Carroll Williams: experiments on the metamorphosis of butterflies
The birth of a butterfly
The metamorphosis of the caterpillar into a butterfly is one of the most common biological phenomena. From an egg laid by the butterfly, a small caterpillar hatches which nourishes truthfully for 6-8 weeks, until maturity; it is then a fat worm of about 8 cm. The larva then spins the cocoon, inside which it turns into a pupa. During a period of hibernation the pupa's body is somehow rearranged to form a mature moth that will find a mate and repeat the cycle. But what causes this incredible body change? What mechanism controls the chronology of these phenomena so that the fragile moth leaves its cocoon only in the warmth of spring? Over the past two decades, some elegant experiments have helped to shed light on the mystery.
From egg to pupa: story of a Cecropia. An egg hatches after 10 days (1). The caterpillar, which has turned green to blend in with the substrate, grows enormously and changes 4 times (2). Spin a cocoon with liquid silk secreted by buccal glands (3). In 4, 5, 6 the transformations of a pupa within the cocoon which here has been removed for a clear vision.
The butterfly comes out of its papal envelope, which begins to split on the side of the head; then, with the help of its legs, the insect gets rid of the envelope completely and shows its folded wings. In short, the butterfly can fly and becomes a machine destined for reproduction, which lives only to find a mate to mate with.
Carroll Williams
In 1942 the biologist Carroll Williams began a series of brilliant experiments that led to the solution of the mystery of metamorphosis. Working on cecropia, Williams soon realised that what directed the phenomenon of metamorphosis was located in the front of the insect. If a pupa was cut into two parts, the front half developed into a half butterfly while the back remained pupa. Williams also found that the cause of the change was located in the head. The experiments of the scientist, some of which are presented here, revealed two independent hormone-producing centers: one in the brain, the other in the chest, just behind the head.
Further experiments showed that an increase of the temperature stimulates the flow of the brain hormone and also initiates the wonderful transformation that occurs in the hibernating moth.
A caterpillar can develop even without hormones. To prove this fact, Williams separated the head from the thorax and then the thorax from the tail with a tight ligature.
With these separations the caterpillar continued to live but without metamorphosing.
With a hormone, development begins. The ligation in the insect was performed after the brain hormone entered the circulation, but before the thoracic hormone was produced. The thorax has developed, but the same cannot be said of the tail.
With both hormones metamorphosis occurred. The head and chest were separated with a ligature after the hormones were produced. Experiments show that cephalic hormone stimulates thoracic hormone and metamorphosis.
A decapitated female develops to mutate and lay an egg.
When hormone-producing centers are introduced into the adult pupa whose nipped end is covered with plastic, the abdomen develops into the posterior part of an adult butterfly.
This female fragment attracted a male, was fertilized and laid numerous eggs (picture below).
By dividing the pupae in half, the effect of the lesions on the metamorphosis can be checked. For a comparative examination Williams used four butterfly pupae of the same age. From left to right: a whole pupa, a divided pupa, whose two cutting surfaces are covered with plastic, a divided pupa but then joined by a tube, and finally a divided pupa with the two halves joined by a tube in which a movable ball can stop any tissue that grows and attempts to rejoin the two segments.
A month later the experiment ended. The first pupa develops normally; the second has a metamorphosed front part and not the back; the third presents a reunion of the pieces by means of a cord of tissue that has grown into the tube and through which the hormones can pass. Both extremities developed. The fourth pupa, in which the movement of the ball prevented tissue growth, did not develop. Williams concluded from these experiments that wounds must heal before metamorphosis.
A fatal flight concluded the experiment. The third pupa, which has become a butterfly, extends its wings and flies away. Although the two sections of the body had developed, the delicate fabric bridge within the tube broke and the insect fell to the ground and died.
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