Caterpillar Instars

"I hear the term 'instar' often when people talk about Monarch caterpillars. What does this term mean, 'first instar' or 'second instar'?"

A caterpillars' skin doesn't grow. When the caterpillar grows too large for its skin, it must crawl out of its old skin (actually a cuticle, not a living skin)to continue to grow. Underneath its old skin is a loose baggier new skin that will stretch only to a certain point before the caterpillar must crawl out of it.

Looking at the photo with a large caterpillar and a tiny one on the right, it is very evident that the tiny caterpillar on the right could never grow to become the larger caterpillar if its skin didn't grow OR if it didn't crawl out of its old skin and have a larger skin underneath. The smaller caterpillar is first instar. The larger caterpillar is fifth instar, about to become a chrysalis.

Caterpillars crawl out of thier skin four times. This process is called 'molt'.

The time period between molts is called an 'instar'. Because Monarch caterpillars molt four times, they have five instars.

1st instar; hatched out of its egg
2nd instar; after the first molt
3rd instar; after the second molt
4th instar; after the third molt
5th instar; after the fourth molt and before becoming a chrysalis.

The last time it sheds its skin is when it pupates into a chrysalis.

The instar of a Monarch caterpillar is measured by its head capsule, not the size of the caterpillar. If a caterpillar doesn't eat much, it will be smaller during an instar.

Tiny caterpillars, hanging by an invisible string ...

"I have a milkweed plant in my garden, and im sure that there arent any pesticides on it or anything. A monarch comes and lays her eggs on the leaves, and i check on them and they look fine. The caterpillars hatch, but they look tinier than i think they're suppose to be. Then a couple of hours later, i look and the baby caterpillar is hanging from an invisible string, and its either dead, or will be in the morning. I'm not sure if its because of wasps, i know there are some lingering around the plant, but i really need some help. How can I save the eggs from the wasps? I cant afford a $30 sock/net thing to put around the plant. PLEASE help! Thank you very much. I would appreciate your suggestions. I am very concerned about the poor little caterpillars."

It's wonderful to hear from someone else who is so concerned about caterpillars. That concern is what turned me into a butterfly farmer. Welcome to the world of worrying about nearly invisible caterpillars and monsterous predators.

On the dime above, two Monarch eggs rest and one hatchling Monarch caterpillar crawls around. If you take a dime out of your pocket and look at it, you can realize how tiny the hatchling really is!

BUT ... a hatchling caterpillar is so tiny it is nearly invisible. I has to be small to fit inside the egg. It eats its way out and doubles in size nearly every day. In fourteen warm summer days, it gains weight over 2,500 times its original hatchling weight. At first, though, it is tiny.

The invisible thread is webbing from its spinnerets. The spinnerets are located under its head, where we have a neck. It lays a line of silk when it travels. When the plant is bumped or a predator comes by, it can drop from the leaf and suspend by its silk thread. It will slowly crawl back up the thread, an amazing sight to watch.


Wasps will kill caterpillars and eat them or carry them away. The only ways to protect them is to enclose the plant in a fine net (like sheet curtain material) or move the caterpillars and some leaves indoors. If you move them indoors, you need to either give them a safe milkweed plant to eat (like you are growing) or add leaves to the container every day. Sleeves can be expensive so simply make one with glue and a piece of fine material. Slide it over the plant or a branch with the caterpillars inside and tie it tightly shut on both ends. When the caterpillar(s) eat the leaves, move the net and caterpillars to another branch or plant.

Nature only allows one or two eggs laid to become adult butterflies. We should expect to lose caterpillars in our gardens. We should expect to see wasps, spiders, lizards, praying mantis, and other predators to carry away our caterpillars. But it is not something that many of us can accept. So we fight it!

Didn't become a full chrysalis; what happened?

"I'm raising Monarch caterpillars inside, and the last two times the caterpillars formed a J and went to shed their last skin, the skin did not fully come off, and the chrysalis did not fully emerge, only the very bottom of it, and they both died before the skin fully came off. Only the very tip of the green chrysalis could be seen, with the dead caterpillar was dangling there with most of his skin on."

I assume your caterpillar/pupa looked somewhat like the first photo. Several things can cause this. According to Dr. Urqhart, a silk strand caught around the J'ing caterpillar can cause it. Sometimes it is simply too weak or it may have other problems. This is common. It is heartbreaking, but it happens a lot.



"Underneath one of them was a little puddle of green fluid, and the other one's abdomen seemed very squishy and liquidy, like there wasn't a chrysalis to come out, only liquid there."

Look at the second photo. The green fluid is blood. Another caterpillar crawled on this one and tore the skin slightly before it pupated. When it was pupating, it pupated at the tear which resulted in a deformed pupa.

It IS squishy. This is normal! When it first pupates, it is basically gel with a cuticle. This gel is already preformed in some ways - you can see the antennae, legs, proboscis, wings, and more. Note the photo with these parts labeled. After an hour, it will have slowly reshaped into the classic Monarch chrysalis shape.



"I had touched both of them one time to get them back into the cage because they wandered off, but just by letting them crawl on a piece of paper and putting them in there. What do you think happened?"

It is safe to touch caterpillars if YOU ARE GENTLE. If one has started laying a silk mat, it should be left alone. If one leaves the plant in a rearing container and has not escaped the container, it should be left alone. They leave to molt (and should be left alone) and they leave the plant to pupate (and should be left alone). They can find their way back as long as they are inside the container with the plant.

"I'm very upset and I've been doing everything I can to keep these caterpillars healthy!!! Any suggestions you have would be really appreciated."

