The egg

The secret of metamorphosis

The egg has many identical cells. Some of which have been chemically tagged to form the caterpillar while others have been tagged to become the butterfly.

The caterpillar is actually only oneof two possible forms of the same creature. Although all the cells of the caterpillar and butterfly are identical they differ in that some cells have the caterpillar genes switched off and the butterfly genes switched on and the rest have the caterpillar genes turned off and the buttefly genes switched on. Initially, it is the cells that have the caterpillar genes switched on that are encouraged to grow under the chemical influence of hormones. When the caterpillar matures the cells with the butterfly genes turned on are stimulated into action at the expense of the caterpillar cells.

You see, once the egg is fertilised, cells within it start to divide and multiply in numbers. Up to this point any cells can contribute to the formation of any part of the adult insect, for each cell has an identical set of genes which are the blueprints for the development of a complete adult.

A chemical, hormone, sweeps across the cells and tags some of them to become caterpillar cells. A while later a second chemical sweeps across and assigns the rest of the cells to the development of the butterfly body parts. Now the egg consists of identical cells with specialised functions. The cells committed to the formation of the caterpillar are now encouraged to grow in size while those cells responsible for the butterfly body parts lie dormant amongst the caterpillar cells. The butterfly cells lie in the body of the caterpillar in small groups called imaginal disks and are prevented from developing by a chemical called juvenile hormone that circulates in the caterpillar's blood. The imaginal disks grow as the caterpillar grows, taking food from the caterpillar in a kind of parasitic existence.
When the caterpillar has fully matured, the secretion of juvenile hormone ceases and the butterfly cells start to develop. The absence of juvenile hormone also triggers the death of the caterpillar cells, their death and eventual breakdown will provide the nourishment to feed the butterfly cells.


Explain what is happening inside the pupa as it rests?

Answer true or false

The caterpillar is a different organism to the butterfly

Inside the pupa, caterpillar cells are breaking down.

Caterpillar cells turn into butterfly cells while in the pupa.

The egg contains only caterpillar cells

Home of butterflies