How Caterpillars Work

Four caterpillars munch on a leaf in Thailand. See more insect pictures.
Four caterpillars munch on a leaf in Thailand. See more insect pictures.

Before it even emerges from its egg, the very first thing a caterpillar does is eat. It chews its way out of its egg, and then it typically eats the rest of the eggshell. After that, it starts devouring the plant it's standing on. The caterpillar eats until it's literally too big for its body, and then it molts, revealing a newer, roomier skin. Some caterpillars will even eat things that could harm them -- for example, the corn earworm (Helicoverpa zea), a moth caterpillar, uses an enzyme in its saliva to break down the nicotine in tobacco plants that would otherwise kill it.

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It's hard to imagine that a chubby, voracious munching machine will eventually become a delicate, winged butterfly or moth. If you hold a caterpillar and butterfly of the same species next to each other, you might not see any similarities at all. It can seem like the only thing they have in common is single-mindedness: Caterpillars are all about eating, and butterflies are all about sex.

This single-purpose lifestyle is why caterpillars and butterflies look so unlike each other. Their bodies have the same basic parts, including a head, a thorax and an abdomen, but these parts look different because they're suited for very different purposes. A caterpillar's body is adapted for eating food, turning it into fuel and storing it. A butterfly's -- or a moth's -- body is adapted for finding a mate and reproducing.

In this article, you'll learn about a caterpillar's journey from egg to chrysalis, and you'll find out exactly what's happening inside the chrysalis as a caterpillar transforms its body into a butterfly. You'll also learn why some caterpillars use their waste as projectiles, eat their own skin or disguise themselves as bird droppings. And if hungry, tent-building caterpillars have commandeered the trees in your backyard, we'll tell you what to do to save your foliage.


Caterpillar Anatomy: A Tube for Food

Egg of red admiral butterfly (Vanessa atalanta)
Egg of red admiral butterfly (Vanessa atalanta)
Kim Taylor and Jane Burton/Dorling Kindersley/Getty Images

A caterpillar's life starts with a textured, patterned egg and ends with a chrysalis -- a protective covering in which the caterpillar pupates, or undergoes a metamorphosis. Once the insect leaves the chrysalis, it's a full-grown butterfly or moth. It's the same species as it was before, but it no longer looks much like a caterpillar.

Between egg and chrysalis are a series of molts, in which the caterpillar sheds its too-tight skin, typically eating it afterward. This gives the caterpillar a little extra nourishment and also gets rid of evidence that could attract predators. The stage between each molt is called an instar, and most caterpillars go through five of them, growing very quickly and consuming lots of food to power their metamorphosis. The length of each instar varies based on the caterpillar's species, its food intake and the weather. Molting gives caterpillars more room to move, but it doesn't wipe out what they've learned about their environment -- most likely, their memory lasts over one or two molts.


A caterpillar's body is basically a tube for processing and storing food. A set of mouth parts lets the caterpillar chew its food -- typically leaves and other plant parts. The mouth empties into a very long intestine with fore and hind parts. Here, the caterpillar's digestive system breaks down the food and eventually stores it in a layer of fat called the fat body.

Most caterpillars are fuzzy (or prickly) on the outside and soft on the inside
Most caterpillars are fuzzy (or prickly) on the outside and soft on the inside
2008 HowStuffWorks

Six legs attached to the thorax let the caterpillar move around. Additional pairs of prolegs support and move the length of the caterpillar's abdomen. These prolegs end in small, hook-like suction cups called crochets. Since prolegs don't have segments or joints, they're not real legs, so even though it doesn't look like it, a caterpillar is a six-legged animal.

Most caterpillars move in one of two ways. Some crawl, moving each pair of prolegs and their true legs in sequence. Others, like the caterpillars of geometer moths, have no prolegs in the middle part of their abdomen. These caterpillars move in little arches, appearing to measure the surface under them.

The rest of the caterpillar's body lets it survive and get around:

  • Spiracles are holes in the caterpillar's sides through which it breathes.
  • Antennae provide sensory input, particularly relating to taste and smell.
  • In many species, false eyes help distract predators while real eyes allow the caterpillar to see.
  • Hairs, spines and quills called setae can deter predators and even carry toxins and irritants.
  • An osmeterium, found on swallowtail caterpillars, produces a foul-smelling substance that deters predators.

Caterpillars also have spinnerets, or silk-producing organs, in their heads. Next, we'll look at how silk can save a caterpillar's life, why it effects how quickly a caterpillar grows and why it's crucial to a successful metamorphosis.

