Diane Campanelli, age 12, of Staten Island, New York, for her question:
What process changes a caterpillar into a butterfly?
When you don't know what goes on, the change from caterpillar to but¬terfly seems like a miracle. In the past feGr decades, biologists have learned quite a lot about this remarkable happening. But what they tell us does nothing to shake our faith in miracles. Far from it. When we know something about this ama>ring process, we tend to regard it as even more of a miracle.
It 1s hard to believe that a grubby green caterpillar and a gorgeous swallowtail butterfly are one and the same insect. But this is true. It is hard to believe that a stodgy stiff chrysalis is busier than a beehive, when it seems to be fast asleep. This also is true. Inside that peaceful¬ looking pupa the miracle of metamorphosis completely remodels the caterpil¬lar to build a butterfly.
During its lifetime, the insect inhabits two entirely different bodies. Each body is built and governed by a different set of biological blueprints. The dramatic changing process that occurs inside the pupa is triggered by endocrinal secretions from glands in or near the head.
As the caterpillar completes its destiny, the blueprint that governed its body becomes inactive. The second blueprint takes over and issues a very different set of instructions. The previously famished caterpillar loses its appetite, its body becomes shorter and fatter. It spins silken ropes to suspend its shoulders from a twig and a tacky wad to attach its tail. In a few hours, its soft skin becomes a crisp, waterproof shell.
What we see is a chrysalis pupa, hanging from a twig and apparently in a deep sleep. However, great changes are happening. The process is governed by endocrinal secretions and organized around tiny units called histo¬blasts. First, the caterpillar's body is disassembled completely and far a time the pupa shell is filled with a soupy mishmash of biochemicals.
The mixture contains the rich nutrients that the feasting caterpillar stored in its cells. The busy biological blueprint organizes the building of entirely new cells from these materials. The new cells are organized around the histoblast centers to build organs and muscles for an entirely different body.
During the remodeling process, the wormy old body is discarded for one that has a distinct head, thorax and abdomen. The stubby caterpillar feet are traded for six long legs. The old chewing mouth parts are traded for sucking mouth parts. Four large wings are added to the shoulders and the remodeled insect now is a butterfly.
The adult butterfly may spend the winter inside her protective pupa. When spring returns, her glands are stimulated, perhaps by warmth or extra hours of daylight. Endocrinal secretions set in motion the final phase of metamorphosis. The pupa shell cracks apart and the butterfly struggles forte to complete the last phase of her life. At first, her wings are small and shriveled: As blood flows into them, they expand and become stiff. Now the adult insect is ready to fly off, mate and lay eggs for the next gener¬ation of small miracles.