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 envelope''s shape is maintained by the pressure of the gas that fills it; there is no framework. The rigid''s control car and engines are suspended from the framework; in the non-rigid they are attached directly to or suspended d irectly 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.
飞艇按其结构可分为软式、半硬式、硬式。

(1)软式飞艇艇体由主气囊和前后副气囊组成。

气囊不仅要求密封，还要有相当
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，外界大气压强降低时，副气囊放气使主气囊增大容积，从而保持主气囊原来的内外压
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。

设置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，飞艇产生抬头力矩；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，但在气囊下部增加刚性的龙骨架，组
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，主要起维
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。

随着飞行高度增加，外界大气压强降低，囊内气体随之膨胀，在达到规定高
度时，气囊恰好胀满气室。

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。

1900年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，因此产生了相互间的水汽密度梯度，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，就使六角形雪片一直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程度下降，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理想状况。

事实上，事物与周围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。

如果周围空气输人水汽较少，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，便因为这个原因经常降落柱状雪晶。

空气里水汽饱和程度较高的时候，出现另外一种情况。

这时周围空气不断地向雪
片输送水汽，使雪片快速地发生凝华作用。

凝华降低了雪片周围空气层中的水汽
密度，反过来又促进外层水汽向内部输送。

这样，雪片便很快地生长起来。

当水
汽快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，顶角上会出现一些突出物和枝杈。

这些枝叉增长到一定程度，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，象宝石一样闪烁着瑰丽的光彩，
人们把它叫做钻石尘。

冰针的生长有二种情况：一种是在严寒下(-30℃以下)湿度很小时水汽自发结晶的结果，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 t hat 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 t hey 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，日冕不可能处于
稳定静止状态，而是稳定地向外膨胀，热电离气体粒子连续地从太阳向外流出，
就形成了太阳风。

近年来的观测表明，存在于日冕中的冕洞同地球附近的太阳风有很好的相关
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测到太阳风的存在，近年来利
用卫星观测近地空间，终于证实了太阳风的存在。

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处理的，这种理
论模型必然导致无结构的太阳风。

但实际上太阳风中很少存在这种状态，几乎所
有观测到的参量都有一种无规则的起伏。

起伏的原因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
结果中可以看出，随着太阳活动程度的降低，太阳风的流速也降低。

当太阳风流
速降至每秒320公里时，可近似认为太阳风处于宁静状态。

既然太阳风起源于日冕，人们有理由认为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”的观测结果，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个数量级之多，成因至今还是个谜。

观测表明，高氦量等离子体常常在日地间激波或地磁扰动突然开始后5—12小时内出现，这说明它与太阳爆发有关。

太阳风中的动力学现象包含许多随时间变化的复杂结构[高速等离子体流、日地间激波、阿尔文起伏等] ，大致可分为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。

常以激波的形式出现，这种激波是由耀斑区抛射出
的快速等离子体压缩太阳风而形成的。

因为等离子体具有较高的电导率，阻止了快速的相互渗透，所以只要抛射出来的快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；日地间激波平均传输的时间约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，因此传输过程中可能有某些微小的减速。

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
帕克完成的。

近年来的理论发展主要集中在研究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同，并且认为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交换项将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异；双流体模型导出电子温度大于质子温度，这个推断与观测结果一致，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。

不论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输是不够的，也许还有另外的能量传输形式，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，可能在日冕外区
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个，它的体积可以容纳1300多个地球。

它的
质量是地球质量的300多倍。

把太阳系所有其他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行一周需要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-150°C。

木星自转很快，自转一周只需9
小时50分30秒，是太阳系中自转最快的一个。

由于快速自转，使木星形状变扁，
不是正圆形，而是中腰鼓起的椭圆形。

很快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。

木星是一个没有固体表面的
星球，表面充满液态的氢。

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/秒的速度才能摆脱木星的引力，飞离木星。

在离木星几十万千米处围绕木星赤道的区域，有一个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小时的周期围绕木星高
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，有一个颜色明亮而鲜艳的大红斑，300多年来，
大红斑的形状几乎没有变化，大小和颜色却经常变幻。

长度最长时达到4万千米，最少也有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：大红斑原来是木星大气云层中的一个大旋涡，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。

它们主要由红磷化合物构成，而且不停地激烈运动。

科学家们批出，这实际上是
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猛烈风暴。

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 -190°F (-124°C) for the visible surface of the
C) at lower cloud levels; localized regions reach as high as
F (-13°
atmosphere, to 9°
40°F (4°C) 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 largest—Io, Europa, Ganymede, and Callisto—were 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 2000. Eleven small previously
unknown satellites were reported in 2001. If these sightings should be confirmed, it
er markings of the
would raise the number of Jovian moons to 28. … The spot and oth
atmosphere also provide evidence for Jupiter’s 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 p roduce 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示它的波长外，一般均用频率代替波长，其单位为
赫（Hz）。

频率范围约在30千赫（kHz）～30000兆赫（MHz）之间。

其波长范
围在10-3～104米之间。

当赫兹发现电磁波以后，首先被用于无线电信之传递试
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
马可尼同时独立地发明天地线制，马可尼且于天线中加接调谐电路，试验越过大
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，误认为波长越长，电力越大，通信距离越远；事实上在短波通信特性未发现
以前，确实如此。

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收率均较小。

在传播途中的衰减大致与距离成正比，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，不过波长
已减至1000米左右。

长波无线电特别适宜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，无法达成远距离通信。

长波通信须高大之天线。

自超短波及微波通信实现后，已可利用轻便之转继站构成通信网，故长波通信已在淘汰。

中波波段为标准广播波段，其波长自200～545米。

适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标准。

中波广播波段自540～1650千赫（kHz）。

中波广播也可用于空中导航。

短波通信之特点是：即使是不太强的无线电波也能跨越大洋，而完成极远距离通信。

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收率甚高，故不论发射电力多大，不出百里以内，
其沿地面进行的电磁波即被吸收以尽，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出现。

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。

围绕地球的这一层离子化气体称之为“电离层”。

无线电波也和光波一样有四种特性：它能被物体吸收、反射、散射及折射。

当电磁波前进时，遇金属物，则有一部分被吸收，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。

当电磁波经过不同的介质时，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；
另外有集中在地中海、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，所以成為活火山。

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，所以這股力量便在地底累積，
等到累積到相當的程度，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
现代人们对季风的认识有了进步，至少有三点是公认的，即：
（1）季风是大范围地区的盛行风向随季节改变的现象，这里强调“大范围”是因
为小范围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。

季风形成的原因，主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，所以到一定高度，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。

在我国为东南
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。

在我国为西北季风，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 Rearing
Natural History
Big-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.
Rearing
Big-eyed bugs are easy to rear. A dults 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 on Earth. Both have the same broad flat profiles, large central calderas, and similar lava flow features. The giant martian shields are also much older than any Earth volcano. The youngest lavas on the martian shields are about 20 to 200 million years old. The oldest lavas are near 2.5 billion years old. Thus, these giant volcanoes were active for billions of years. This may explain their large size. On Earth, plate tectonics is always moving volcanoes away from their magma sources.。