托福听力考试背景知识综合辅导2

托福听力考试背景知识综合辅导(三十四)第一次世界大战中AIRSHIP的背景知识By the start of the Great War the airship had been developed into two main types - rigid and non-rigid (technically, the latter is more accurately described as pressure-rigid). Cylindrical in cross-section, both were given buoyancy by gas and motion by engine-driven propellers and were controlled by vertical rudders and horizontal elevators. In the rigid type, a solid framework, which might be likened to a skeleton, supports an external covering of fabric called the envelope (a very few experimental types had a metallic covering). Within the framework are contained bags of gas called balloonets. In the non-rigid, the envelopes shape is maintained by the pressure of the gas that fills it; there is no framework. The rigids control car and engines are suspended from the framework; in the non-rigid they are attached directly to or suspended directly from the envelope. (In some later rigids the engines were mounted internally, driving the propellers by transmission belts.) There was an intermediate type, now long vanished, called the semi-rigid. It had a pressure-rigid envelope but a solid keel. The rigid airship, employed so extensively by Germany during the Great War, was perfected soon after the turn of the century by a former Wurttenberg army cavalry officer, Ferdinand Adolf August Heinrich Graf von Zeppelin, who had been inspired by a balloon ascent he had made in the United States on 19-Aug-1863. 2 His craft were, naturally , known as Zeppelins. Zeppelin is a proprietary, or trade, name (such as Kodak, Ferris Wheel and Autogiro), and is applied properly only to craft constructed by Luftshiffbau Zeppelin G.m.b.H. or firms licensed to use its patents. It should thus be spelled with a capital Z; modern writers who spell it in lower case are mistaken in their apparent belief that it is generic. 3 It has become customary to refer to all German rigid airships of the Great War as Zeppelins, but in fact not all were. Some were constructed by a rival firm, Luftschiffbau Schutte-Lanz G.m.b.H., which employed a framework of laminated plywood instead of the aluminum alloy used in Zeppelins. 4 The vast amount of literature devoted to German airships can easily, and has, led to the belief that these were the dominant LTA craft of the Great War. Actually, the British navy was the greatest exponent of the airship, receiving more than 200 non-rigids during 1915-1918 for anti-submarine patrolling. This type was also used, although in far fewer numbers, by the U.S. Navy and the French and Italian armies and navies. The German army and navy also had a handful. The rigid airship no longer exists; none has been built since the 1930s. 5 The demise is usually blamed on the Hindenburg disaster 6 although that had been preceded by the loss of a number of American, British, French and Italian rigids. All airships flying today are non-rigid types, popularly known as blimps. 