What are they? What do they do?
First, they are the bottom two black dots that are near the cremaster. They are hooked or club shaped.
When a caterpillar is preparing to pupate, it separates itself from the cuticle/skin. The body drops down in the cuticle, much like a heavy ball in a sock. Once the cuticle splits and moves above the head (as it pupates), the only thing holding the body inside the skin are the holdfast tubercles. The anal prolegs, attached to the silk button, are no longer there. It is only the cuticle that is attached to the silk button at this point. The prolegs are drawn up into the body and are not in the area of the cuticle where they were located when it first dropped into a J.
In the photos below, the white arrows indicate how, when the body separated from the cuticle, it dropped out of the anal prolegs. The first photo is when it first hung in a J. The second photo is just before it actually pupates, showing the empty anal prolegs. The green arrows indicate the rear filaments. Because the cuticle is now separated from the body, the filaments are empty skin. Hemolymph no longer flows through them. They looked ragged, like a dog’s old chew toy. The yellow arrows point to some of the other prolegs. In the first photo, hemolymph is still running through them and the body is still attached to the cuticle. In the second photo, one can see how the prolegs point upward from the weight below them. The body is no longer attached to the prolegs and they are empty.
At the point the second photo was taken, the body was no longer attached to the cuticle. It rested in the cuticle like a ball in a sock. The holdfast tubercles are hooked to the cuticle and over the clear band. When the cuticle is pulled back to reveal the holdfast tubercles, the band must break. That’s why the band cannot be seen under the holdfast tubercles. In the photo below, the broken clear band is visible. Click on the photo for a closer view of the band.
As it pupates, the cuticle splits on the back of its thorax and it moves like an accordion to work the cuticle up toward the silk button. The head capsule splits in half and the cuticle clears the bottom of the new chrysalis, continuing to work upward. At this point, it is supported by the holdfast tubercles. The anal prolegs portion of the cuticle is still attached to the silk button. The chrysalis is holding to the cuticle with the holdfast tubercles.
That’s somewhat like us being inside a huge pillowcase, suspended from a hook in the ceiling. When the bottom of the pillowcase is cut open, we must hold on or we will fall. While we are holding on to the inside of the pillowcase with one hand, we reach around the pillowcase to grab tightly onto the hook from which the pillowcase is suspended. In the same way that we aren’t attached to the pillowcase, the chrysalis isn’t attached to the cuticle. (Except, in the case of a caterpillar/butterfly, it is upside down in the cuticle.)
As the cuticle moves higher up on the new forming chrysalis, it begins to do ‘crunches’, working the cremaster from under the cuticle, reaching around with the cremaster, and hooking it into the silk pad. You can see a video of the process here. [In the video, the chrysalis is yellow instead of green because of a missing gene.] Learn more about yellow Monarch chrysalises here.
Now their instinct causes them to turn and twist. This works more of the hooks into the hundreds of loops of silk in the silk button. As it turns and twists, the holdfast tubercles lose their grip on the cuticle and band. This same turning and twisting movement also knocks off the old crumpled cuticle. Every now and then the old cuticle will remain but it can be easily knocked off the new chrysalis. You can see a video of much of the process here.
Because the work of the holdfast tubercles is always done out of sight, underneath the cuticle, we gently pulled back the cuticle of a few that were half-way pupated. Once the cuticle cleared the legs, proboscis, and antennae, we could move the cuticle without moving any of their essential body parts.
The one thing you cannot see in these photos is my hand about three inches below each chrysalis. If it falls to the floor below, it will die. When we pulled the cuticle back, it did place the chrysalis at risk of falling. We can happily say that none fell and died while we took these photos. We only did this for educational purposes. This is not something we normally do or do for fun. It is risky unless you are experienced with handling Monarchs at this stage.
The photo below shows what happens when the cuticle stays attached to both the silk button and the holdfast tubercles. The holdfast tubercles are not released. The chrysalis is held a bit high at the point of the holdfast tubercles. When the cuticle is pulled away a few days later, the chrysalis is slightly bent out of shape just below that point, where stress was placed on the new chrysalis. This doesn’t cause damage to the resulting adult.