How Caterpillars Work

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.

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:

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.

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.

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.

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.

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.

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:

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.