飞艇按其构造可分为软式、半硬式、硬式。 (1)软式飞艇艇体由主气囊和前后副气囊构成。气囊不仅规定密封，还要有相称强度能承受一定的压强。气囊上装有安全活门，压强超过规定值时能自动放气保证气囊不被胀破。主气囊内充浮升气体，副气囊内充空气。副气囊的作用是在不排放主气囊内气体的条件下，保持主气囊内外压强差为定值。当飞艇爬高，外界大气压强减少时，副气囊放气使主气囊增大容积，从而保持主气囊本来的内外压强差。当外界大气压强增大时，向副气囊内充气使它膨胀，从而压缩主气囊的容积，使主气囊的压强仍能保持略高于外界大气压强。设立前后副气囊还可调节浮力中心的位置。仅向后副气囊充气时，重心后移，飞艇产生昂首力矩；反之，产生低头力矩。 (2)半硬式飞艇气囊构造与软式飞艇相似，但在气囊下部增长刚性的龙骨架，构成半硬式飞艇的艇体。 (3)硬式飞艇艇体由刚性骨架外罩蒙布或薄铝皮构成整个艇体不密封，重要起维持流线型和连接各部分的作用。艇体内部由隔框分割成许多小气室，每个小气室内放有密封的小气囊，内充比空气轻的气体。在地面时，小气囊没有完全胀满气室。随着飞行高度增长，外界大气压强减少，囊内气体随之膨胀，在达到规定高度时，气囊正好胀满气室。众多小气囊可提高飞艇的抗损性和安全性。部分小气囊受损，整个飞艇的浮力不会完全丧失。 第一艘飞艇是法国的H吉法尔于1852年制成的。19德国的齐柏林公司开始制造大型硬式飞艇，在第一次世界大战期间，曾多次用飞艇进行远程轰炸托福听力考试背景知识综合辅导(三十五)雪花对于一片六角形雪片来说，由于它表面曲率不等(有凸面、平面和凹面)，各面上的饱和水汽压力也不同，因此产生了互相间的水汽密度梯度，使水汽发生定向转移。水汽转移的方向是凸面平面凹面，也就是从曲率大的表面，移向曲率小的表面。六角形雪片六个棱角上的曲率最大，边棱部分的平面次之，中央部分曲率最小。这样，就使六角形雪片始终处在定向的水汽迁移过程中。由于棱角上水汽向边棱及中央输送，棱角附近的水汽饱和限度下降，因而产生升华现象。中央部分由于获得源源不断的水汽而达到冰面饱和，产生凝华作用。这种凝华结晶的过程不断进行，六角形雪片逐渐演变成为六棱柱状雪晶。 这是假定外部不输送水汽的抱负状况。事实上，事物与周边环境保持着密切的联系，空气里总是或多或少存在着水汽的。如果周边空气输人水汽较少，少到不够雪片的棱角向中央输送水汽的数量，那么雪片向柱状雪晶的发展过程继续进行。在温度很低水汽很少的高纬和极地地区，便由于这个因素常常降落柱状雪晶。 空气里水汽饱和限度较高的时候，浮现此外一种状况。这时周边空气不断地向雪片输送水汽，使雪片迅速地发生凝华作用。凝华减少了雪片周边空气层中的水汽密度，反过来又增进外层水汽向内部输送。这样，雪片便不久地生长起来。当水汽迅速向雪片输送的时候，六个顶角首当其冲，水汽密度梯度最大。来不及向雪片内部输送的水汽，便在顶角上凝华结晶；这时，顶角上会浮现某些突出物和枝杈。这些枝叉增长到一定限度，又会分叉。次级分叉与母枝均保持60的角度，这样，就形成了一朵六角星形的雪花。 在高山或极地的晴朗天气里，还可见到一种冰针，象宝石同样闪烁着瑰丽的光彩，人们把它叫做钻石尘。冰针的生长有二种状况：一种是在寒冷下(-30如下)湿度很小时水汽自发结晶的成果，另一种是在温度较高(-5左右)湿度较大时沿着雪片某一条辅轴所在的顶角特别迅速生长的产物，是雪花的畸形发展托福听力考试背景知识综合辅导(三十六)KEYSTONE SPECIES A keystone is the stone at the top of an arch that supports the other stones and keeps the whole arch from falling. A keystone species is a species on which the persistence of a large number of other species in the ecosystem depends. If a keystone species is extirpated from a system, the species it supported also will disappear, as will the other dependent species. Keystone species may be top carnivores that keep prey in check, large herbivores that shape the habitat in which other species live, important plants that support particular insect species that are prey for birds, bats that disperse the seeds of plants, and many other types of organisms. An example of a keystone predator is the sea otter, which is a keystone in the kelp forest ecosystem. Kelp forests are marine forests of seaweed that provide food and shelter for large numbers of species of fish and shellfish. They also protect coastlines from damaging wave action. Sea otters prey on sea urchins and keep their numbers in check. If the sea otters are extirpated, large numbers of sea urchins feed