"There is nothing in a caterpillar that tells you it's going to be a butterfly," said R. Buckminster Fuller, one of my favorite authors, and he is surely right. If I ask you or anyone else whether you see a potentially lovely butterfly in a caterpillar, you would surely squirm and scratch your head in utter denial saying, “I could hardly recognize a butterfly in it. But the stark truth is clear—that the caterpillar cycle will soon lead into a colorful fluttering butterfly. “How could that be?” you would ask, “How can a crawler—that undergoes caterpillars stages called ‘instars’ become a flyer, and a lovely flyer?” I guess nothing is impossible in this world, and we can only marvel at how nature has made it possible for caterpillars to become butterflies.
The wormlike caterpillar is called the butterfly larva, so when the butterfly is in its caterpillar form, it is in the larval stage of the life cycle of the butterfly . The larval stage is the transitional stage from the ovum or egg stage to a fully-grown butterfly. The caterpillar stage also has substages called “instars,” and the average caterpillar usually undergoes four to five instars. In each of these instars, it molts.
Why Does Caterpillar Molt or Change its Skin?
We can compare a caterpillar to a black hole. Like a black hole that gobbles up everything in its path, the growing caterpillar — like a tiny eating machine — also eats up every leaf in its path. Because the caterpillar is in a transitional stage, it needs to eat and eat, so that it will quickly grow. So, within a few days after wiggling its way out of the eggshell, it usually devours enough food to double its size. Because of this rapid growth, it overstretches its skin to a point that its skin can no longer expand. This explains why a caterpillar sheds its skin in between instars.
Defense Mechanisms of a Caterpillar
Nature or the law of physics may have determined the number of moltings a caterpillar must undergo. Typically, a caterpillar undergoes 5 instars . In each of these developmental stages, the caterpillar must defend itself against predators, and so it has evolved various defense mechanisms to ward off dangers.
Caterpillars are desirable food for some predators. The body of a caterpillar is rich in protein. So, predators surely salivate for a hefty caterpillar. But caterpillars also have various defenses, and one of these defenses is their appearance. Some caterpillars look very poisonous and appear to be larger than their actual sizes. Some caterpillars also mimic their environment by adapting their colors to their surroundings. The Nemoria Arizonaria, for example, camouflages itself to avoid predators.
Some caterpillars, however, aggressively defend themselves. They usually have spiny bristles equipped with detachable tips. These tips lodge into the skin of any predator. Other caterpillars can get toxins from plants that make them poisonous or not good to taste; others use bristles with venomous glands.
Some caterpillars, such as the Amorpha Juglandis, for example, develop whip-like organs. They can wiggle these organs to create sounds that can ward off predators. Other caterpillars, however, can form a long link like a train or a chain, making themselves appear large enough to scare off predators.
How Many Instars Does a Caterpillar Undergo?
The caterpillar stages are called “instars,” and there are four to five instars in the caterpillar cycle depending on the species. In between instars is a molting process because a caterpillar rapidly grows, and it must quickly grow because it is vulnerable to the attacks of predators. Moreover, it should enter the pupal stage as quickly as possible. For this reason, it needs to be nutritionally ready to metamorphose into a beautiful butterfly.
A caterpillar spends most of its time looking for food to devour. Its mouth is designed for constant chewing! A Monarch caterpillar, for instance, can eat up to 20 milkweed leaves in one day!
Most caterpillars are herbivores, meaning they only eat leaves. Some, however, are predators like the Spalgis Epius caterpillars that eat insects; other caterpillars eat ant larvae.
Every instar ends with the shedding of the caterpillar’s outer covering. This molting happens with the help of the caterpillar’s neurohormones. During the molting process, the outer skin, called the "cuticle," is shed; it is detached from the softer under skin or epidermis. This epidermis then starts to form a new outer skin or cuticle. This molting or shedding of old skin starts with a split on the back skin of the caterpillar.
Molting with the Help of Hormones
Ecdysone , a hormone that prompts molting, is produced by the caterpillar’s body. This hormone enables the shedding of the old exoskeleton. The new exoskeleton is soft at first. Once the old skin is fully shed, the caterpillar inhales a lot of air to expand its body, after which the new soft skin hardens and becomes strong. Afterward, the caterpillar exhales the air to allow for more growth. The caterpillar cycle of molting generally undergoes four to five instars depending on the species of butterfly. Additionally, the caterpillar stages of instars usually last from 10 to 21 days.
The First Instar
The female butterfly searches for a hidden place where she will lay her egg. This place is usually the underside of a leaf so that when the little caterpillar comes out of the egg, it will have something to feed on. Then, the egg hatches after 3 to 8 days. As the egg cracks, the tiny crawling caterpillar emerges and feeds on the eggshell first. It then chews on the leaf and it quickly grows. The size of the larva usually depends on the type of butterfly. The Monarch larva during the first instar, for example, is usually around 1/8” and is as big as an ant.
The larva has a translucent body and doesn’t have banding or coloration. It also doesn't have tentacles. Its only instinct is to eat and eat and eat! Once it has consumed the egg case, it then gobbles up the leaf, chewing in a circular motion so that it will not get trapped by the flowing latex from the damaged leaf.
