad assumption; one generally makes the upper stages as large as they can be given the throw-weight of the stages below them. In this case one would not make a drop-in version of the S-IVB, but a larger stage who's overall weight was the same as the S-IVB. Following that line of reasoning, we can envision a replacement S-IVB stage that weights 119,900 kg fully fueled, which would require much larger tanks. Assuming that the tankage mass triples, we have a m1 of 31,500 + 6,800 = 38,300 kg, and since we have fixed m0 at 119,900 kg, we get = (850 s * 9.81) ln (119,900 / 38,300), or 9,500 m/s. Thus, given the same mass as the original S-IVB, one can expect a moderate increase in overall performance using a nuclear engine. This stage would be about the same size as the S-II stage used on the Saturn. Of course this increase in tankage might not be easy to arrange. NASA actually considered a new S-IVB replacement, the S-N, built to be as physically large as possible while still being able to be built in the VAB. It weighed only 10,429 kg empty and 53,694 kg fueled (suggesting that structural loading is the dominant factor in stage mass, not the tankage). The combination of lower weight and higher performance improved the payload of the Saturn V as a whole from 127,000 kg delivered to low earth orbit (LEO) to 155,000 kg. It is also worth considering the improvement in stage performance using the more advanced engine from the STNP program. Using the same S-IVB baseline, which does make sense in this case due to the lower thrust, we have an unfueled weight of (m1) of 10,500 + 1,650 = 12,150 kg, and a fueled mass (m0) of 22,750 + 12,150 = 34,900 kg. Putting these numbers into the same formula we get a of just over 10,000 m/s – remember, this is from the smaller S-IV-sized stage. Even with the lower thrust, the stage also has a power-to-weight ratio similar to the original S-IVB, 34,900 kg being pushed by ~350 kN, as opposed to 253,000 lb pushed by ~250,000 lbf thrust. The STNP-based S-IVB would indeed be a "drop-n replacement" for the original S-IVB, offering higher performance from much lower weight. [edit] Risks There is an inherent possibility of atmospheric or orbital rocket failure which could result in a dispersal of radioactive material, and resulting fallout. Catastrophic failure, meaning the release of radioactive material into the environment, would be the result of a containment breach. A containment breach could be the result of an impact with orbital debris, material failure due to uncontrolled fission, material imperfections or fatigue and human design flaws. A release of radioactive material while in flight could disperse radioactive debris over the Earth in a wide and unpredictable area. The zone of contamination and its concentration would be dependent on prevailing weather conditions and orbital parameters at the time of re-entry. [edit] See also NERVA Project Prometheus Project Timberwind Project Pluto nuclear pulse propulsion spacecraft propulsion [edit] External links Dumbo (PDF) picture of the EX' engine Rover Nuclear Rocket Engine Program: Final Report - NASA 1991 (PDF) Neofuel Proposal for steam-based interplanetary drive, using off-earth ice deposits Project Prometheus: Beyond the Moon and Mars Nuclear propulsion (German page) Nuclear propulsion Edit Spacecraft Antimatter catalyzed nuclear pulse propulsion • Bussard ramjet • Fission-fragment rocket • Fission sail • Fusion rocket • Gas core reactor rocket • Nuclear electric rocket • Nuclear photonic rocket • Nuclear pulse propulsion • Nuclear salt-water rocket • Nuclear thermal rocket • Radioisotope rocket Sea vessels Nuclear marine propulsion • Nuclear navy Aircraft Nuclear aircraft v·d·e Nuclear Technology Nuclear engineering Nuclear physics*| Nuclear fission*| Nuclear fusion*| Radiation*| Ionizing radiation*| Atomic nucleus*| Nuclear reactor*| Nuclear safety Nuclear material Nuclear fuel*| Fertile material*| Thorium*| Uranium*| Enriched uranium*| Depleted uranium*| Plutonium Nuclear power Nuclear power plant*| Radioactive waste*| Fusion power*| Future energy development*| Inertial fusion power plant*| Pressurized water reactor*| Boiling water reactor*| Generation IV reactor*| Fast breeder reactor*| Fast neutron reactor*| Magnox reactor*| Advanced gas-cooled reactor*| Gas cooled fast reactor*| Molten salt reactor*| Liquid metal cooled reactor*| Lead cooled fast reactor*| Supercritical water reactor*| Very high temperature reactor*| Pebble bed reactor*| Integral Fast Reactor*| Nuclear propulsion*| Nuclear thermal rocket*| Radioisotope thermoelectric generator Nuclear medicine PET*| Radiation therapy*| Tomotherapy*| Proton therapy*| Brachytherapy Nuclear weapons History of nuclear weapons*| Nuclear warfare*| Nuclear arms race*| Nuclear weapon design*| Effects of nuclear explosions*| Nuclear testing*| Nuclear delivery*| Nuclear proliferation*| List of countries with nuclear weapons*| List of nuclear tests Retrieved from "http://en.wikipedia.org/wiki/Nuclear_thermal_rocket" Category: Nuclear spacecraft propulsion Views Article Discussion Edit this page History Personal tools Sign in / create account if (window.isMSIE55) fixalpha(); Navigation Main Page Community Portal Featured articles Current events Recent changes Random article Help Contact Wikipedia Donations Search * Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this article This page was last modified 11:59, 25 August 2006. 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08-29-2006, 02:12 AM
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#30 (permalink) |
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Vendor
Join Date: Sep 2005
Location: Broken Arrow Oklahoma
Posts: 5,734
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here's something I found about eartheorms,...pretty interesting stuff actually.....