heavily on the kelp forests, causing severe declines in the kelp forests and their associated species. Wherever otters have been reintroduced, kelp communities have returned. Ironically, many fishers resent sea otters because they view them as competitors for valuable commercial fish and shellfish, such as abalone. They do not realize that without the otters, the abalone would not have kelp forest habitat. Protecting keystone species is a priority for conservationists. Unfortunately, the keystone functions of a species may not be known until it has been extirpated and the ecosystem changes. Keystone species may occur at any level of the ecosystem, from plants and herbivores (plant eaters), to carnivores (meat eaters), and detritivores (waste eaters). Where a keystone species has been identified, efforts to protect it also will help protect the other species in delicately balanced ecosystems.托福听力考试背景知识综合辅导(三十七)Sunwind 从日冕不断发射出的稳定的粒子流。日冕具有极高的温度，作用于日冕气体上的引力不能平衡压力差，因此日冕中很难维持流体静力平衡，日冕不也许处在稳定静止状态，而是稳定地向外膨胀，热电离气体粒子持续地从太阳向外流出，就形成了太阳风。 近年来的观测表白，存在于日冕中的冕洞同地球附近的太阳风有较好的有关性，而长寿命的冕洞 M区 更是太阳风的风源。 开始人们是从慧尾总是背向太阳这种天象中猜想到太阳风的存在，近年来运用卫星观测近地空间，终于证明了太阳风的存在。太阳风的理论模型，是按稳定态球对称的日冕向外扩张的物质流解决的，这种理论模型必然导致无构造的太阳风。但事实上太阳风中很少存在这种状态，几乎所有观测到的参量均有一种无规则的起伏。起伏的因素可归诸于空间的不均匀性或随时间变化的因素，是日面上发生的天体物理现象在行星际空间的反映。相对宁静的太阳风只有在太阳活动极小年才会存在。从太阳活动水平不同的年份的观测成果中可以看出，随着太阳活动限度的减少，太阳风的流速也减少。当太阳风流速降至每秒320公里时，可近似觉得太阳风处在宁静状态。 既然太阳风来源于日冕，人们有理由觉得太阳风的化学成分和日冕的化学成分相似。奇怪的是，根据“水手2号”、“探险者34号”、“维拉3号”的观测成果，长时期的平均氦丰度约为氢的4.5%左右，低于太阳光球中的氦氢丰度比。这个事实意味着氢在日冕膨胀过程中也许比氦更加容易从太阳中逃逸，也就是说，不同荷质比的离子在日冕膨胀中会分离，导致日冕重粒子的引力沉淀。此外，太阳风中氦氢丰度比变化很大，升降幅度有时可达一种数量级之多，成因至今还是个谜。观测表白，高氦量等离子体常常在日地间激波或地磁扰动忽然开始后512小时内浮现，这阐明它与太阳爆发有关。 太阳风中的动力学现象涉及许多随时间变化的复杂构造 高速等离子体流、日地间激波、阿尔文起伏等 ，大体可分为2类： 同日面上长寿命的活动区有关的； 同日面上爆发过程有关的。常以激波的形式浮现，这种激波是由耀斑区抛射出的迅速等离子体压缩太阳风而形成的。由于等离子体具有较高的电导率，制止了迅速的互相渗入，因此只要抛射出来的迅速等离子体与太阳风的相对速度超过声速，就会形成这种激波波阵面。在地球附近，这种激波的平均传播速度约每秒500公里；日地间激波平均传播的时间约55小时，由此算出平均传播速度每秒为760公里，较地球轨道附近实测激波速度略大，因此传播过程中也许有某些微小的减速。 太阳风的大尺度性质可用流体模型来描述，其初级理论是美国天体物理学家帕克完毕的。近年来的理论发展重要集中在研究2种模型上：单流体模型 假设能量方程中电子温度和质子温度相似，并且觉得在日冕底层区域之外唯一的能源来自热传导。双流体模型 假设电子温度和质子温度不同，需要分别建立电子气体和质子气体的能量方程，并且通过电子和质子间的库仑碰撞互换项将2个能量方程耦合起来。 目前尚难判断哪种模型更好。单流体模型所预言的温度值与观测值较为吻合，但未能导出电子和质子的温度差别；双流体模型导出电子温度不小于质子温度，这个推断与观测成果一致，但是与实际观测值比较起来，电子温度的理论值偏高，质子温度的理论值过于偏低。不管是单流体模型还是双流体模型，只靠来自热传导和对流的能量传播是不够的，也许尚有此外的能量传播形式，如激波、磁流体力学波、磁湍流等。同样，太阳附近对日冕增温有影响的机制，也许在日冕外区域仍起作用。托福听力考试背景知识综合辅导(三十八)Jupiter木星是太阳系九大行星中最大的一种，它的体积可以容纳1300多种地球。它的质量是地球质量的300多倍。把太阳系所有其她行星的质量全加起来还不及木星质量的一半。 木星在椭圆轨道上绕太阳运营一周需要11.86年，与太阳平均距离是7.78亿千米.由于木星离太阳遥远, 木星表面温度比地球表面低得多。根据“先驱者”11号宇宙飞船测得的温度约为-150C。木星自转不久，自转一周只需9小时50分30秒，是太阳系中自转最快的一种。由于迅速自转，使木星形状变扁，不是正圆形，而是中腰鼓起的椭圆形。不久的旋转速度带动它的大气层顶端的云层，竟以约35400千米/小时的速度旋转，这种高速产生的离心力就把云层拉成线丝，从而使木星赤道上空高高隆起。木星圆面上有许多带状纹，每条带状纹都与木星的赤道平行。这些带状纹是木星的大气环流。木星是一种没有固体表面的星球，表面布满液态的氢。