Monarch caterpillar behavior

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Previous posts on my pet monarchs
Raising monarchs from the egg
Monarchs are growing steadily

Most people have heard of the miracle of metamorphosis, when a caterpillar transforms itself into a moth or butterfly.  But not many have watched the day-to-day behavior of a caterpillar.

Perhaps that's because it's about as exciting as watching molasses flow.

But wait!  Caterpillars really do have behavior!  They just have a clock that runs more slowly.  Finding out what they do is a challenge.

Portrait of George,with head to right.

Update on Elizabeth and George

When they hatched a day apart, both of my caterpillars were tiny, but Elizabeth, who hatched second, grew much more slowly.  After a few days, she stopped eating, becoming lethargic.

Fearing she had caught a disease (common in captive caterpillars), I wanted to protect George, so I put Elizabeth on a milkweed plant in my garden--on top of a leaf.  I marked the plant with a little flag, figuring that if she survived, I'd be able to find her.

But when I returned two hours later,  Elizabeth was gone without a trace.  I had placed her on top of a large leaf, out of sight under a fragment of an old leaf she had been eating.  There wasn't a breath of wind, so she didn't blow away.  She must have fallen victim to a passing bird.   Normally, we cheer the birds on as they snap up insects.  But not when they eat your pet.

George continued to grow, faster and faster.  He had periods of activity, when he'd eat large holes in his fresh milkweed leaf, then periods of rest--like your uncle after Thanksgiving dinner.

Every now and then, he went into a more prolonged rest--after which I'd find his old caterpillar skin, like a shriveled, wrinkled panty hose thrown onto the floor by a teenager.  Monarchs shed or molt their caterpillar skin 5 times, to allow for growth.

Towards the end of his caterpillarhood, George was eating an entire milkweed leaf every half day.

Caterpillar behavior

George always heads for the underside of the leaf he is working on.  Obviously, that's for hiding from predators and protection from the sun.

I wondered what would happen if a caterpillar falls off the milkweed he lives on.  They can only eat milkweed, so he'd have to climb back--a long, perilous journey for a tiny Monarch.  But I quickly discovered, when trying to shake him onto a fresh leaf, that he spins a strand of silk he can hang from.  He attaches that to the leaf he's on, like a safety line.

But once George grew large and became a buzz saw, ripping through a leaf in a few hours, I began to wonder--Is George going to saw off the leaf he's sitting on?

No, caterpillars are "smarter" than that.  George might start eating a random spot on the leaf, but as he progressed, he always aligned himself with his rear end towards the plant's main stem, with his rows of feet clinging to the central vein of the leaf.   After aligning himself safely, he only ate portions of the leaf further from the base of the leaf.

It's hard to tell the front end of George from his rear.  Both ends have small antennae, but the antennae on the front end are a bit larger.   There's a small head, then three pairs of spiny "true legs".  After a gap, there are four pairs of fleshy "prolegs," followed by a gap.  Then there is another pair of prolegs at the rear end.  More anatomy.

When searching, George extends his flexible front end (left) and waves it slowly about, sensing the environment.

He can do a U-turn on a narrow twig with his flexible body.

If you disturb George by touching him, coming very close, or shining a bright light, George stops moving, except for little vibrations of his antennae.  After a few minutes of quiet, he resumes moving or eating.

So much to eat, so little time.

Time for a change

I hadn't been keeping close track of his molts or the time, so I didn't know when George would transform himself into a chrysalis.  But I did know that he would need more room--and a stick to crawl up.  That's because the chrysalis has to hang from something.  And the butterfly has to hang after it emerges, while it's wings dry.

George had never emerged from his mixing bowl--always content to just eat and rest.  So I no longer bothered to keep a lid (with air holes) on the bowl.

But yesterday, when I came home, George was missing!  Nothing but a leaf fragment and frass in his bowl.  I panicked--had I already stepped on him?

Panic gave way to a cooler head...  His bowl was on my dining room table.  How far can a caterpillar hump in two hours?  There was one frass on the floor near the table.  Is that the direction he headed?

To find a missing caterpillar, you have to think like one.  I began to reason... either he's still on the table, or he had to climb down the legs.  So I checked the table top, and several house plants sitting there.  No George.

Next step--check the legs and underside of the table.  And there he was, underneath the table, a few inches from the edge.  (The frass was nearly underneath him on the floor.)  He was completely inactive.

Perhaps George had left searching for more food.  But when placed on a fresh leaf, he quickly left it, and climbed out of the bowl again.  I watched him for about an hour.  He was restless, searching from something he couldn't find.  He'd climb out on a branched stick I provided only to climb back down.  I decided he wanted to change to a chrysalis, and that he needed to do this out of sight, underneath something.  So I placed him close to the edge of the table, and near a leaf he could go under.

But instead, he crawled a short distance back onto the twig that projected horizontally past the edge of the table.  He went slowly back and forth, till he seemed to settle on certain spot on the twig, where he finally settled down.

After a few hours, George had attached his rear end to the twig with silk, and contracted his body into a shorter, thicker shape.  It was obvious he was going to form a chrysalis.  By the following morning, George was dangling from the twig by his hind end, curled into the shape of a question mark.  He was still responsive when I touched him, reacting with a jerk.  Five hours later, by 1:00 pm, no change--George is still in the shape of a question mark, and moving when I approach.  But no sign yet of his striped skin splitting to reveal the chrysalis inside.

Looking back, I think George escaped his home bowl because he had to change into a butterfly.

Imagine yourself in his predicament.  You are getting very sleepy.  In a short time, you are going to lose consciousness.  You have to find the perfect spot--not just safe for a very long sleep--but a place with the requirements for emerging as a butterfly--hanging upside down!  You have short stumpy legs, and eyes that barely form an image.  Your entire survival depends on getting this right.

No wonder he was restless!

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More on caterpillar behavior.