It was awesome! And very easy to observe using a small, bright flashlight, along with a 10X magnifying hand lens.
I was lucky to observe the hatching of two caterpillars. The first was already out. He was very tiny but appeared clear and bright with the equipment I used. I was surprised at how pale his body was, contrasting with his huge and shiny black head. There were short pimples on the caterpillar's skin, each apparently with a tiny black hair protruding. The two sets of antennae were very small, barely visible.
The caterpillar was eating his eggshell. The shell seemed tough, because he had to make twisting or prying motions, while not making very rapid progress. One time the caterpillar twisted so hard that its whole body rotated, while the little guy raised and cocked his tail at an angle, apparently to keep his balance. His two mandibles (chewing mouth parts) moved every 1-2 seconds.
Planet milkweed
The scene visible through the lens was striking. It looked like another planet! The leaf surface was a green plain stretching into the distance-- with repeating, tile-like units, that looked somewhat like the cracked mud of a dried-up swamp. The leaf was covered with dense, curly, silvery fur (the trichomes that protect the leaf). On the plain crouched a pale alien with bumps on its skin, toiling away at a huge, transparent sphere.
He took several short naps between prying and tugging at the shell. He turned away from the shell several times, before turning back to pry at it some more. Finally, after consuming the entire shell, the caterpillar crawled over to the down-facing side of the leaf.
The last egg on the leaf was the next one to hatch. At first, I saw nothing but the black head inside. When I looked a minute later, I saw a tiny hole, a bit off-center. I could see tiny motions of the mandibles inside. Gradually the hole expanded, extending in a ring around the side of the shell, leaving the very top of the shell intact. This makes sense since, seen from the top, the mandibles are on one side of the head.
After the hole was large enough and 11 minutes after the first puncture, the head emerged. Four minutes more, he emerged completely after eating 20% of the shell from the inside. Once out, while facing away from the shell, he explored his immediate environment by turning his head from side to side. Then he turned completely around and started eating the shell. In five minutes after emerging, 30-40% of the shell was gone. Between bouts of eating, the caterpillar moved a short distance away from the shell, sometimes to rest, and then back to eat some more. , Thirty minutes after emerging, he had eaten about 60% of the shell. In another 15 minutes, 80% of the shell was gone, and he was resting again. Thirty-six minutes later, there was no further progress on the shell-- the caterpillar had moved to the safer, down-side of the leaf to join the other.
Puffing up
One striking observation: both caterpillars seemed to enlarge substantially--perhaps by as much as 50%-- during the time they dined on the shell and rested. I had no way to measure them, so this was an impression. But this consistent with what happens during molting and eclosure. Because insects have a rigid skin (the "exoskeleton"), the new skin after they shed the old, smaller one has to be inflated. To do this, the whole caterpillar takes on air, after which the new skin hardens. This way, they have room to grow inside until the next molt.
You might say that shedding the eggshell is like molting, in that inflation and hardening is required after both transitions. Another similarity is that the caterpillar eats some of his old skin after each molt. In this way, they conserve skin material--they don't have to spend energy to convert milkweed into skin. They are indeed very efficient growing machines.
- Laying
- hatching
- molting (5 times)
- forming the "J," and
- eclosure (butterfly adult emerging).