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2008.12.13雅思A类阅读 最后一篇文章  

2008-12-13 18:40:58|  分类: 雅思历次考试笔试 |  标签: |举报 |字号 订阅

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12.13阅读 最后一篇的文章:

 都是填空和T,F,NG,最后还有一个加标题的选择题!

 The typical antnest begins with a single individual, the queen. This queen flewfrom her home nest a day or two earlier, together with other queensand males from her nest and other nests in the area. The queensearches for a mate, often being attracted to large, distinctiveobjects such as especially tall trees, large shrubs or hill tops.These sites act as meeting places for queens and males from manynests, ensuring that they can find each other. The queen then mateswith one or a few males while still in the air or on low vegetationor the ground. Once mated, she searches for a suitable nest site.Where she searches will vary with the species and can range fromthe tops of trees to open soil.

Either during her search or once asuitable site is found she bites off her wings as they are nolonger needed. She then seals herself into a small chamber or othersecluded place and lays a small batch of eggs. The queen remains inthe nest with her brood while it develops, feeding the growinglarvae special trophic (unfertilised) eggs which she laysspecifically as food for them. These first workers (callednanitics) are often much smaller than subsequent workers as thequeen can only provide a limited amount of food compared to thatwhich foraging workers can provide. Once these initial workersmature, they leave the nest and begin to forage, returning capturedprey to the queen and her additional brood.

The colony grows as more workersmature, these new workers taking over the care of brood as well asbringing in additional food. At this stage, the queen reduces heractivities to egg laying and the workers assume all other tasks inthe nest. The queen is still essential for normal colony life,however, as the chemical messages she produces controls theactivities of all workers in the nest.

The above pattern of nest foundingis one of the most common and widespread, however many species varyfrom it. For example, in some Rhytidoponera mating may take place on or in existing nests。It is also common for several queens to establish a nest togetherand then either live together or later fight among themselves todetermine which queen remains in the nest, the others being forcedout or killed . In other species, new colonies are established whena new queen leaves an existing nest together with a number ofworkers and relocates to a new location some distanceaway.

Queens may forage outside the nestbefore the first workers emerge. They may remain in their parentalnests or form small satellite nests which share workers with themain nest . In others, brood may be removed from the main nest andplaced in small groups together with a few workers near foodsources 。The specific details of nest founding is as varied as theants themselves. For a more comprehensive discussion, seeH?lldobler and Wilson (1990).

As the colony reaches maturity, itbegins to produce the queens and males (called alates) which willform the next generation. Numerous factors determine when queensare produced, including the time of year, the food available to thegrowing larvae, the size and contents of the egg laid, pheromonesor hormones produced by the queen and the age of the queen. Theproduction of males has a much simpler controlling mechanism. As itturns out, females (queens and workers) are diploid. That is, theyhave two copies of each chromosome. Males, on the other hand, arehaploid and have only a single copy of each chromosome. Because ofthis, fertilised eggs, where an egg and sperm combine, form femaleswhile unfertilised eggs develop into males. These factors combineto give the colony a high degree of control over when queens andmales are produced, as well as the relative numbers ofeach.

The larvae of these new queens andmales are similar to the larvae of workers but are generallylarger. Once they emerge, they remain in the nest waiting forenvironmental triggers to initiate their leaving the nest. Thesesame triggers will cause the simultaneous release of queens andmales from the vast majority of nests of a given species in a givenarea. Because of this, huge numbers of queens and males can bereleased on the same day, sometimes over hundreds of kilometres. Inopen country with scattered shrubs, large numbers of flying antscan be seen hovering over each shrub over wide areas. These massemergences last only a few days, with the queens mating andattempting to establish new nests while the males generally diewithin several days of leaving their nests.

Once established, individual nestscan last many years. Queens of the common "sugar ant" ofsouth-eastern Australia, Camponotus consobrinus, have beenknown to live as long as 23 years in captivity 。although they are likely to be more short-lived in nature. In somespecies, established colonies will produce or accept new queens ifthe existing queen dies. Other species add additional queens as thecolony grows. In these species, nests can potentially lastindefinitely as new queens replace older queens, thus ensuring theconstant production of new workers. Individual workers generallylive a few years but detailed studies are few. Males can remain innests for some months and in some species they will over winter.However, most will die within a few days of leaving theirnests.

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