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Hatching and Newborn...

Baby caterpillars eat their way out of their eggs.

In the video below, the baby caterpillar within the egg is engaged in a repetitive motion, and it may be the initial nibbling of the inside surface of the eggshell. In the last 1/2 second of the video, it appears that the caterpillar has broken though the eggshell. Best viewed with full screen.

A couple of minutes later, the caterpillar has clearly broken through the surface of the egg and is nibbling away, enlarging the hole. The video lasts 7 minutes, so you can skip through it at intervals if you wish to more quickly see the progress made by the caterpillar. Best viewed with full screen.

The same caterpillar is shown below continuing to enlarge the hole. The eggshell flexes as the caterpillar moves. The video lasts about 8 minutes, and starting at about 4:47 the caterpillar starts to crawl out and onto the fennel stem. A very cute little guy, and about 3mm long. Best viewed with full screen.

What has happened to the liquid which appeared to fill the egg prior to hatching, and in which a bubble was clearly present? It's a mystery.

A few minutes after hatching and walking away from the eggshell and seeming to rest, the caterpillar will turn around, return to the eggshell, and finish what it started when it ate its way out of the egg. The video below shows the caterpillar getting started with the task of devouring the rest of the eggshell.

Below is the baby caterpillar romping around after finishing its meal of eggshell. Like youngsters of other species, he (or she) is either moving around actively or pausing to seemingly rest. The back-and-forth head motions are usually an indication that silk is being deposited on the stems of the fennel plant, but in this case, there is no visible silk. Caterpillars lay down a lot of silk, presumably to help themselves keep a grip on the stem.

For scale, below are webcam pics of another newly-emerged caterpillar. He is pointed away from the egg shell, having emerged head-first:

The caterpillar then turns around and starts devouring the eggshell:

Good progress after 20 minutes or so:

Some baby caterpillars eat every last bit of the eggshell, while others leave some behind. The one below didn't finish its eggshell (the nibbled white transparent material just to its right). Note the green aphid lurking behind the next flower stem, and that the caterpillar has what appears to be a bit of eggshell tangled in its hairs.

Once the caterpillar has finished its meal of eggshell, there is a radical change in its diet---to fennel fronds or whatever other part of the fennel plant on which the caterpillar happens to have been born. In the above case, the butterfly laid the egg on a developing flowering head, so the developing flowers and their stems will become food.

To get a better sense of the size of the newborn, here's a big guy, about ready to roam, alongside a newborn. There's a 15- to 20-fold difference in length, depending on how stretched out each caterpillar is when measured.

A little closer in...remnant of eggshell barely visible behind the newborn caterpillar which is facing to the right.