Life Cycle of the Promethia Moth

Hello everyone!

Longhorn beetle on my window

So, in case you were curious, and even if you aren’t, the reason this blog has come to life is so that I can share my knowledge of the arthropods in Maine with you…And so I have a place to put all my knowledge, instead of just note books that clutter the shelf.  I have been collecting insect specimens, documenting their behavior, their life cycles, their everything that I can think to jot down in the notes I take on them after their daily feeding, since I was about 7. Many people ask me why I love insects. The best answer I have found so far is that they are simply a source of endless fascination. I love to tell the story of the “frog” in the bathtub, which ended up being a giant scarab beetle covered in cat hair. The next day, he had rolled all the hair that was on his back into a nice, neat little ball. I believe that was one of the first times I remember being curious about insects. After watching the movie Microcosmos – here is the link to it Microcosmos – From Netflix, I was totally enthralled. I highly suggest this movie to anyone, whether they like insects of not, it is simply a superb film, set in France I believe, it uses microscopic cameras and super sensitive microphones to pick up the sound and movement of various insects as they go about their normal lives.

So, I’ve had enough of the intro stuff. Lets get down to business.

Callosamia promethia moth adult

Early this summer, my mom and I were sitting on the porch one evening, marveling at the what a beautiful day it was, as well as the super fast, large dark insects that kept flying by too high to catch with a net. Looking up into the trees next to the house, we saw a huge cloud of these large dark insects. At first glance they looked like butterflies, but the coloration seemed only to fit a super dark swallowtail such as a black swallowtail (Papilio polyxenes) or a pipevine (Battus philenor), neither of which are very common in our area. After about a half hour of neck craning and swatting black flies while waiting for one to come down low enough that we could net it, we gave up for the day. We guessed that at least one would get hit by a car in our area that night, and we were right! The next day, walking up the road, we ran across two, that were still alive. To our surprise, they were not butterflies. Instead, we identified them as Promethia moths, (Callosamia promethia).  We had seen at least 30, swarming at the top of a pine and ash tree clump, right before sunset. As a side note, the identification guide, Caterpillars of Eastern North America, by David L. Wagner is excellent, and what I use for most of my identifications.

Out of the two adults we found, one flew off, and the other, which I brought home intending to add it to my insect collection of which I will post pics later, started laying eggs on my hand. This is not the first time a large moth has laid eggs on my hand. When I was about 12, I found one while running to the outhouse at night, and bringing it inside, it had laid a bunch of eggs on my thumb. However I had wrecked the eggs trying to pry them off after the mother’s egg glue had dried. But this time, I set the moth on a tissue, and got the eggs off before they dried. The next day, that moth had laid me about 43 eggs.  The eggs are creamy white, with purplish blotches on them in random spots. The glue that the mother uses to attach the eggs to a surface is glopped on and dries like a hard brown resin. I have vowed to comment or make notes on these little guys every day such that each new development is noticed for future reference.

Promethia moth larvae right after hatching – about 3 mm

The eggs opened June 27. The larvae, about 3 mm long, were extremely cute. Yellow as the basic color, with tiny black stripes along the whole body, which are more prominent at the thorax and A13. There is a slight suggestion of spiny protrusions along the length of the body. The larvae proceeded to eat most of the egg shells , though avoiding the parts where they were glued down. First feeding was of Ash (Fraxinus nigra), which they began chewing right off, instead of scraping like many small larvae begin with. First head count resulted in 29 hatched, with a few eggs still to go. Using a fine paint brush to move them to petri dishes, where I will keep them until they get big enough for a jar. The ash I feed them has to be cut into small squares in order to fit into the dishes. The whole feeding process is going to be kinda tedious, but hey, it’s in the name of science…and it is fun, believe it or not.

June 28. I counted 41 live larvae, and two dead. Live larvae are lively and have expanded to about 5 mm in length. So far it appears that they are night feeders, and group feeders, as they line up on the edge of leaves and carve it down, with

Underside of larvae – So cute!

all their tiny frass collecting in a pile behind them. I have split them up into petri dishes of 12:12:17, to make it easier to count, and so they are not too crowded. I use a small magnifying glass with measurements in inches and millimeters when taking pictures so that my camera can spot the details.

June 29. Head count is down to 39. Most are visibly longer and thicker, while a couple are still weaklings, looking like they just hatched. Next feeding I will have to divide them up again to even out the numbers, as the dishes are down to 10:12:17. The longest larva is already 6.5 mm. Most are remaining in rows to feed.

