During complete metamorphosis, the larvae of moths, butterflies, and a few other kinds of insect occur in the form of wormlike caterpillars. In other words, caterpillars are the immature stage of certain kinds of insects, especially butterflies and moths. You might enjoy looking at a series of photos showing Mourning Cloak eggs and caterpillars emerging from them.
The picture at the right shows a caterpillar of the Question Mark butterfly, Polygonia interrogationis, found right outside my door. One neat thing about that picture is that right below the caterpillar you can see the caterpillar's just-discarded old skin. Well, this discarded skin needs an explanation:
Butterfly caterpillars may molt several times before they become large enough to metamorphose to the next stage, the chrysalis. There's more about chrysalises on our Insect Pupa page. The chrysalis of the Question Mark Butterfly is gray-brown with olive mottling and it hangs down camouflaged like a shriveled leaf. Eventually, if all goes well, a Question Mark Butterfly will emerge from the chrysalis.
If you are interested in seeing a couple of caterpillars pupate (turn into a chrysalis) then check out this page at Captain's European Butterfly Guide and scroll down to the bottom.
The mother Question Mark Butterfly laid an egg, the egg hatched and a tiny caterpillar emerged. Caterpillar skin can't grow the way human skin can so, in order to grow, the tiny caterpillar needed to molt by splitting through its skin-husk to become a larger caterpillar. Therefore, what you are seeing in the picture above is the caterpillar resting as its new skin and spines harden. There is more about caterpillars molting on our Caterpillar Instar page.
Another very common and conspicuous kind of caterpillar is shown at the right -- at least the home of a bunch of caterpillars. Here you see the silken tent of a colony of tent caterpillars. Such tents protect the caterpillars from predators and to some degree from the weather.
Overwintering eggs hatch early in the season, then the caterpillars start eating like crazy and building their tent. They become fully grown by late spring or summer, and during the growing process they can actually eat every leaf from a tree, so that the tree must redevelop all of its leaves. Of course that's pretty hard on any tree and can even kill an already-weakened tree.
Once the caterpillars are grown they wander off and hide themselves in well protected places such as a piece of tree bark laying against a rock. There they spin silken cocoons for themselves, then moths emerge from those cocoons in a few weeks. The most common tent caterpillar in Eastern North America is the Eastern Tent Caterpillar, Malacosoma americanum. This is one insect species in which the caterpillar is better known than the adult. Adult tent-caterpillar moths are small, brownish items you wouldn't look at twice.
At the left you see the caterpillar of a Luna Moth, Actias luna, found on a Pecan tree in southern Mississippi. When I found it, it was very quiet, in the slightly curled position you see, and its colors suggested that it was ready to make a cocoon (only butterflies produce chrysalises). The picture is interesting not only because it shows the many tiny, sharp spines protecting its body but also because in this picture you can learn a bit about caterpillar legs.
Insects have six legs, right? But how many legs does this caterpillar seem to have? In the picture, the caterpillar's head is on the right. The yellow-framed inset shows a close-up of the front of the caterpillar, from below. In the inset, the large, brown thing is the head. To the head's left are six black jointed legs. However, in the larger picture notice that midway the body there appear to be eight more legs. Those stubby, mid-body legs are not real, jointed legs -- they're not the legs we refer to when we talk about an insect's six legs. They are called prolegs and they function very well as legs but they're not to be counted among those six legs... Finally, at the rear end there's a very special pair of prolegs, called the anal prolegs, and they make pretty good legs, too, but also they are not to be counted as "real legs."
At the right you see a much-magnified close-up of two of the above caterpillar's prolegs. The items looking like black combs are minute hook-like affairs called hooklets or crotchets. You can visualize a caterpillar clamping onto something with these hooklets. If you ever pull a caterpillar from your clothing you'll notice that as the hooklet-bearing prolegs come unattached, it feels like removing something stuck there with Velcro. In fact, proleg hooklets behave very much like Velcro!
This is so interesting that we have a special Caterpillar Camouflage Page.
