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第1段
1 .Listen to part of a lecture in a biology class.
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2 .Okay, let\'s talk about butterflies.
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3 .As most of you probably know a butterfly undergoes radical changes in the course of its transformation from an egg to an adult butterfly.
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4 .People have been fascinated by this metamorphosis for 1000s of years.
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5 .The butterfly life cycle has four stages.
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6 .The first stage is the egg; the second stage is the larva.
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7 .That\'s what most people refer to as a caterpillar.
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8 .The third stage is the pupa, and the fourth stage is the adult butterfly.
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9 .So for the first stage, the female butterfly has to deposit her eggs on a plant that has a lot of edible leaves.
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10 .That\'s because when the eggs hatch, the larva is basically eating machine.
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11 .It needs tremendous amounts of food to sustain its rapid growth.
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12 .Once it\'s grown to its full size, which takes about two or three weeks.
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13 .The larva attaches itself to a twig or a blade of grass and becomes a pupa.
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14 .The pupa is encased in a hard outer shell.
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15 .From the outside it may appear to be totally lifeless.
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16 .But inside the shell, some pretty amazing transformations are taking place.
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17 .The structure of the larva is breaking down
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18 .And being replaced by the structure of an adult butterfly.
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19 .If this process proceeds without any interruption
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20 .an adult butterfly can emerge within two weeks
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21 .but some pupae may remain dormant.
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22 .That is, they may remain in a resting state for a lot longer than that
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23 .and we\'ll talk more about that in a minute.
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24 .When the adult butterfly emerges from the pupa, its wings dry out and harden
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25 .and the transformation is complete, you then have a fully formed adult butterfly.
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26 .Now, typically, a large group of butterflies from the same species will emerge at around the same time, say, in the spring.
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27 .each one of these groups that emerge is called a brood that b, r, o, o, d.
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28 .butterfly species in cooler climates, where the summers shorter
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29 .generally produce only one brood per year.
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30 .A nice example is the brown elfin butterfly.
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31 .In the spring, the brown elfin females lay their eggs.
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32 .In the summer, the eggs hatch and the larvae feed until they turn into pupae.
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33 .During the fall and winter, the pupa rests, and then the following spring, the next brood emerges.
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34 .so one brood per year.
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35 .But in warmer climates, most butterfly species will produce two broods per year.
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36 .the second brood emerging late in the summer.
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37 .Now what\'s interesting is that the same species can be single brooded in cooler climate zones and double brooded in warmer climate zones.
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38 .And what\'s also interesting is that the number of broods produced is often tied in with the size of the butterflies.
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39 .In other words, you can have the same species and when it produces just one brood.
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40 .the butterflies that emerge are often bigger than when that species produces two broods per year.
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41 .you know, maybe in a warmer climate zone.
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42 .So it\'s often the case that the fewer the broods.
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43 .the bigger the butterflies.
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44 .The more broods, the smaller the butterflies.
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45 .And speaking of size, when you see a small butterfly.
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46 .It\'s not a baby butterfly.
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47 .it\'s a small, full sized adult.
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48 .That\'s because butterflies reach their full adult size before they emerge from their shell.
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49 .Now you have a lot of possible variations in butterfly life cycles and variations can give a species new avenues to develop and evolve.
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50 .It\'s possible, for example.
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51 .that at some time, you\'ll have a unique set of circumstances.
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52 .maybe some unusual environmental conditions.
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53 .and the next thing you know, you have a brown elfin butterfly flying in the fall.
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54 .Just because we haven\'t yet observed a brown elf and butterfly flying in the fall doesn\'t mean it can\'t happen.
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55 .I mean, we\'re now discovering that even in warmer climate zones.
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56 .some pupae may remain dormant for two, three or even four years
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57 .if there are unfavorable conditions, say a drought or a flood.
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