June 30. Lost two more last night, hope these casualties slow down soon. The remaining 37 have an average length of about 6 mm, and small spines are visible on the thorax and end of the abdomen. They are very hungry…understatement. They are ravenous! In addition to feeding in rows, they also only feed on the bottoms of leaves. The cool part is that they find the true bottom of the leaf weather it is pointing up or down.

Promethia larvae feeding in rows

In other words, they do not feed based on gravitational pull, rather they prefer the softer underside to grab onto. That is my thinking. I am sure there is some other reason, but I have not yet found it. Changed dish numbers to 12:9:8:8.

July 1. No casualties! Yay! They are still feeding in rows, though consuming leaf area at a much faster rate. It is amazing how quickly these little creatures expand and develop in just a few days after hatching. Coloration is more evident now. Black and bright yellow bands, with the black bands being in pairs, and thicker yellow bands between the pairs, (see picture at right). Some of the yellow bands have two black bumps, placed on the back of the larva, where thin branching spines appear to originate. The head is black, with two horizontal yellow stripes across “forehead” and upper lip area . True legs are black, pro-legs and belly are pale yellow. Largest of the larvae is about 8 mm long and 1.5 mm thick. If jostled or picked up awkwardly, they will spit up brown liquid…I believe it is their stomach contents. Eew.

Close up of patterning and color – Promethia moth larvae

July 2. The larvae are doing very well, still no casualties and holding steady at 37 individuals. Interesting observation: Out of the four dishes of numbers 12:9:8:8, only the dish of 12 still eats in a large group, side by side. The other dishes are eating singly or in pairs. Larvae range from 5-8 mm and still growing, pooping, and eating a lot.

July 3. Now a week old, still 37, no visible size change, though eating a lot more.

July 4. Hope everyone had a good and safe July fourth. Personally, I was too busy to spend the long hours it takes to feed all my larvae (I am rearing more than just the tulip tree larvae), so I simply cut up some fresh leaves and added them to what was already in the dishes. I will clean and properly feed them tomorrow.

July 5. It is late and I’m very tired. I managed to clean and feed the tulip larvae before falling asleep, (needless to say, I am noting all this on the 6th…). Promethia moth larvae are almost all pumping themselves up to shed, or actually shedding, so the whole process of feeding them was very delicate, and I didn’t even bother trying to move them off the old leaves. I just changed the leaves that were bare of larvae.

Larva on my finger…this is the one I squished, oops.

July 6. Everyone is healthy after their busy night of molting. No casualties, (of unknown causes) or complications. However, one did die, due to a very stupid mistake of mine which I will now know never to do again. I was marveling at how large they are getting, and that they can actually hold on to my skin now. But that was the mistake. I picked one up and when I tried to pull him off like I do with larger larvae, his feet and guts stuck while the body moved…it was terrible. I felt really bad too, no deaths in a long time and I have to up and go mutilate one. Oh well, won’t do that again! On a separate note, the dish of 12 is getting crowded again, so I will be dividing it up again soon.

July 7. After their molts, the larvae have grown even more, and now measure 6-13 mm. Had to split up the dishes, now in numbers of 6:6:8:7:9. Might split them up again soon. Some are still molting and soft. So far their coloration and patterning is very similar to the past molt, their spiny protrusions are just a little bigger.

Just a nice picture – shows colors and camaraderie

July 8. Now that they are done molting, they are getting snappish about their space, so I have them in groups of 4 and 5. Still holding on at 36 individuals, I should probably switch them to jars soon. One of them is still at 6 mm, while two are at 14 mm, and the rest are between somewhere.

July 9. All are very healthy. My mom is working an ESL program for Jr. High kids from China, and I brought one of my dishes of larvae in to show them, as they have never seen a caterpillar.

I have tagged the dish so I know which one it was and I am going to show them the larvae again right before they leave, to exemplify how much these caterpillars can grow in just a week. Starting size is still that of July 8.

Molting larva in petri dish – bright yellow is new skin

July 10. Too tired to do a full cleaning. I cut up leaves and added them to jars, will do a better job tomorrow. Everyone looked healthy though.

July 11. Everyone is large, fat, and look like they should molt again. Most are over 10 mm  long, though two are still 6.5 mm. They are all very hungry, and starting to make and use silk wherever they go.