At this point, the tiny caterpillar eats up cardenolides. This is a form of steroid present in the plant leaves. It makes use of this substance to ward off "would-be" predators. Then, as its body grows, its skin expands. As the skin expands, it becomes overly extended and can no longer keep up with the growth of the caterpillar. At this point, the caterpillar begins to rest to molt its skin.
Its body produces neurohormones called ecdysones that enable the old skin to molt or detach from the softer epidermis underneath. Soon after, the caterpillar inhales lots of air, expanding its body as it sheds the old exoskeleton. Afterward, it exhales to allow its body to further grow. As a caveat, you should not assume that a caterpillar is dead when it stops moving. It may be immobile, but it is undergoing the molting process in between every instar. Don’t touch it because its body is very vulnerable at this point.
The Second Instar
The onset of the second instar begins when the old exoskeleton has been completely molted. Bands of different colors begin to appear on the body of the caterpillar as it devours more food. Tentacles also appear. After the first instar, the caterpillar begins to sport a larger body frame at around 1/4" in length. Its bands then begin to become more visible.
The Difference in the Mouth!
The main difference between the first and second instars is not the size of the caterpillar but the [size of the mouth] https://msu.edu/~shingle9/Documents/Instars.pdf). During the first instar, the mouth of the larva is very small and looks like two pairs of very small black spots. The caterpillar's mouth during the second instar, however, has larger hooks and highly defined structures.
Once again, the exoskeleton becomes highly expanded and the caterpillar begins to rest. Neurohormones are once again secreted by the caterpillar so that it can shed its exoskeleton. The exoskeleton then detaches leaving the underlying epidermis as the new covering of the caterpillar. The third instar then begins.
The Third Instar
The third instar begins and shows remarkable changes in the bodily structure of the caterpillar. Distinctive bands now become apparent, and tentacles continue to grow. During this stage, the caterpillar starts to eat along the edges of the leaf. The remarkable difference between the second instar and third instar lies in the structure of the caterpillar’s spiracles. The second-instar caterpillar has an anterior spiracle that appears club-like, while in the caterpillar of the third instar, the anterior spiracle appears branched. The third instar's posterior spiracles also have a dark orange ring, usually at the tip. This dark orange ring is not present in the caterpillar of the second instar. If you are a butterfly enthusiast, you can use this difference to figure out if the caterpillar is in the second or third instar.
The caterpillar of the third instar is also larger than the second-instar caterpillar, but its size usually depends on which species of butterfly it belongs to. Monarch caterpillars of the third instar are usually around 10 to 14 mm.
Once again, the same process happens! The caterpillar’s body expands, and it secretes neurohormones that enable molting. Its skin breaks and sheds, and it enters the fourth instar.
The Fourth Instar
The caterpillar then inhales more oxygen and molts again. As it sheds its exoskeleton, it exhales the air, allowing for more growth. It begins to eat and eat to grow. Its banding becomes distinctively different, while its tentacles further lengthen. It also develops prolegs. The prolegs also develop white dots. At this point, the caterpillar is almost 1 inch or around 13 mm to 25 mm long.
Most fourth-instar caterpillars eat around 1 leaf every hour. You can just imagine how voracious the fourth instar caterpillar is and how mad you would be upon seeing that your garden plants losing so many leaves.
Once again, the fourth-instar caterpillar begins to rest and look for a place to molt. Its body becomes inappropriately large for its exoskeleton; so, it breaks free of its skin via the help of neurohormones that enable molting. The old skin detaches then, and the underlying skin becomes the new skin. For some butterfly species, this is the last instar, but for many others, a fifth instar has just begun.
The Fifth Instar
During the fifth instar, the caterpillar develops highly intricate patterns of banding, White dots become obvious on its prolegs. The caterpillar reaches its maximum size and is around 2,000 times bulkier than when it first came out of the egg. Its length ranges from 25 mm to 45 mm, and it is around 5 to 8 mm wide.
The caterpillar still devours whole leaves every hour and sometimes it will sever the leaves from the branch by munching and cutting its petiole. It is preparing itself for the intense processes of the pupal stage.
As the fifth-instar caterpillar reaches its maximum size, it begins to be listless and starts to look for a safe and secluded place to spend its pupation period. Once it has found a good place, it will then latch itself onto a leaf by first weaving a silk pad. It then attaches its hind legs to this silk pad, called the “cremaster.” Afterward, it begins to molt for the last time as a caterpillar. It then encloses itself inside a sturdy yet transparent chrysalis.
The caterpillar stages of instars are truly periods of intense growth for the would-be butterfly and are necessary for the caterpillar cycle for reaching the ultimate stage of becoming a flying butterfly. The caterpillar eats and eats to get the nutritional sustenance needed to undergo the metamorphosis process. This intense metamorphosis from a wormlike creature to a birdlike creature is indeed magical. Some people might not like caterpillars because of their bristles and toxins that can sometimes give us rashes. But caterpillars need to make use of defense mechanisms to survive so that there will be new lovely butterflies to help in the pollination of flowers and to make this world a more colorful one.
You can read more about the caterpillar’s transition into a butterfly with this special issue of National Geographic Kids .