Silk and Caterpillar Behavior: Tents and Trellises

This spicebrush swallowtail (Papilio Troilus) caterpillar has eye spots that make it look like a snake.
This spicebrush swallowtail (Papilio Troilus) caterpillar has eye spots that make it look like a snake.
©iStockphoto/Mark Ross

A caterpillar's vast food intake makes it an attractive meal, so many species use camouflage to protect themselves from predators. Some swallowtail caterpillars, for example, bear a strong resemblance to bird droppings through their first few instars. When they get too big for this to be convincing, they molt and reveal green bodies with eyespots that make them look like snakes. Other caterpillars use color to blend in to their surroundings or warn predators that they taste bad or cause illness.

But caterpillars use more than just the color and shape of their bodies to avoid predators. Some feed from the undersides of leaves, making it harder for airborne birds to see them. Others make small meals of lots of leaves or eat only partway through the leaf's structure, cutting down on visible damage. In addition to eating their shed skin, a few species make an extra effort to clean up behind themselves. The silver-spotted skipper (Epargyreus clarus), for example, launches its waste away from its leaf shelter to keep predators from tracking it back to its source.


To build its leaf shelter, Epargyreus clarus takes advantage of one of caterpillars' most useful tools -- silk. Many use their silk like a rock climber's rope and harness. If they fall, they can climb back up a fine thread to the leaf. Some species do this deliberately to escape predators.

Gregarious tent caterpillars rest inside their silken nest.
Gregarious tent caterpillars rest inside their silken nest.
© iStockphoto/Kris Hollingsworth

This might be all that a solitary caterpillar does with its silk during its day-to-day life. It hatches from a single egg and leads an independent existence, using silk only to rescue itself, make a small nest or -- in rare cases -- restrain prey. But for gregarious species, which live in large groups, silk is crucial to survival. Gregarious caterpillars hatch from large clumps of eggs that completely encircle limbs or trunks of trees. As a group, they build large tents, bivouacs or other shelters using their silk and neighboring plant material.

Gregarious caterpillars also lay trails of silk from the tent to the food, and they use silk to build a network of trellises to make it easier to get around the tree where they live. They usually travel in single-file lines, following trails marked with silk and scents. Younger caterpillars rely on the silk left by their older siblings. As a result they grow and develop faster because they don't have to spend energy they could be using to grow on making silk.

One vital use for silk has to do with the caterpillar's metamorphosis -- most species use their silk during this process in one way or another. Next, we'll look at how it's possible for a caterpillar to transform completely and exactly what silk has to do with it.

Caterpillar Metamorphosis: The Magic Within the Chrysalis

A monarch caterpillar twists to embed its cremaster in a silk pad. Then, it sheds its skin one final time, revealing its chrysalis.
A monarch caterpillar twists to embed its cremaster in a silk pad. Then, it sheds its skin one final time, revealing its chrysalis.
©iStockphoto/Kathy Keifer

A caterpillar spends most of its life crawling on -- and devouring -- its food source. But when it's time to become an adult, most caterpillars start to wander away from what they've been eating. They find a sheltered, safe spot in which to pupate, or transform into an adult. In all caterpillars, this happens inside a protective shell known as a chrysalis, but the specifics differ from species to species.

Let's look at the monarch caterpillar (Danaus plexippus) as an example. After wandering for a while, the caterpillar makes a simple silk pad on the underside of a branch or twig. It uses a hook-covered appendage called a cremaster to attach itself to this pad. It twists around, embedding its cremaster firmly in the silk. Then, it sheds its skin, revealing the chrysalis. The chrysalis hangs upside down from the cremaster until the butterfly is ready to emerge, or eclose.


Other caterpillars use variations on this process when they pupate. Instead of hanging upside down, some make a silk sling from a tree branch to support themselves as they pupate right-side up. Others create a silk hammock to support their chrysalis. The chrysalis starts out soft and skin-like, but gradually hardens to form a protective shell. Often, moth caterpillars spin a cocoon to protect their chrysalis, which starts out soft and skin-like. However, it will gradually harden to form a protective shell. The moth caterpillar may also disguise the cocoon with leaves or other debris.

Caterpillars use different types of support for their chrysalis. Clockwise from the top right: a silk hammock, a silk pad and supportive loop, a silk cocoon on a leaf, and a suspended chrysalis.
Caterpillars use different types of support for their chrysalis. Clockwise from the top right: a silk hammock, a silk pad and supportive loop, a silk cocoon on a leaf, and a suspended chrysalis.
2008 HowStuffWorks

It's easy to think of a chrysalis as something a caterpillar makes, like a cocoon, but this isn't really true. The body that the caterpillar reveals when it sheds its skin for the last time is the chrysalis. Along with the ability to occasionally twitch in response to threats, this shell is what protects the caterpillar while it transforms.

