Metamorphosis: A Caterpillar Becomes A Butterfly Help
Introduction to Metamorphosis: A Caterpillar Becomes A Butterfly Help
No doubt you have seen a humble caterpillar crawling slowly across a leaf. But hidden from your gaze, at the end of summer the caterpillar begins to undergo a dramatic metamorphosis ( met -uh- MOR -fuh-sis) – a “changing over” (meta-) of its body shape or “form” (morph).
In most cases, metamorphosis is a mechanism whereby an embryonic stage of an animal (such as an insect) changes into a strikingly different adult body form. One of the most beautiful examples of metamorphosis is the transformation of a caterpillar into a gracious, delicate-winged butterfly. Some of the major steps in this process are shown in Figure 11.7 for the well-known monarch ( MAHN -ark) butterfly. Like a king or queen or some other type of “ruler-alone” (monarch) in the world, the adult monarch butterfly stands alone in its striking orange-and-black body pattern.
The process begins with a larva (caterpillar) stage. The black-striped monarch caterpillar (larva) likes to feed on the milkweed plant, named for its white, milky juice. The caterpillar just keeps gorging itself on milkweed leaves and growing throughout the summer, molting its skin several times. Finally, at the end of the growing stage, the caterpillar larva firmly attaches itself to a branch. Here it molts several more times, then encases itself within a cocoon (kuh- KOON ). A cocoon is an external “shell or husk” of silky material that caterpillars spin around themselves during their preparation for metamorphosis.
When the larva is encased within a tough, protective cocoon, its new stage of development is called the pupa ( PYOO -puh) or chrysalis ( KRIS -uh-lis). In Latin, chrysalis literally means “golden pupa of a butterfly.” Likewise, pupa means “girl or doll.” So, the pupa (chrysalis) is poetically described as a “golden girl or doll” that will eventually mature into a queen-like, elegant butterfly! Perhaps the reference to “golden” is due to the rather shiny appearance of the outer wall of the cocoon enveloping the chrysalis (pupa).
Within the pupa, a precisely timed genetic program is now turned on. The tissues of the larva are broken down, then replaced by other cells that undergo mitosis and differentiation ( dif -uh- ren -she- AY -shun) – “the process of becoming different” or specialized. Certain body cells, for example, migrate to the sides of the pupa in a bilaterally symmetrical manner. The cells in these locations then differentiate ( dif -uh- REN -she-ate) into the highly specialized structures of the developing butterfly wings.
Soon after the arrival of spring, the adult begins to emerge from the cocoon. At first, the wings are flat and wet and pressed against the sides of the butterfly body. But a pumping process pushes fluid out into the veins of the wings, stiffening and opening them. The result is a beautiful, orange, black-striped, bilaterally symmetrical monarch butterfly – one of the most impressive displays of Biological Order known to humankind!
Practice problems for these concepts can be found at: Arthropod Test
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