The juvenile hormone acts like a delay for the metamorphosis throughout the whole larva stage. It works by blocking the genes in the imaginal discs – tiny disc-shaped bags of cells that kick into action when the caterpillar wraps itself in the chrysalis, eventually turning intoan antenna, eye, wing or other butterfly bit. As such, the juvenile hormone is essential to the caterpillar’s survival prior to metamorphosis. You see, once the larva reaches its final moult and begins its metamorphosis, strange things happen to its body. Cells in the larva’s muscles, gut and salivary glands are digested and act as spare parts for the soon-to-be butterfly. Each cell is programmed to self-destruct through the activation of enzymes called caspases.
Imaginal discs
The caspases tear through the cell’s proteins, releasing prime butterfly-making material. Were it not for the juvenile hormone, this could have happened at any time killing the caterpillar. Instead, nature programmed the hormone to lower in levels at exactly the right moment, when metamorphosis is ripe. With less juvenile hormone around, instead of inducing a regular moult the ecdysone now drives the caterpillar to pupate. Once a caterpillar has disintegrated all of its tissues except for the imaginal discs, those discs use the protein-rich soup all around them to fuel the rapid cell division required to form the wings, antennae, legs, eyes, genitals and all the other features of an adult butterfly or moth. The imaginal disc for a fruit fly’s wing, for example, might begin with only 50 cells and increase to more than 50,000 cells by the end of metamorphosis.
