The so-called "complete metamorphosis" of moths and butterflies from larva to pupa to adult must be considered one of the most astounding transformations in nature.
I have been following this process in Hyalophora cecropia, commonly known as the "Cecropia moth" and one of North America's largest moths, with wingspans reaching 6 inches or more.
Last year I purchased some eggs. The cecropia moth caterpillars feed on a wide variety of host plant leaves including cherry, birch and maple - and they feed on the leaves of the lilac bush that grows just outside my door here on the Front Range of the Rocky Mountains.
The eggs are tiny - about 2 mm in diameter, and the hatchling caterpillars are black with spiky protuberances called "scoli" that apparently serve a defensive purpose.
Hatchling caterpillars and eggs on the surface of a paper bag.
The caterpillars eat their way through the egg shells and immediately begin consuming leaf material - an endeavor which they will pursue without pause for about six weeks until they spin cocoons and pupate within.
Two 2nd instar caterpillars. The one on the right has just shed its skin which is collapsed near its tail at right. After resting for about 30-45 minutes, the caterpillar will turn around and eat the discarded skin.
A 2nd instar caterpillar after it has assumed its normal appearance - yellow with black scoli and spots.
As they feed and grow they shed their skins to accommodate growth. The hatchlings are called "1st instar" larvae, the largest caterpillars are called "5th instar".
3rd instar larva devours its just-shed skin. The 3rd instar caterpillar's scoli are red, blue, yellow, and black, depending on location on the body.
Close up of the scoli near the head of a 3rd instar caterpillar showing the colored scoli.
There is an interesting article here whose abstract states, "The morphology of the variously coloured scoli (bristle-bearing structures on the integument, producing an exocrine secretion) on caterpillars of Hyalophora cecropia and the secondary chemistry of the discharged secretions have been investigated for the first time and compared. According to our scanning electron microscopic study, the red/orange, yellow and blue coloured groups of these glands differ morphologically. Gas chromatographic-mass spectrometric analyses showed that the patterns of secondary compounds in the respective glandular secretions are also different. Furthermore, the secretion of the penultimate larval instar is chemically distinct from that of the last instar, as are both secretions from the respective haemolymph. The results favour the idea that the differences in scoli colour, morphology and chemistry could affect various predator species differently."
So the above paper suggests that not only are the variously-colored scoli different in form and structure, but that the chemical compounds they release are different [gas chromotography/mass spectrometry is a method for separately complex mixtures and then determining the chemical structures of the components]. A pretty impressive defensive tool box for the caterpillars.
5th instar caterpillar begins pulling together lilac leaves and fashioning the cocoon in which it will pupate and overwinter.
About 13 hours later, the cocoon is taking shape, but the caterpillar is still working.
The 5th instar larvae [who have shed their skins 4 times] spin cocoons, and shed their skins again to become pupae. They overwinter inside their cocoons and the the pupae shed their skin and "eclose" from the cocoons as adult moths.
Ten or eleven months later
During the long time the pupa are in the cocoon, dramatic changes are taking place. Body structures such as the scoli, the mouthparts, and even the fleshy pairs of prolegs that support most of the caterpillar's movement and support the rear two-thirds of its body disappear. Completely new structures appear such as antennae, large compound eyes, and most dramatic of all the large cinnamon-colored furry-looking wings.
Around mid-day, the pupa sheds its skin, and the adult moth pushes through the loosely-woven top of the cocoon and crawls out. Within about half an hour, its wings expand and set.
Adult moth with wings almost fully expanded.
Male adults have plumose ["feathery"] antennae that are used to detect the pheremones secreted by female adults. The female antennae, in contrast, are much less expansive.
Male moths have plumose["feathery"] antennae that are capable of guiding them to the source of a particular chemical compound that the females moths produce.
By far the most striking feature of adult Cecropia moths are the wings: large, strong, and "furry", with dramatic features on both the fore and hind wings. All of the coloration is from tiny scales that reflect different colors of light.
Male Cecropia moth in all his glory.
Closeup of moth forewing showing eye spot and other patterns are formed from tiny scales of various colors. A sort of pointillistic painting.