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The pupa
This is the stage that makes the difference between an insect with a complete metamorphosis and an incomplete metamorphosis, it places the lepidopteran among the insects with complete metamorphosis. For the duration of this phase, the pupa is not mobile, legs, eyes, digestive tract and mouth parts are completely lacking. Inside the pupa a number of muscles, the heart and nerves still function, enough to stay alive during the period of one week to sometimes four years depending on the species. Occasionally on a rotating movement of the abdomen is the only thing that seems to happen but nothing is further from the truth.
Development
During embryonic development of the caterpillar in the egg, unspecified clusters of cells have been inactive till now these are called imaginal discs. Hormonal activation causes the beginning of cell division and specification. The nutritious soup containing amino acids and fats, provide plenty of building blocks for the growth of new cells. The specified cells will grow out to parts specific to the appearance of the lepidopteran like compound eyes, wings, antennae, new legs and in many cases a proboscis. In addition, new organs are developed as well like a new digestive tract and reproductive organs, a new external skeleton and muscles so the lepidopteran can fly.
Appearance
After the caterpillar has shed its skin, a soft, green or yellow pupa is left, the contours of the folded up lepidopteran are often clearly visible. In most cases, wings and antennae are visible as separate components immediately after moulting, hereafter they are carefully folded against the body and together with the rest of the skeleton, harden to create a solid pupa. Moth pupae that stay underground, hide in the litter layer or make a cocoon are smooth and coloured brown to black. Pupae of butterflies are usually quite different. Because these pupae are visible to everything that thinks of it as a nutritious meal, camouflage is most important. Therefore, the pupae often have the same colour as the surface they are attached to. The smooth appearance is exchanged by ridges and extensions so they are harder to find, they often look like a curled leave, thorn or a broken twig. They can also have very remarkable colours to show their toxicity but that is quite rare. At the end of the development of the lepidopteran, a discoloration of the pupa could be visible, sometimes stretching of the pupa is visible which makes the pupa longer and forces the abdominal segment to get further apart. In many cases, the patterns on the wings of the lepidopteran can be seen through the shell of the pupa, this is a sign that the lepidopteran is fully grown and will emerge within a few days. When it's time to get out, the pupa will open up near the head and the butterfly or moth crawls out of its pupa shell. For moths who emerge in a cocoon or underground , there is still a tough job ahead.