July 12. This is perhaps the neatest day so far. I began cleaning and feeding them right as their third molt began. In addition to some awesome pics, I managed to get 6 or more minutes of good video, which I will try to post later, showing one of the larvae shedding, and his buddy next to him pumping up to burst. Everyone is shedding well and easily, still 36 individuals! So yesterday I had mentioned that the larvae were using silk. Well the reason is that they spin silk pads on which to attach their anal prolegs. Then when they molt, their old skin is actually attached and they can simply pull out of it. Some times I wonder if they actually do have brains because that is a great idea!

Darkening after molt, can see some of later colors

July 13. Took a day off of feeding, as they are still soft, and some are still molting. Very delicate.

July 14. Simply added more leaves…will do a full cleaning tomorrow.

New contraption – can see water, leaves and big bucket

July 15. Today I got fed up with cutting up all the leaves into tiny squares, and then adding them to the petri dishes with tweezers, while making sure the larvae don’t get out with a paint brush…it was all taking WAY TOO LONG. Also, the summer heat keeps drying out the ash leaves before the caterpillars even get around to eating them. So, thanks to the internet, I have devised a much better system. Mom lent me a 5 gallon bucket that is clear and has a good lid, and then we took a smaller plastic container with a thin lid and punched holes in that smaller lid. We then put small branches with clean leaves into the holes, and filled the small container with water, and put the whole contraption into the 5 gallon bucket. Now the leaves won’t dry out, the larvae have a small bush to eat before I have to add more food, and everyone has a ton more room to grow. And I am very happy with my new-found free time. All 36 larvae made it through their molt. Almost all of the larvae are in their third molt, with two still in the first, and two still in the second. All are eating very well. The only problem that seems to be occurring with this new contraption is keeping the humidity down, but they don’t want to leave the leaves, so leaving the lid off is no problem.

July 16. Everyone is quite happy with the new arrangement, humans and larvae both. My mom is also happy because now she can see them, as opposed to when I kept them in little dishes in my room, out of her sight. I believe just over night they have all doubled in weight and almost size. The third instar has different patterning and coloration. The base color is a white-ish/blue-ish/green-ish color, very pretty, but can appear any of those three colors in changing light.

After first night in large container

Their heads are now yellow, with black spots, and about twice the size of the second instar’s head. Spiny protrusions are yellow on the thorax and A12, and have a shape like a  crown. All other segments are dotted with tiny black spines, in pairs of two on the top and side of the larva. Prolegs are yellow and much bigger, while true legs are still black.

After second night in big jar – about an inch long

July 18. I swear that each day they double in size, and get fatter and fatter! All third instar larvae are over an inch long, while the second instar larvae are quickly catching up. I still have two runts that refuse to exit the first instar, however they seem healthy enough. Most of the third instars are preparing to molt. They are still gathering in groups for this activity, which I don’t understand because it just seems to annoy them having something so poky nearby while molting. They are attached to large vein in a leaf, and tuck their

3rd instar before molt

heads under, similar to the pose often seen in pictures of the later instars. They are beginning to pump up the head and thorax region to the bursting stage. This might be a two day process.

Larvae of 2nd and 3rd instar getting ready to molt

July 20. Yup. It is a two day process. The first one popped this morning, and the rest are quickly following. It is a pretty insane to watch. A couple hours before their skin splits open at the head and the head separates, they begin this agitated thorax swinging thing where they drag their head across the leaf like they are trying to relieve an itch. Then their bodies start convulsing to pressurize the thorax and head region, and with each convulsion the old skin can be seen, bag like, sagging to their sides. A little while later, the four yellow knobs on the thorax begin to turn reddish, and a few minutes after that they pop. By pop I mean a cut opens up right above the head, the old head falls off, and all the pressure the larva has built up pushes the old skin back to where the larva simply steps out of it and is free. For the next half day or so, the larva will sit still, expanding to fill the new skin, and getting color to all its parts, as they are bright yellow when they first molt. So far, it appears the four thoracic knobs are going to be reddish-orange, and the one on A12 is yellow. All are much bigger. There are still rows of smaller black spines along the body length. I am going to wait until tomorrow to start changing their food, as they are still touchy.

4th instar right after molt…gaining color



4 thoughts on “Life Cycle of the Promethia Moth

  1. The Pal Guy says:

    Wow, this is great. I am glad people like you do this. I have raised Monarch Butterflies from caterpillars many times in the past.

  2. Thanks! I have also raised many monarchs, as well as sphinx, tussock and some brushfoot butterflies. I hope to get notes up on them at some point in the future.

  3. Hi,

    I’d like to use this photograph in an art project I”m working on. Please let me know if this is a problem.

    Many thanks,


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