When the larva is ready to molt, it becomes very still and latches itself to a leaf with silk it produces from a spinneret located on its lower lip. The head capsule is the first to molt, and the cuticle (outer skin) then splits down the back. Below you can see the caterpillar just after it escaped from its cuticle (the grey mass behind it). It has now moved into its fifth instar. You can tell which instar a monarch larva is in by the size and shape of its tentacles. To learn more about monarch larva and instars, click here for a pdf version of "A Field Guide to Monarch Caterpillars (Danaus plexippus)," by Karen Oberhauser and Krista Kuda, or here for the monarch life cycle on the University of Minnesota's website.
|Our monarch caterpillar has just molted into its 5th instar. You can see the thin, crumbled cuticle behind it.|
In the following video, you can see the very end of the caterpillar shedding its skin. The quality isn't great, because I took it with a cell phone instead of my camera, but you get the idea...
Monarch caterpillar shedding its cuticle (outer layer of skin) from Kelly Riccetti on Vimeo.
Where is its head?
At first it's a little hard to tell the front end from the back end of a monarch caterpillar. Both ends have black fleshy tentacles that resemble antennae, but when you look a little closer, it's easy to tell the difference. First look at the tentacles. The front pair are much longer than the back pair. The front tentacles function as sense organs similar to antennae, but the back pair are thought to be defensive to confuse predators on which end is the head. Second, look at the legs. Three pairs of jointed "true legs" are in the front. Five pairs of "false legs" (prolegs) are attached to the abdomen all the way to its rear.
What's the difference between true legs and prolegs?
Caterpillars have three pairs of true legs attached to the thorax. These legs are segmented and often have a claw on the end. They help the caterpillar hold its food. If you look closely in the photo below, you can see the tiny claws at the tips of the true legs on the monarch caterpillar. True legs transform into the legs of the adult insect during metamorphosis. Caterpillars also have prolegs attached to the abdominal segments, but the number of pairs varies among species (monarch caterpillars always have five pairs). Prolegs are stumpy, cylindrical, and unsegmented. They also have microscopic hooks at their tips that work like suction cups to help the caterpillar cling to stems and leaves. The prolegs dissolve during metamorphosis.
|In this close-up you can see why monarch tentacles are always described as "fleshy." They are not segmented like an adult insect's antennae, they are fleshy. You can also see a close-up of the three segmented true legs on the thorax.|