10 Fascinating Facts About Caterpillars

Interesting Behaviors and Traits of Caterpillars

Monarch caterpillar eating milkweed
Adam Skowronski / Flickr / CC BY-ND 2.0

Surely you've seen a caterpillar in your lifetime, and you've probably even handled one, but how much do you know about Lepidopteran larvae? These cool facts about caterpillars will give you new respect for what remarkable creatures they are.

1. A caterpillar has just one job – to eat.

During the larval stage, the caterpillar must consume enough to sustain itself into adulthood. Without proper nutrition, it may not have the energy to complete its metamorphosis, or may be unable to develop eggs as an adult.

Caterpillars can eat an enormous amount during a life cycle stage that typically lasts several weeks. Some consume 27,000 times their body weight during this life phase.

2. Caterpillars increase their body mass by as much as 1,000 times or more.

The larval stage of the life cycle is all about growth. Within the span of a few weeks, the caterpillar will grow exponentially. Because its cuticle, or skin, is only so pliable, the caterpillar will molt multiple times as it gains size and mass. The stage between molts is called an instar, and most caterpillars go through 5-6 instars before pupating.

3. A caterpillar's first meal is usually its eggshell.

In most cases, when a caterpillar ecloses (hatches) from its egg, it will consume the remainder of the shell. The outer layer of the egg, called the chorion, is rich in protein, and provides the new larva with a nutritious start.

4. A caterpillar has as many as 4,000 muscles in its body.

That's one seriously muscle-bound insect!

By comparison, humans have just 629 muscles in a considerably larger body. The caterpillar's head capsule alone consists of 248 individual muscles, and about 70 muscles control each body segment. Remarkably, each of the 4,000 muscles is innervated by one or two neurons.

5. Caterpillars have 12 eyes.

On each side of its head, a caterpillar has 6 tiny eyelets, called stemmata, arranged in a semi-circle.

One of the 6 eyelets is usually offset a bit, and located closer to the antennae. You would think an insect with 12 eyes would have excellent eyesight, but that's not the case. The stemmata serve merely to help the caterpillar differentiate between light and dark. If you watch a caterpillar, you'll notice it sometimes moves its head from side to side. This most likely helps it judge depths and distances.

6. Caterpillars produce silk.

Using modified salivary glands along the sides of their mouth, caterpillars can produce silk as needed. Some caterpillars, like gypsy moths, disperse by "ballooning" from the treetops on a silken thread. Others, such as eastern tent caterpillars or webworms, construct silk tents in which they live communally. Bagworms use silk to join dead foliage together into a shelter. Caterpillars also use silk when they pupate, either to suspend a chrysalis or to construct a cocoon.

7. Caterpillars have 6 legs, just as adult butterflies or moths do.

But wait! There are way more than 6 legs on most caterpillars you've seen, right? Most of those legs are false legs, called prolegs, which help the caterpillar hold onto plant surfaces and allow it to climb. The 3 pairs of legs on the caterpillar's thoracic segments are the true legs, which it will retain in adulthood.

A caterpillar may have up to 5 pairs of prolegs on its abdominal segments, usually including a terminal pair on the hind end. The

8. Caterpillars move in a wavelike motion, from back to front.

Caterpillars with a full complement of prolegs move in a fairly predictable motion. Usually, the caterpillar will first anchor itself using the terminal pair of prolegs, and then reach forward with one pair of legs at a time, starting from the hind end. There's more going on than just leg action, though. The caterpillar's blood pressure changes as it moves forward, and its gut, which is basically a cylinder suspended inside its body, advances in sync with the head and rear end. Inchworms and loopers, which have fewer prolegs, move by pulling their hind ends forward in contact with the thorax, and then extending their front half.

9. Caterpillars get creative when it comes to self defense.

Life at the bottom of the food chain can be tough, so caterpillars employ all kinds of strategies to avoid becoming a bird snack. Some caterpillars, such as the early instars of black swallowtails, look like bird droppings. Certain inchworms in the family Geometridae mimic twigs, and bear markings that resemble leaf scars or bark. Other caterpillars use the opposite strategy, making themselves visible with bright colors to advertise their toxicity. A few caterpillars, like the spicebush swallowtail, display large eyespots to deter birds from eating them. If you've ever tried to take a caterpillar from its host plant, only to have it fall to the ground, you've observed it using thanatosis to thwart your efforts to collect it. A swallowtail caterpillar can be identified by its smelly osmeterium, a special defensive stink gland just behind the head.

10. Many caterpillars use the toxins from their host plants to their own advantage.

Caterpillars and plants co-evolve. Some host plants produce toxic or foul-tasting compounds meant to dissuade herbivores from munching their foliage. But many caterpillars can sequester the toxins in their bodies, effectively using these compounds to protect themselves from predators. The classic example of this is the monarch caterpillar and its host plant, milkweed. The monarch caterpillar ingests glycosides produced by the milkweed plant. These toxins remain within the monarch through adulthood, making the butterfly unpalatable to birds and other predators.


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