The transformation itself is amazing. Think of it as recycling -- if you drop a plastic bottle off in the recycling bin, it can be melted down into an entirely different shape. This is what happens inside the chrysalis. Much of the body breaks itself down into imaginal cells, which are undifferentiated -- like stem cells, they can become any type of cell. The imaginal cells put themselves back together into a new shape. A few parts of the body, such as the legs, are more or less unchanged during this process.

This process of complete transformation is known as holometabolism. The amount of time required to transform completely varies from one species to another, but in general it takes about two weeks. For species that survive the winter by staying in the chrysalis, it can take months.

According to research at Georgetown University, the caterpillars and butterflies that emerge from the chrysalises may remember some of their lives as caterpillars [source: Blackiston]. When caterpillars in the study were conditioned to avoid specific smells, they remembered to keep away from those scents as adults -- but only if the conditioning happened in the last instars of the caterpillars' lives. If you want to learn more what happens once the chrysalis opens, read How Butterflies Work.

Aside from the damage hungry moths can cause to clothing, most people think of the butterflies and moths that emerge from the chrysalis as beautiful and beneficial. But not everyone feels the same way about caterpillars. Read on to learn what to do if hungry caterpillars are destroying the plants in your yard.

Managing Caterpillars

The caterpillars in this tent have stripped the leaves from many branches of a tree.
The caterpillars in this tent have stripped the leaves from many branches of a tree.

In general, caterpillars are beneficial insects. They become butterflies and moths, which are important pollinators. Butterflies and caterpillars alike are food sources for other animals -- and even for people. Although all species damage plants as they eat, this damage is often minimal, and caterpillars' important role in ecosystems offsets holey, unattractive leaves.

But swarms of gregarious caterpillars can do more harm than good, completely defoliating large swaths of forest. They can cause other problems as well. Gypsy moth caterpillars (Lymantria dispar) can do so much leaf damage that trees eventually die. Eastern tent caterpillars (Malacosoma americanum) have been linked to mare reproductive loss syndrome (MRLS), which causes miscarriages and stillbirths in horses. The hair-like setae of some species can cause allergies and extreme irritation in animals and humans -- sometimes so severe that it requires hospitalization.


In these situations, caterpillar control or extermination can be important. Here's what to keep in mind if you're trying to protect your trees or plants from caterpillar pests:

  • In spite if what you may have heard, attempting to burn tents from the trees is not a good idea, particularly in dry conditions. The risk of fire overshadows the immediate extermination of the nest.
  • Keep in mind that wasps often prey on caterpillars and keep their population in control. Parasitic wasps will lay their eggs on or in caterpillars, and when these eggs hatch, the larvae will use the caterpillar's body as food. If you exterminate all the wasps in your area, you may inadvertently cause caterpillars to thrive.
  • Focus on prevention rather than control. Look for egg masses in the fall. If you see a ring of eggs encircling a tree branch, remove and destroy it before caterpillars hatch.
  • If you want to remove tents or nests from your plants, wear gloves -- the caterpillars' hairs can be irritating or cause allergic reactions. If you find that caterpillars' spines have become embedded in your skin, try pulling them out with tape. See a doctor if irritation persists or if you show symptoms of an allergic reaction.
  • Use insecticides as a last resort. Often, they're not particularly effective against hungry larvae, although they may kill the adults who are preparing to lay the next generation of eggs. Preparations of the bacterium Bacillus thuringiensis (Bt) will usually control caterpillars, but its effect can be widespread, wiping out beneficial caterpillars as well as pests. Consult your local agricultural extension office or a licensed exterminator to find out which chemicals are recommended and legal in your area.

If you see caterpillars in your yard, keep in mind that only a few species are true pests. Damage from hungry caterpillars, even gregarious species, is usually temporary -- and a yard full of butterflies can be the reward for tolerating it.

To learn more about caterpillars, butterflies and other insects, read through the links on the next page.

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Related HowStuffWorks Articles

More Great Links

  • Batzer, Harold O. and Robert C. Morris. "Forest Tent Caterpillar." U.S. Department of Agriculture Forest Service. (4/22/2008)
  • Bessin, Ric. "Stinging Caterpillars." University of Kentucky College of Agriculture. 1/2004 (4/22/2008)
  • Blackiston, Douglas J. et al. "Retention of Memory through Metamorphosis: Can a Moth Remember what it Learned as a Caterpillar?" PLoS One. Vol. 3, issue 3. March 2008.
  • Collman, Sharon J. "Biology and Control of Tent Caterpillars." Washington State University. (4/22/2008)
  • Despland, Emma and Alice Lee Huu. "Pros and Cons of Group Living in the Forest Tent Caterpillar: Separating the Roles of Silk and Grouping." Entomologia Experimentalis et Applicata. Vol. 122. 2007.
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