地球上的物体只要获得11.2千米/秒的速度就能飞离地球，木星上的物体必须具有60千米/秒的速度才干挣脱木星的引力，飞离木星。在离木星几十万千米处环绕木星赤道的区域，有一种由黑色碎石块构成的环，叫做木星环。木星环的厚度约30千米，宽数千千 米，以7小时的周期环绕木星高速旋转。每个石块的直径从数十米到数百米。这个木星环的外缘距离木星中心约12.8万千米。在木星的南半球，有一种颜色明亮而鲜艳的大红斑，300近年来，大红斑的形状几乎没有变化，大小和颜色却常常变幻。长度最长时达到4万千米，至少也有1万多千米，一般保持在2万千米左右，宽度变化不大。大红斑颜色有时鲜红，有时略带棕色或淡玫瑰色。当它的位置在东西方向上时会有漂移。木星探测器探明：大红斑本来是木星大气云层中的一种大旋涡，其中飘浮着五颜六色的云，有棕红色的、棕黄色的、橙色的、白色的，五彩缤纷。 它们重要由红磷化合物构成，并且不断地剧烈运动。科学家们批出，这事实上是木星大气中的带电粒子，在木星旋转磁场作用下的螺旋运动中形成的剧烈风暴。托福听力考试背景知识综合辅导(三十九)JUPITER Jupiter has thick, gaseous atmospheres and low densities. It has no solid rock surface. The temperature ranges from about -190F (-124C) for the visible surface of the atmosphere, to 9F (-13C) at lower cloud levels; localized regions reach as high as 40F (4C) at still lower cloud levels near the equator. Jupiter radiates about four times as much heat energy as it receives from the sun, suggesting an internal heat source. This energy is thought to be due in part to a slow contraction of the planet. At least 17 natural satellites are known to orbit Jupiter. They are conveniently divided into three groups. The four largestIo, Europa, Ganymede, and Callistowere discovered by Galileo in 1610, shortly after he invented the telescope, and are known as the Galilean satellites. A second group is comprised of the four innermost satellites. The final group consists of the eight satellites with orbits outside that of Callisto. The seventeenth satellite, 1999J1, was discovered in . Eleven small previously unknown satellites were reported in . If these sightings should be confirmed, it would raise the number of Jovian moons to 28. The spot and other markings of the atmosphere also provide evidence for Jupiters rapid rotation, which has a period of about 9 hr 55 min.托福听力考试背景知识综合辅导(四十)MUSICAL TERMINOLOGY PITCH is the perceived highness or lowness of a sound. Variations in frequency are what we hear as variations in pitch. The greater the number of sound waves produced per second, the higher the sound we hear; the fewer sound waves per second, the lower the sound. INTERVAL is the distance between two different pitches. ? A tone (or a note) is a musical sound of definite pitch. MELODY is the element that in many musics makes the most direct appeal to the listener. It is often what moves us emotionally, what we remember and whistle, and hum. A melody is a succession of single pitches or tones perceived by the mind as a unity. Just as we hear the words of a sentence not singly but in relation to the entire thought, so do we perceive the pitches of a melody in relation to each other. INTENSITY (amplitude, dynamics, volume) is the amount of energy affecting the vibrating body, and is heard as the loudness or softness of a sound. Terms to