Though only 9 of 26 insect orders are holometabolic, this accounts for 80 per cent of all insects butterflies, beetles, moths, flies, bees, wasps and ants are majority stakeholders. In short, this is a very successful strategy for growth and development. Want to see the process for yourself? How do I rear a caterpillar? Even in the smallest caterpillar, just hatched from the minuscule egg, bundles of cells are already primed, destined to become adult features such as antennae, wings, legs and genitalia.
Called imaginal discs being flat and round , they are prevented from growing and developing by a constant wash of a juvenile hormone. As the larva feeds, its gut, muscles and some other internal organs grow and develop, but the imaginal discs are temporarily suppressed and remain dormant. The caterpillar behaves like a free-living, eating, growing but developmentally repressed embryo. When it reaches a critical size, a burst of moulting hormone, ecdysone, is released.
It will shed its skin several times in response to ecdysone, each time forming a new instar stage , but juvenile hormone keeps it a caterpillar, preventing onward development until, as it nears full size, concentrations of the latter hormone decline.
In the fifth and final caterpillar instar, the imaginal discs have already begun to emerge from their enforced dormancy and started to grow. Certain highly organized groups of cells known as imaginal discs survive the digestive process. Before hatching, when a caterpillar is still developing inside its egg, it grows an imaginal disc for each of the adult body parts it will need as a mature butterfly or moth—discs for its eyes, for its wings, its legs and so on. In some species, these imaginal discs remain dormant throughout the caterpillar's life; in other species, the discs begin to take the shape of adult body parts even before the caterpillar forms a chrysalis or cocoon.
Some caterpillars walk around with tiny rudimentary wings tucked inside their bodies, though you would never know it by looking at them. 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, cells by the end of metamorphosis.
Depending on the species, certain caterpillar muscles and sections of the nervous system are largely preserved in the adult butterfly. One study even suggests that moths remember what they learned in later stages of their lives as caterpillars.
Getting a look at this metamorphosis as it happens is difficult; disturbing a caterpillar inside its cocoon or chrysalis risks botching the transformation. But Michael Cook, who maintains a fantastic website about silkworms , has some incredible photos of a Tussah silkmoth Antheraea penyi that failed to spin a cocoon. You can see the delicate, translucent jade wings, antennae and legs of a pupa that has not yet matured into an adult moth—a glimpse of what usually remains concealed.
Ferris Jabr is a contributing writer for Scientific American. The job of the caterpillar is to eat and eat and eat. As the caterpillar grows it splits its skin and sheds it about 4 or 5 times. Food eaten at this time is stored and used later as an adult. Caterpillars can grow times their size during this stage. For example, a monarch butterfly egg is the size of a pinhead and the caterpillar that hatches from this tiny egg isn't much bigger. But it will grow up to 2 inches long in several weeks.
When the caterpillar is full grown and stops eating, it becomes a pupa. The pupa of butterflies is also called a chrysalis. Depending on the species, the pupa may suspended under a branch, hidden in leaves or buried underground. The pupa of many moths is protected inside a coccoon of silk.
This stage can last from a few weeks, a month or even longer. Some species have a pupal stage that lasts for two years. It may look like nothing is going on but big changes are happening inside. Special cells that were present in the larva are now growing rapidly. They will become the legs, wings, eyes and other parts of the adult butterfly. Many of the original larva cells will provide energy for these growing adult cells. The adult stage is what most people think of when they think of butterflies.
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