Earthworm From Wikipedia, the free encyclopedia Jump to: navigation, search ?Earthworms Scientific classification Kingdom: Animalia Phylum: Annelida Class: Clitellata Subclass: Oligochaeta Order: Haplotaxida Suborder: Lumbricina Families * Acanthodrilidae * Ailoscolecidae * Alluroididae * Almidae * Biwadrilidae * Eudrilidae * Exxidae * Glossoscolecidae * Lumbricidae * Lutodrilidae * Megascolecidae * Microchaetidae * Ocnerodrilidae * Octochaetidae * Sparganophilidae Earthworm is the common name for the larger members of the Oligochaeta (which is either a class or subclass depending on the author) in the phylum Annelida. In classical systems they were placed in the order Opisthopora, on the basis of the male pores opening to the outside of body posterior to the female pores, even though the male segments are anterior to the female. Cladistic studies have supported placing them instead in the suborder Lumbricina of the order Haplotaxida. Folk names for earthworm include "dew-worm", "night crawler" and "angleworm". Earthworms are also called megadriles (or big worms), as opposed to the microdriles, which include the families Tubificidae, Lumbriculidae, and Enchytraeidae, among others. The megadriles are characterized by having a multilayered clitellum (which is much more obvious than the single-layered one of the microdriles), a vascular system with true capillaries, and male pores behind the female pores. Contents [hide] 1 Overview 2 Anatomy 3 Dissection 4 Reproduction 5 Regeneration 6 Behavior 6.1 Rainstorms 7 Locomotion and importance to soil 8 Benefits 9 Earthworms as invasives 9.1 North America 9.2 Australia 10 Special habitats 11 Ecology 12 Threats to earthworms 13 Economic Impact 14 Taxonomy and main geographic origins of earthworms 15 See also 16 External links //<![CDATA[ if (window.showTocToggle) { var tocShowText = "show"; var tocHideText = "hide"; showTocToggle(); } //]]> [edit] Overview There are over 5,500 named species known worldwide, existing everywhere but Polar and arid climates. They range in size from two centimeters (less than one inch) to over three meters (almost ten feet) in the Giant Gippsland Earthworm. Amongst the main earthworm species commonly found in temperate regions are the reddish coloured, deep-burrowing Lumbricus terrestris, In temperate zone areas, the most commonly seen earthworms are lumbricids (Lumbricidae), mostly due to the recent rapid spread of a relatively small number of European species, but there are many other families, e.g. Megascolecidae, Octochaetidae, Sparganophilidae, Glossoscolecidae, etc.. These other families are often differ from the lumbricids in behavior, physiology and habitat. Anatomy of the earthworm [edit] Anatomy Earthworms have a closed circulatory system. They have two main blood vessels that extend through the length of their body: a ventral blood vessel which leads the blood to the posterior end, and a dorsal blood vessel which leads to the anterior end. The dorsal vessel is contractile and pumps blood forward, where it is pumped into the ventral vessel by a series of "hearts" (aortic arches) which vary in number in the different taxa. A typical lumbricid will have 5 pairs of hearts; a total of 10. The blood is distributed from the ventral vessel into capillaries on the body wall and other organs and into a vascular sinus in the gut wall where gases and nutrients are exchanged. This arrangement may be complicated in the various groups by suboesophageal, supraoesophageal, parietal and neural vessels, but the basic arrangement holds in all earthworms. [edit] Dissection The classroom dissection of the earthworm and other animals has become controversial in recent years. One response to this has been the development of online virtual dissections. [edit] Reproduction Mating earthworms Earthworms are hermaphrodites (both female and male organs within the same individual) but generally cannot fertilize their own eggs. They have testes, seminal vesicles and male pores which produce, store and release the sperm, and ovaries and ovipores. However, they also have one or more pairs of spermathecae (depending on the species) that are internal sacs which receive and store sperm from the other worm in copulation. Copulation and reproduction are separate processes in earthworms. The mating pair overlap front ends ventrally and each exchanges sperm with the other. The cocoon, or egg case, is secreted by the clitellum, the external glandular band which is near the front of the worm, but behind the spermathecae. Some indefinite time after copulation, long after the worms have separated, the clitellum secretes the cocoon which forms a ring around the worm. The worm then backs out of the ring, and as it does so, injects its own eggs and the other worm's sperm into it. As the worm slips out, the