Some caterpillars, opposite to being camouflaged, bear bold markings that make them very conspicuous. The gorgeous caterpillar at the left is the Saddleback Caterpillar, Sibine stimulea, the larval stage of a small, brown moth not at all outstanding in appearance. Why doesn't the Saddleback's bright colors cause birds and other critters to notice it and eat it? It's because those stiff hairs you see on the caterpillar are poisonous spines! Contact with them causes a burning sensation and inflammation that can be as painful as a bee sting.
Therefore, this caterpillar's survival strategy is the opposite to being camouflaged. It actually "wants" predators to notice its bright colors, for those colors warn anyone who might eat it that it can sting lips and tongues in a way not to be forgotten. In this case, bold colors and patterns warn predators to stay away.
The Monarch Butterfly's caterpillars eat the plant known as milkweed. It happens that milkweeds contain a bitter chemical known as an alkaloid that keeps most insects from eating it, but which Monarch caterpillars just love. Consequently, Monarch caterpillars and adult butterflies contain the same alkaloid, and they themselves then are considered by most caterpillar-eating and butterfly-eating animals as too bitter to eat. This explains why Monarch caterpillars, instead of being camouflaged, are boldly marked. As is the case with the Saddlebacks mentioned above, the Monarch caterpillar's striking appearance announces to the world I'm dangerous, don't try to eat me!
Certain caterpillars of the commonly encountered, eye-catching Swallowtail Butterfly Family, the Papilionidae, defend themselves when they feel threatened by suddenly producing a conspicuous, forked, fleshy, antler-like gland from the segment right behind the head. You can see the orange one on a Black Swallowtail caterpillar being annoyed with a finger poking him from the left in the picture below:
Do you see the orange, wet-looking, Y-shaped gland above the head? The caterpillar is bending around to rub the gland on the finger because the gland produces strong-smelling chemicals that often stop birds and other animals who may be about to eat the caterpillar.
The gland is called an osmeterium (plural osmeteria). Not all caterpillars have osmeteria; they're just found among swallowtail butterflies. However, if you spot a swallowtail caterpillar you might enjoy nudging it gently to see if it will put out its osmeterium.
Some caterpillars hide in sneaky ways. Above, on the right, you see the top and bottom views of part of a single Giant Ragweed leaf. Notice where the leaf margin has been fixed in a curled-under position by a caterpillar who used silk to tie the two leaf surfaces together, to form a "tunnel" inside which it could hide. In the enlarged picture at the left you can see both the silk strands holding the leaf in its curled-up position, and one end of the greenish caterpillar snugly hidden in its tunnel. Why spin a cocoon if you can just curl a leaf around you?
Finally, I want to show you, at the right, what might be North America's weirdest-looking caterpillar. It's the Monkey Slug (Hag Moth Larva) - Phobetron pithecium, of the family Limacodidae, found near my home in southwestern Mississippi. It's about the size of a man's thumbnail. The curly "arms" are actually just lobes of hairy flesh atop the caterpillar. If you turn the caterpillar over you'll see the usual legs and head.
Why would a caterpillar look so strange? I've read that it is thought to mimic cast skins of tarantulas, but I doubt that, especially because I've never seen a tarantula skin sticking to a tree leaf, which is where Monkey Slugs are usually found. My thought is that maybe it's meant to look like a gall. Or maybe it's just meant to look so strange that a bird wouldn't even think of eating it. Who knows? By the way, those hairs on the "arms" are capable of stinging, so this caterpillar is very well defended, indeed. I include this image mostly to remind you to always keep an open mind, and to expect just about any kind of surprise from the insect world!
The North American Moth Photographers Group is not only an excellent site for moth identification, but they also have a section on Larvae of North American Lepidoptera. You can also browse identified pictures at Bugguide.net after doing a search on the word "caterpillars." Another good place on the Web to identify caterpillars is Bill Oehlke's Butterfly and Moth Caterpillars: Identification Guide. If you know the family of your caterpillar, you might try the Caterpillars of Eastern Forests website, click on the family name, and view the thumbnails. You may also like to take a look at some caterpillars that Bea Laporte found in her own neighborhood at Some Caterpillars Found in Ontario.
You may be interested in reviewing books about caterpillars available at Amazon.com in the US, Canada and the UK by clicking here. Also, take a look at those below: