I'll list their behaviors below. You can use it as a checklist--how many of these behaviors can you see them do? Here's a link to more info on this website. Link to how behaviors change as caterpillars grow.
- Hatching. The tiny caterpillar must unfurl itself from a very compact posture and eat its way out of the shell. Video of hatching by Dave Hinterberg.
- Once hatched, its first job is to eat the eggshell (probably not for energy, but to save shell material for making its next, larger skin when it molts in a few days).
- Cannibalism. Sometimes the first eggs to hatch may snack on neighboring eggs. This may not happen much in the wild, since there is seldom more than one egg per plant. Once I saw a large caterpillar eat part of a chrysalid that had formed nearby.
- Shaving trichomes. These are the tiny hairs on leaves that protect milkweeds and other plants. Before the tiny caterpillar can start to eat, it has to first shave off the trichomes where it wants to feed. Since there are fewer trichomes on the upper surface of the leaf, tiny caterpillars sometimes move from the bottom of the leaf, where they hatched, to the top, to begin feeding.
- Carving crescents on the leaf. Smaller caterpillars eat a crescent-shape on the leaf around where they are located. This reduces sap flow to the part of the leaf they are eating. They need to exclude sap from their meal because sap is toxic and very sticky.
- Locomotion. Caterpillars crawl about on two different kinds of legs. Their legs have claws, and the legs on either side are "opposable" like your thumb and fingers--meaning they can grip either side of a leaf when they crawl along the edge. They are also good at walking on flat surfaces or stems. Their two hind legs, with the best grip, are often used as an "anchor." So, if you want to pick up a stubborn caterpillar, first tickle it's rear end with a brush, to make it release its rear anchor.
- Curling to land on feet. When you move a caterpillar and put it down, let's say it lands on its back. The caterpillar immediately curls in a way that causes his feet to face downward--his claws then grab the leaf. Someone should make a video that clarifies this rapid motion. Does the body twist as well as curl? They also curl sometimes when dangling from a silk line.
- Exploration. When a caterpillar hatches, or when it runs out of good food, it needs to look around for a better place to dine. Link to why caterpillars move about on the plant. Exploratory behavior is exaggerated when the caterpillar is ready to form the chrysalis; it needs to find a good spot. They generally head away from the light; they like overhangs (probably both for shelter from weather and hiding from predators.) When you see a large caterpillar wandering widely or persistently, it is probably ready to form the "J."
- Head waving. Caterpillars have primitive eyes that can't form an image. But they can distinguish light from dark. By rearing up off their front legs and waving their upper body about, they may be able to distinguish large shadows and bright areas. They might also be "tasting" the air with their antennae for aromas, such as the scent of a nearby milkweed plant.
- "Stretching" is similar to head waving. When trying to reach a nearby leaf that the caterpillar senses, it may stretch further than it does while waving. It can stretch out about half a body length.
- Antenna waving. Caterpillars have a pair of longer antennae behind the head, and a shorter pair in the rear. They can direct them somewhat. They undoubtedly have a sensory function, but I don't know the details of how they are used or what they can sense.
- Excreting. Their excrement is called "frass." Tiny caterpillars excrete tiny pellets almost immediately after they start to eat. Often the frass comes out in double pellets. On a large caterpillar, it's almost like a garage door opens in their rear. Before it can form the "J," a 5th instar caterpillar must excrete all the remaining frass. Caterpillars do not urinate, but the adults do excrete excess fluid.
- "Freezing." It was recently discovered that monarch caterpillars can hear. They also respond to vibrations. For example, if you enter the room at night, they may all stop eating and "freeze." I seem to recall an article about how caterpillars avoid predatory paper wasps when they hear the wasps. Large monarch caterpillars freeze when you place them in a very unfamiliar environment.
- Threats. Caterpillars have at least two threat behaviors they use in conflicts with other caterpillars. One is "butting," where they butt the other caterpillar with their upper body. They can also swing very fast from side to side, as if to strike their opponent. Sometimes caterpillars are very tolerant of nearby caterpillars, sometimes not. Personality differences?
- The other behavior is "jerking," where they make a repeated butting motion without actually hitting the other. But the jerking motion is probably conducted along the stalk and felt by other caterpillars. I have never seen a caterpillar "bite" another (excepting cannibalism).
- "Recoil" is a rapid, jerky withdrawal when you touch a caterpillar. Possibly an antipredator behavior or avoidance of unpleasant sensations. The "J" can also recoil violently. Butting, jerking, and recoil could all startle a predator.
- Spinning silk. Caterpillars do not use silk to make a cocoon, as many moths do, but they do use it to make a silk pad that the chrysalid attaches to.
- They also use silk to lay down a "safety line" as they crawl about (as many spiders do). They use this safety line to return to the leaf if they are knocked off. In other words, they can climb up the silk line. Or, if they want to leave a dried up leaf, they may lower themselves down.
- They probably also spin smaller silk pads to anchor themselves each time they shed their caterpillar skin (molt). I suspect they anchor the old skin with silk--then crawl out of it.
- Cutting a notch in the leaf's stem. Fifth instar caterpillars cut a notch in the upper backbone of the leaf they want to eat. (I don't know if 4th instars also do it.) This causes the leaf to hang over, concealing the caterpillar from predators (and perhaps also protecting it from hot sun). A major predator of large caterpillars are paper wasps, which can carve up and carry away caterpillars. The notch might also help by limiting the flow of latex.
- Drinking? Caterpillars undoubtably get their water from the leaves they eat. I don't know if they actually "drink" mist or water droplets on the leaf, but they must ingest it accidentally. In any case, in captivity it's important to "mist" their food plant, since they normally live in a humid environment, among plants which are actively "transpiring" (breathing out water). If their food plant dries out, misting it can help them get enough water.
- Resting/molting. While caterpillars eat day and night, they do go into prolonged periods of "inactivity" lasting about 24 hours. Some of this is preparation for molting (shedding their skins), and resting afterwards while they inflate their bodies (to make a larger, new skin) and harden the new skin. Sometimes they leave the leaf they are chewing on to molt. I don't know if they rest for other reasons. Don't mistake the inactivity of molting for sickness.
- "Dropping" from the leaf is an antipredator behavior of larger caterpillars. They might even roll up into a ball to move more quickly--if so, they seem to have invented the wheel. I'm not sure if they actively let go and roll into a ball, or if they become accidentally dislodged and drop. It all happens so fast. Here's a masters degree thesis study waiting for you.
- Behaviors that help regulate body temperature (thermoregulation): Caterpillars move from the top of the leaf to the bottom, or vice versa, to find the most favorable temperature. On hot, sunny days, remaining on the top of the leaf could prove fatal. But on cold days, moving up into the sun can result in higher body temperature and faster growth. You can see them "basking" broadside to the sun, especially during inactive periods.
- Molting,
- forming the "J," and
- eclosure (emerging from the chrysalis) are all extremely complex and fascinating behaviors. They are too complicated to detail here.
- Even the chrysalis has a little bit of behavior, because it squirms before it solidifies.
How many of these behaviors can you observe? Challenge your students!
Perhaps the most noticeable thing about caterpillar behavior is how slowly and deliberately they move. They have only a few rapid movements: butting and jerking, curling, recoil, dropping off the plant, and twisting the stalk (cremaster) of the chrysalid into the silk pad. How can slow movement be a good survival strategy?
Remember, the monarch is protected from much potential predation because of its bad taste and toxicity. Moreover, it's an eating and growing machine--it doesn't waste energy or time building fancy eyes, legs or wings to move about quickly. In place of rapid movement, its growth is extremely fast. If a human baby grew as fast, in two weeks it would be as big as an elephant.
The monarch has an extremely specialized, dual lifestyle: the caterpillar is specialized for growing, the adult for dispersing and reproducing. Two species in one. This is a pretty successful strategy. We can use total biomass to assess "success," because you can't compare numbers of tiny insects to numbers of much larger humans. If you weighed up the total biomass of all insects that have the "complete metamorphosis" lifestyle (the beetles, bees and ants, flies, butterflies and moths), their biomass would probably weigh more than the biomass of all humans combined.
We humans think the learning/intelligence/technology path is the peak of evolution. The insects would disagree.
Other animals have evolved the slow approach to living. The tortoise has the protection of its shell. Slow-moving skunks don't worry about escaping predators, for obvious reasons. (That's why they get hit by cars so often--skunks think they are immune to attack.) Sloths are notoriously "slothful."
The study of animal behavior is called "ethology." Ethologists sometimes publish very detailed studies, hundreds of pages long, on subjects like the behavior of monarch caterpillars. I haven't seen such a study of monarchs, and don't know if one exists. If you know of one, contact me. The above list is based on my ten years of observation.
It's very likely that the behavior of a few species of caterpillars have been studied in detail--and this would give us a lot of clues about monarchs. Certainly, silk moth caterpillars have been studied in huge detail, because they have been domesticated for 5,000 years.
Behavior of tiny caterpillars after they hatch. In process, not published.
Link to more information on butterfly senses and behavior.