ends of the cocoon seal to form a vaguely lemon-shaped incubator (cocoon) in which the embryonic worms develop. They emerge as small, but fully formed earthworms, except for lacking the sexual structures, which develop later. Some earthworm species are mostly parthenogenetic, in which case the male structures and spermathecae may become abnormal, or missing. [edit] Regeneration Earthworms have the facility to replace or replicate lost segments, but this ability varies between species and depends on the extent of the damage. Stephenson (1930) devoted a chapter of his great monograph to this topic, while G.E. Gates spent 10 years studying regeneration in a variety of species, but “because little interest was shown”, Gates (1972) only published a few of his findings that, nevertheless, show it is theoretically possible to grow two whole worms from a bisected specimen in certain species. Gates’s reports included: Eisenia fetida (Savigny, 1826) with head regeneration, in an anterior direction, possible at each intersegmental level back to and including 23/24, while tails were regenerated at any levels behind 20/21. Lumbricus terrestris Linneus, 1758 replacing anterior segments from as far back as 13/14 and 16/17 but tail regeneration was never found. Perionyx excavatus Perrier, 1872 readily regenerated lost parts of the body, in an anterior direction from as far back as 17/18, and in a posterior direction as far forward as 20/21. Lampito mauritii (Kinberg, 1867) with regeneration in anterior direction at all levels back to 25/26 and tail regeneration from 30/31; head regeneration was sometimes believed to be caused by internal amputation resulting from Sarcophaga sp. larval infestation. An unidentified Tasmanian native shown growing a second head is reported here: [1]. [edit] Behavior [edit] Rainstorms One often sees earthworms come to the surface in large numbers after a rainstorm. There are three theories for this behavior. The first is that the waterlogged soil has insufficient oxygen for the worms, therefore, earthworms come to the surface to get the oxygen they need and breathe more easily. However, earthworms can survive underwater for several weeks if there is oxygen in it, so this theory is rejected by some. Secondly, some species (notably Lumbricus terrestris) come to the surface to mate. This behavior is, however, limited to a few species. Thirdly, the worms may be using the moist conditions on the surface to travel more quickly than they can underground, thus colonizing new areas more quickly. Since the relative humidity is higher during and after rain, they do not become dehydrated. This is a dangerous activity in the daytime, since earthworms die quickly when exposed to direct sunlight with its strong UV content, and are more vulnerable to predators such as birds. Lumbricidae [edit] Locomotion and importance to soil Earthworms travel underground by the means of waves of muscular contractions which alternately shorten and lengthen the body. The shortened part is anchored to the surrounding soil by tiny claw-like bristles (setae) set along its segmented length. (Typically, earthworms have four pairs of setae for each segment but some genera are perichaetine, having a large number of setae on each segment.) The whole process is aided by the secretion of a slimy lubricating mucus. In more compacted soils the earthworm actually eats its way through the soil, cutting a passage with its muscular pharynx and dragging the rest of the body along. The ingested soil is ground up, digested, and the waste deposited behind the worm. This process aerates and mixes the soil, and is constructive to nutrient uptake by vegetation. In addition, earthworms often come to the surface and graze on the higher concentrations of organic matter present there, mixing it with the mineral soil. Because a high level of organic matter is associated with soil fertility, an abundance of earthworms is beneficial to the organic gardener. In fact as long ago as 1881 Charles Darwin wrote: “ It may be doubted whether there are any other animals which have played so important a part in the history of the world, as have these lowly creatures „ —Charles Darwin,*The formation of vegetable mould through the action of worms, with observations on their habits [edit] Benefits The major benefits of earthworm activities to soil fertility can be summarized as: Biological. The earthworm is essential to composting; the process of converting dead organic matter into rich humus, a medium vital to the growth of healthy plants, and thus ensuring the continuance of the cycle of fertility. This is achieved by the worm's actions of pulling down below any organic matter deposited on the soil surface (eg, leaf fall, manure, etc) either for food or when it needs to plug its burrow. Once in the