Describe Intensity: 1. Very Loud -fortissimo 2. Loud - forte 3. Medium Loud - mezzo forte 4. Medium Soft - mezzo piano 5. Soft - piano 6. Very Soft - pianisimo 7. Gradually getting louder 8. Gradually getting softer 9. Suddenly loud 10. Suddenly soft托福听力考试背景知识综合辅导(四十一)TIMBRE is the tonal quality or color of a sound. This quality is determined by the wave shape, which is determined in turn by factors such as the shape of the vibrating body, its material (metal, wood, human tissue), and the method used to put it in motion (striking, blowing, plucking). 1. Vibrating Stimulus (Drum Stick, Bow, fingers, pick) 2. Vibrating Body (Drum Head, String, Metal, Air) 3. Resonator (Body of Drum, body of violin or guitar) 4. Other (Snares, guitar distortion, spider-web casing on balafon) Each tone we hear is actually a composite of tones called the harmonic series. The pitches produced simultaneously by the vibrating sections are called partials, overtones, or harmonics. The first partial, often called the fundamental, is the lowest frequency and is perceived as the loudest. This is what the ear identifies as the specific pitch of the musical tone. TIMBRE: CLASSIFYING MUSICAL INSTRUMENTS IDIOPHONES Instruments made of naturally sonorous materials not needing any additional tension as do strings or drumskins. The basic question is how they are set into vibration. Methods: concussion (struck together) struck stamped shaken scraped plucked rubbed (friction) AEROPHONES Aerophones include what are usually called “wind instruments,” with the addition of a few instruments with a different acoustical principle called free aerophones. There are two essential factors: a tube enclosing a column of air, and a devise for setting that air into vibration托福听力考试背景知识综合辅导(四十二)MEMBRANOPHONES Membranophones produce sound by stretching a membrane over an opening. Most, but not all, membranophones are called drums. They are classified according to the following features: material shape skins (head) fastening of the skins playing positions manner of performance CHORDOPHONES Chordophones are instruments with strings. The number of stringed instruments can be reduced to four functional types: zither lute lyre harp ELECTROPHONES Instruments where vibrations are produced mechanically or reproduced mechanically. DURATION is the length of time a pitch or tone is sounded. Musical time is usually organized in terms of a basic unit of length, known as a BEAT or regular pulsation. Three terms related to duration are meter, rhythm, and tempo. Meter refers to regularly recurring pulses of equal duration, generally grouped into patterns of two, three, four, or more, with the first pulse in each group accented. These patterns of strong and