Shed skin, head parts and mandibles, oh my!

As caterpillars grow, they outgrow their skin and need to shed it, or molt, from time to time so that they can continue inflating as they eat and eat and eat and eat....

Below is a former black-instar caterpillar that has just shed its last black skin, revealing a greenie, the final instar, underneath:



After the molt, the caterpillar stays motionless for 10-20 minutes.

If you observe the caterpillar very carefully as it sheds or just after, you will notice something interesting: the hard capsule that covers the head is shed separately from the rest of the skin. In the above image, just beneath the caterpillar's head is an indistinct black shape. That is the shed head capsule dangling tenuously from the caterpillar by a strand of silk. At some point it will fall away from the caterpillar to the ground below, while the rest of the shed skin will remain on the stem.

Below is another caterpillar at the same stage showing the old head capsule more clearly, the caterpillar with its new white (as yet unpigmented) head capsule, and the shed skin.



When the caterpillar resumes moving, it will usually turn around and devour the shed skin. The video below shows the first caterpillar above in the process of eating its shed skin:




It took the caterpillar about 5 minutes to eat the entire skin.

The discarded head capsule is not eaten. It is made of a tough-as-nails type of chitin and would probably be very resistant to slicing with the caterpillar's mandibles (see below), even though they are also made of a very tough form of chitin. 

Below is the discarded head capsule of the first caterpillar shown above . It is viewed from the top with the front facing down the page.



Below is a view of the underside showing that this is a hollow shell. The front points to the left. It looks like an elaborate molded polycarbonate bike helmet!



Below is the head capsule again but mounted on a sharpened tip of a wooden barbeque skewer. Can a caterpillar head ever look a little ferocious?? We think so!



Especially closer up!



Right in the center of the capsule are two reddish-brown serrated structures, seemingly joined at the top, a little like a pair of open serrated scissors. These are the mandibles that do the chopping of fennel stems. Surprised that they are shed with the head capsule? We were!

They are also much bigger than they appear above where only a portion of the cutting edge is visible. It turns out that they are very easy to pry away from the capsule and can be examined by themselves. They are marvelous tiny cutting tools.

Take a look at one of the above mandibles, or what amounts to a caterpillar tooth, below. It is shown end-on, with the serrated edges pointing up at the camera. This one is about 0.75mm wide.



At higher magnification, below. The surface of this dangerous piece of anatomy is glossy smooth.



Viewed from the side, below. Yeow! Be glad you're not a fennel stem. That's some serious cutting edge. The big serrations even have little serrations.



In these two shots you can get a better sense of the beautiful glossy surface.



But not all mandibles stay glossily perfect. Below is the above mandible next to two larger mandibles removed from the shed head capsule of a pupated full-grown caterpillar. They look a lot more "used", although with some polishing they'd probably look much better. Now what'd I do with the toothpaste.....



The last molt is also different from the others: the head capsule is not shed separately. Instead, it splits open along what appear to be "seams", which are shown intact on the inner surface of an earlier instar capsule below:



Is it possible that this splitting of the head capsule starts the final molt by spreading to the skin? Below is a split capsule still attached to the shed skin of a pupated caterpillar:



What is the somewhat "wormy"-looking tissue between the two halves? It looks like it might be a remnant of the upper section of the digestive tract of the caterpillar.