burrow, the worm will shred the leaf and partially digest it, then mingle it with the earth by saturating it with intestinal secretions. Worm casts (see below) can contain 40% more humus than the top 6" of soil in which the worm is living. Chemical. As well as dead organic matter, the earthworm also ingests any other soil particles that are small enough—including stones up to 1/20 of an inch (1.25mm) across—into its 'crop' wherein minute fragments of grit grind everything into a fine paste which is then digested in the stomach. When the worm excretes this in the form of casts which are deposited on the surface or deeper in the soil, a perfectly balanced selection of minerals and plant nutrients is made available in an accessible form. Investigations in the US show that fresh earthworm casts are 5 times richer in available nitrogen, 7 times richer in available phosphates and 11 times richer in available potash than the surrounding upper 6 inches (150 mm) of soil. In conditions where there is plenty of available humus, the weight of casts produced may be greater than 4.5 kg (10 lb) per worm per year, in itself an indicator of why it pays the gardener or farmer to keep worm populations high. Physical. By its burrowing actions, the earthworm is of great value in keeping the soil structure open, creating a multitude of channels which allow the processes of both aeration and drainage to occur. Permaculture co-founder Bill Mollison points out that by sliding in their tunnels, earthworms "act as an innumerable army of pistons pumping air in and out of the soils on a 24 hour cycle (more rapidly at night)" (Permaculture- A Designer's Manual, Tagari Press, 1988). Thus the earthworm not only creates passages for air and water to traverse, but is itself a vital component in the living biosystem that is healthy soil. It is important that we do not take the humble earthworm for granted. Dr. W. E. Shewell Cooper observed "tremendous numerical differences between adjacent gardens" (Soil, Humus And Health), and worm populations are affected by a host of environmental factors, many of which can be influenced by good management practices on the part of the gardener or farmer. Darwin estimated that arable land contains up to 53,000 worms per acre (13/m²), but more recent research from Rothamsted Experimental Station has produced figures suggesting that even poor soil may support 250,000/acre (62/m²), whilst rich fertile farmland may have up to 1,750,000/acre (432/m²). Professor I. L. Heiberg of State University of New York College of Environmental Science and Forestry has stated that in optimum conditions, the worm population may even reach 250,000,000 per acre (62,000/m²), meaning that the weight of earthworms beneath the farmer's soil could be greater than that of his livestock upon its surface. One thing is certain however: rich, fertile soil that is cared for organically and well-fed and husbanded by its steward will reap its reward in a healthy worm population, whilst denuded, overworked, and eroded land will almost certainly contain fewer, scrawny, undernourished specimens. [edit] Earthworms as invasives [edit] North America Lumbricid earthworms are not indigenous to North America and not only have displaced native earthworms in much of the continent, but have invaded areas where earthworms did not formerly exist. There are no native earthworms in much of North America, especially in the north, and the forests there developed relying on a large amount of undecayed leaf matter. The worms decompose that leaf layer, making the habitat unsurvivable for certain species of trees, ferns and wildflowers. Currently there is no economically feasible method for controlling earthworms in forests, besides preventing introductions. Earthworms normally spread slowly, but can be widely introduced by human activities such as construction earthmoving, or by fishermen releasing bait, or by plantings from other areas. Soils which have been invaded by earthworms can be recognized by an absence of palatable leaf litter. For example, in a sugar maple - white ash - beech - northern red oak association, only the beech and oak leaves will be seen on the forest floor (except during autumn leaf-fall), as earthworms quickly devour maple and ash leaves. Basswood, dogwood, elm, poplar and tuliptree also produce palatable foliage. [edit] Australia Australia has many native species of earthworm, which generally survive only in nutrient-poor conditions. As a result, only introduced species are commonly found in agricultural environments. The introduction of earthworms has probably been accidental in Australia. [edit] Special habitats While, as the name earthworm suggests, the main habitat of earthworms is in soil, the situation is more complicated than that. The brandling worm Eisenia