weak pulses are called beats. Rhythm is sometimes used, like duration, to refer to all phenomena related to the sounding of a tone through time. Specifically, it can refer to the uneven durations of particular notes, overlaying or operating within the meter. Tempo: speed of the pulse or meter. TEXTURE MONOPHONIC, or single-voiced. (Voice refers to an individual part or line, even when we are talking about instrumental music). HETEROPHONY usually results in a melody combined with an ornamented version of itself. POLYPHONY occurs when two or more lines are combined, distributing the melody among the voices. HOMOPHONY, where a single voice takes over the melodic interest, while the accompanying voices take a subordinate role.托福听力考试背景知识综合辅导(四十三)无线电波（即赫兹波）赫兹波除微波波段兼用厘米表达它的波长外，一般均用频率替代波长，其单位为赫（Hz）。频率范畴约在30千赫（kHz）30000兆赫（MHz）之间。其波长范畴在10-3104米之间。当赫兹发现电磁波后来，一方面被用于无线电信之传递实验。最早的无线电讯，借控制火花放电时间，构成电码讯号。火花放电是一种波长很短的减幅波，它的振幅衰减极快，且干扰极大，故不能用它做长距离通信。后经改良用电弧放电以获得长波的等幅波，使通信距离稍增，但其副波干扰仍不能免，且能量较小不能作远距离通信。后来俄国人波波夫与意大利业余无线电家马可尼同步独立地发明天地线制，马可尼且于天线中加接调谐电路，实验越过大西洋电码通信获得成功，至此无线电通信开始进入实用阶段。由于弧光电波的影响，误觉得波长越长，电力越大，通信距离越远；事实上在短波通信特性未发现此前，的确如此。欧洲各国甚至竟用波长10000米以上，耗费巨资建设电台，并用频率很低的交流发电机供应电磁波能直接发射。长波无线电之传递，以地波为主。其折射率在海面与平原之吸取率均较小。在传播途中的衰减大体与距离成正比，因受气候影响甚微，在有效距离内通信可靠，故迄今仍在使用中，但是波长已减至1000米左右。长波无线电特别合适于极地通信及海上导航，因短波无线电在极地失去电离层反射作用，无法达到远距离通信。长波通信须高大之天线。自超短波及微波通信实现后，已可运用轻便之转继站构成通信网，故长波通信已在裁减。中波波段为原则广播波段，其波长自200545米。合用于国内及邻接地区间作广播之用。国土广大之国家均规定其最大电力及广播服务区。最大电力以天线发射之电场强度为原则。中波广播波段自5401650千赫（kHz）。中波广播也可用于空中导航。短波通信之特点是：虽然是不太强的无线电波也能跨越大洋，而完毕极远距离通信。短波无线电讯，由于频率较高，其电磁波由天线发出后，因地球表面矿物质之吸取率甚高，故不管发射电力多大，不出百里以内，其沿地面进行的电磁波即被吸取以尽，其衰减率以对数率随距离而加快。但在数百里以外的地区，因向空发射之天波受高空电离层的反射而再度浮现。这种高空反射波的反射体是天空中大气因受紫外线辐射所形成的离子化气体层。环绕地球的这一层离子化气体称之为“电离层”。无线电波也和光波同样有四种特性：它能被物体吸取、反射、散射及折射。当电磁波迈进时，遇金属物，则有一部分被吸取，有一部分即反射，且金属物与电子线波在不垂直又不平行之方向者，电磁波就发生散射。当电磁波通过不同的介质时，将发生折射的现象。托福听力考试背景知识综合辅导(四十四)火山世界上60以上的活火山都分佈在太平洋沿岸新形成不久的巨大山脈和島嶼；此外有集中在地中海、非洲東部和大西洋中部的地殼發生巨大破裂的地方。因為這些區域的岩漿可以很容易的持續流出，因此成為活火山。噴出的岩漿經過冷卻凝結，如果將火山口堵塞，則需要一段時間，在這段期間，地底的運動仍然活躍的進行著，因為火山口塞住，因此這股力量便在地底累積，等到累積到相當的限度，就又產生另一次的火山爆發，此種類型的火山就稱為休火山。如果在地殼堅硬的地方，所有的裂縫都被堵住了，地底的岩漿再也不能爆發出來，這類的火山就稱為死火山。托福听力考试背景知识综合辅导(四十五)季风现代人们对季风的结识有了进步，至少有三点是公认的，即：（1）季风是大范畴地区的盛行风向随季节变化的现象，这里强调“大范畴”是由于小范畴风向受地形影响很大；（2）随着风向变换，控制气团的性质也产生转变，例如，冬季风来时感到空气寒冷干燥，夏季风来时空气温暖潮湿；（3）随着盛行风向的变换，将带来明显的天气气候变化。季风形成的因素，重要是海陆间热力环流的季节变化。夏季大陆增热比海洋剧烈，气压随高度变化慢于海洋上空，因此到一定高度，就产生从大陆指向海洋的水平气压梯度，空气由大陆指向海洋，海洋上形成高压，大陆形成低压，空气从海洋海向大陆，形成了与高空方向相反气流，构成了夏季的季风环流。在国内为东南季风和西南季风。夏季风特别温暖而湿润。冬季大陆迅速冷却，海洋上温度比陆地要高些，因此大陆为高压，海洋上为低压，低层气流由大陆流向海洋，高层气流由海洋流向大陆，形成冬季的季风环流。在国内为西北季风，变为东北季风。冬季风十分干冷。