First, realize that butterflies have a rough time in nature. Most likely the problem is NOT your fault.

Nature is deadly. Only 1 or 2 out of 100 eggs will become an adult butterfly. Disease is a serious issue in nature. Between predators, parasitoids, disease, windshields, weather, and more ... very few become adult butterflies.

Because of the prevalence of disease in nature, ALWAYS disinfect a rearing container between batches of caterpillars. If your last batch of caterpillars did fine, still disinfect the containers and everything else that you touch or touches caterpillars. It is safest.

Enjoy and please let us know if we can do anything to help you.

Wriggling Butterfly Chrysalids

"Our Painted Lady caterpillars made chrysalises yesterday. Today one of them was wiggling like crazy for a little more than a minute. Could you tell us why?"

Many butterfly and moth chrysalises (pupae) wriggle in response to touch or movement. It is assumed that this is a natural instinctive response that discourages predators and parasitoids.

Some butterfly chrysalises do not have a jointed abdominal segment. Monarch and Queen chrysalises are not jointed and cannot move. Species that are jointed often move.

Click here to see Painted Lady chrysalises wiggle, hundreds of them! Other butterfly species like Julia, Gulf Fritillary, Tawny Emperor, Hackberry Emperor, American Lady, and especially Red Admiral chrysalises are extremely active when touched by anything. A Red Admiral chrysalis that has been laid on a table will often wiggle for minutes, often working itself off the table and falling to the floor.

People often think that this wiggling is a sign that that the adult butterfly is about to emerge. This is is an understandable misconception.

If you are in an area of the world where you can raise Julia butterflies, listen to their chrysalises when they move. They squeak!

OE in Monarch Caterpillars

"I live just outside of Orlando and have been a butterfly garden enthusiast for a few years. This year I have collecting cats from my and a relatives butterfly gardens' milkweed plants and raising them in butterfly pavilions. I feed them milkweed from my yard and that I purchase from a local nursery. I have had some issues with OE and need some advise. My wish is to contribute to the Monarch conservation movement and want to make sure I have proper information. Is there any way to tell if a cat I collect has OE? I've read info about testing adult butterflies and of the bleach solution for eggs but little about the cats themselves which is usually when I get them. Any help you could provide would be greatly appreciated."

There isn’t a way to tell definitely if a cat has OE or not without killing it and checking it through a high powered microscope. Some people report that OE in cats will cause splotching but we found that other things also cause the same type of splotching. We did isolation tests with spotted/splotched larvae. This was during a time of feeding larvae milkweed infected with plant pests like thrips. The Monarchs with spots and splotches emerged without OE. Out of the thousands we raised each week, none had OE.

(OE is the abbreviation for Ophryocystis elektroscirrha, a protozoa disease that affects Monarch, Queen, and Soldier butterflies and other butterflies in the Danaus family. OE is MUCH easier to say than Ophyrocystis elektroscirrha!)

It is the cats that contract the disease – the cat has to eat spores to become infected. One cat can’t give it to another cat – the spores are only transferable from adult butterflies to another butterfly or object – as surface contamination. When a cat EATS the spore, it contracts OE. A pupa or adult cannot contract the disease although an adult can become surface contaminated by mating with an infected adult butterfly. Somewhat like touching a door handle after someone with cold pathogens on their hands touch it – the pathogens can contaminate surfaces such as bodies and plant leaves. Spores fall off like pixie dust when infected adult butterflies fly.

You can tell, sometimes, if a pupa is heavily infected. It does show splotching that I’ve not seen otherwise. Any I’ve had with these splotches (pupae) were OE infected.

The photo above is OE infected. Notice the splotching?

The photo below is of pupae that are not infected. Notice the smoother green color near the cremaster area?


Of course, a light infection won’t show so it always pays to check adults to be sure.

On the encouraging side, it is reported that in the southern tip of Florida, 85% of all Monarchs are reported to be OE infected. If it was as deadly in the wild as we are often led to believe, the population in southern Florida should be extinct by now. BUT it is deadly in nature … we’ve seen milkweed gardens in Florida that were full of dead pupae – from OE. We know it is deadly and in a rearing operation where Monarch butterflies are kept together and bred together, infection can build up if it is not eliminated. That’s why it’s so important for those who raise Monarchs and Queens and mate captive breeding stock to check for OE and to disinfect eggs. We don’t raise them like nature with a 98% death rate BEFORE they become adults – we raise them to have a high survival ratio!

I imagine you already do this but just in case, remember to always disinfect a pavilion after one set of cats are raised in it. OE is only one disease and is one of the least dangerous dangerous. Some other diseases are far worse than OE and they can be transferred from cat to cat.

You can wash milkweed with a light bleach solution and rinse thoroughly before putting cats on it. That will help with surface contamination of milkweed leaves – one of the primary sources of infection. The other source is spores on the eggs themselves. When the cat eats its way out of the egg, it eats spores that the adult female Monarch butterfly left on the egg surface when she laid the egg.

The same goes for Queens too. Queen cats are so cool – one of my very favorite larvae.

Caterpillar on milkweed with extra antennae?

"I have found on my milkweed, among "normal" monarch cats, a caterpillar with a slightly different color pattern and an extra set of antennae about mid-back. Please forgive my ignorance, but what is this?"

You have found a Queen butterfly caterpillar. Queen butterflies are also in the Danaus family, like Monarch butterflies. In the extreme south, Soldier butterflies can also be found.

Learn more about Queen butterflies!