fetida lives in decaying plant matter and manure. Arctiostrotus vancouverensis from Vancouver Island and the Olympic Peninsula is generally found in decaying conifer logs or in extremely acid humus. Aporrectodea limicola and Sparganophilus and several others are found in mud in streams. Even in the soil species, there are special habitats, such as soils derived from serpentine which have an earthworm fauna of their own. [edit] Ecology Earthworm populations depend on both physical and chemical properties of the soil, such as soil temperature, moisture, pH, salts, aeration and texture, as well as available food, and the ability of the species to reproduce and disperse. One of the most important environmental factors is pH, but earthworms vary in their preferences. Most earthworms favor neutral to slightly acid soil. However, Lumbricus terrestris are still present in a pH of 5.4 and Dendrobaena octaedra at a pH of 4.3 and some Megascolecidae are present in extremely acid humic soils. Soil pH may also influence the numbers of worms that go into diapause. The more acid the soil, the sooner worms go into diapause, and remain in diapause the longest time at a pH of 6.4. Earthworms form the base of many food chains. They are preyed upon by many species of birds, e.g. starlings, thrushes, gulls, crows, and robins. Mammals such as hedgehogs and moles eat many earthworms as well. Earthworms are also eaten by many invertebrates such as Ground beetles and other beetles, snails, slugs and flatworms. Earthworms have many internal parasites including Protozoa, Platyhelminthes, nematodes. They are found in many parts of earthworms' bodies such as blood, seminal vesicles, coelom, intestine, or in the cocoons. [edit] Threats to earthworms The application of chemical fertilisers, sprays and dusts can have a disastrous effect on earthworm populations. Nitrogenous fertilisers tend to create acid conditions, which are fatal to the worms, and often dead specimens are to be found on the surface following the application of substances like DDT, lime sulphur and lead arsenate. In Australia, the use of superphosphate on pastures almost totally wiped out the giant Gippsland earthworm. In addition, as earthworms are processors of large amounts of plant and mineral materials, even if not killed themselves they can accumulate pollutants such as DDT, lead, cadmium, and dioxins at levels up to 20 times higher than in the soil, which in turn are passed on at lethal dosages to the wildlife which feed upon them such as foxes, moles or birds. Therefore, the most reliable way to maintain or increase the levels of worm population in the soil is to avoid the application of artificial chemicals, as well as adding organic matter, preferably as a surface mulch, on a regular basis. This will not only provide them with their food and nutrient requirements, but also creates the optimum conditions of heat (cooler in summer and warmer in winter) and moisture to stimulate their activity. A recent threat to earthworm populations in the UK is the New Zealand Flatworm (Artiposthia triangulata), which feeds upon the earthworm, but in this country has no natural predator itself. At present sightings of the NZFW have been mainly localised, but this is no reason for complacency as it has spread extensively since its introduction in 1960 through contaminated soil and plant pots. Any sightings of the flatworm should be reported to the Scottish Crop Research Institute, who are monitoring its spread. [edit] Economic Impact Various species of worms are used in vermiculture, the practice of feeding organic waste to earthworms to decompose (digest) it, a form of composting by the use of worms. These are usually Eisenia fetida or the Brandling worm, also known as the Tiger worm or Red Wriggler, and are distinct from soil-dwelling earthworms. Earthworms are sold all over the world. The earthworm market is sizeable. According to Doug Collicut (see "Nightcrawler" link below), "In 1980, 370 million worms were exported from Canada, with a Canadian export value of $13 million and an American retail value of $54 million." [edit] Taxonomy and main geographic origins of earthworms Main families*: Lumbricidae*: temperate areas of Northern Hemisphere, mostly Eurasia Hormogastridae*: Europe Sparganophilidae*: North America Almidae*: Africa, South America Megascolecidae*: South East Asia, Australia and Oceania, western North America Acanthodrilidae*: Africa, southeastern North America, central and South America, Australia and Oceania Ocnerodrilidae*: Central and South America, Africa Octochaetidae*: Central America, India, New Zealand, Australia Exxidae*: Central America Glossoscolecidae*: central and Northern South America Eudrilidae*: Africa and South Africa |
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