托福听力考试背景知识综合辅导(四十六)Big eyed Bug RearingNatural HistoryBig-eyed bugs are true bugs in the order Hemiptera. The two most common species are Geocoris pallens and Geocorcis punctipes. Both are predators and occur in many habitats, including fields, gardens, and turf grass. Big-eyed bugs are considered an important predator in many agricultural systems and feed on mites, insect eggs, and small insects such as pink bollworm, cabbage loopers and whiteflies. Adult Big-eyed bugs are small (about 3 mm) black, gray, or tan with proportionately large eyes. Eggs are deposited singly or in clusters on leaves near potential prey. They develop with incomplete metamorphosis (there is no pupa) and take approximately 30 days to develop from egg to adult depending on temperature. Both nymphs and adults are predatory, but can survive on nectar and honeydew when prey are scarce. Big-eyed bugs, like other true bugs, have piercing-sucking mouthparts and feed by stabbing their prey and sucking or lapping the juices. Although their effectiveness as predators is not well understood, studies have shown that nymphs can eat as many as 1600 spider mites before reaching adulthood, while adults have been reported consuming as many as 80 mites per day.RearingBig-eyed bugs are easy to rear. Adults can be purchased (see Suppliers) or collected in the wild by using a sweep net in a local alfalfa field, roadside or other lush area that may have potential insect prey. A Petri dish provides adequate housing when a piece of paper towel or filter paper is placed in the bottom and cut to fit the dish. Adults and nymphs should be housed separately as the larger individuals will eat the smaller ones. If you plan on raising large numbers of them, you may want to invest the time in making an artificial diet (see Cohen in the References), however they will also do well with fresh insect eggs (purchased), whitefly nymphs or aphids provided every few days. For moisture, place a one-inch piece of fresh green bean (replaced every couple of days) in the dish. A small piece of cotton placed in the adult cage provides a place for oviposition as well as a hiding place. Periodically remove the cotton from the cage (eggs are pale orange) and place it in a new cage with a fresh piece of green bean. In a week to ten days, nymphs will emerge and a protein source (eggs, diet, etc.) will need to be added. When cages become dirty or moldy, transfer adults and nymphs to a fresh cage. Adults can be handled with soft forceps or with a small soft paintbrush. Nymphs should be handled with a soft brush only.托福听力考试背景知识综合辅导(四十七)Giant Shield Volcanoes The giant shield volcanoes on Mars are truly huge. The largest are three times as high as the biggest Earth volcanoes. They also are bigger in diameter. Thus, the biggest volcano on Mars is comparable to a pile of nearly 100 Hawaiian volcanoes. Despite this difference in size, the Mars shields look a lot like shield volcanoes o