After two weeks, a sharp drop in the juvenile hormone allows cells in the imaginal discs to replicate triggering the 5th and final molt that results in pupation (the formation of a chrysalis). Monarchs go through 4 of these molts, and thus have 5 developmental stages called instars. Once a caterpillar completes molting, as its younger self had done before, the freshly molted caterpillar turns to its old cuticle for its first meal. The molting stage is a vulnerable time for caterpillars because their new cuticle offers little protection and their new mouthparts are too soft for them to eat. First, the head capsule pops off (imagine your face falling off and regrowing) then the caterpillar wiggles out of its old skin, pulling one pair of legs out at a time like an old pair of skinny jeans. Once it outgrows its skin (called the cuticle ), the hormone ecdysone is released triggering the insect to molt. As a caterpillar, a special hormone called the juvenile hormone keeps these clusters of cells from replicating and ensures our caterpillar stays a caterpillar. These blueprints, called imaginal discs, are dormant clusters of cells for each structure present in the adult body—ie. Even though it is only 2-6 mm long, our caterpillar already contains the blueprint for the butterfly to come. When our monarch caterpillar hatches from its egg, its first act is typically to eat its own egg shell. (Photo credit: Wendy Caldwell) A hungry first instar caterpillar nibbling away at its egg shell shortly after hatching. This doesn't appear until it starts consuming its milkweed host plant. Notice that it lacks the characteristic bright yellow coloration. The BeginningĪ 1st instar monarch caterpillar that has just hatched and consumed its egg shell. But how exactly does a chubby little caterpillar manage to recreate itself as a majestic ornament of the skies? Because monarchs happen to be my particular area of expertise, they will serve as our primary subject as we examine what really happens when an insect transitions from caterpillar to butterfly. This probably sounds like a ridiculously strange and poorly thought out horror movie, but for many insects this is the norm thanks to the spectacular process known as metamorphosis. Over the next fourteen days your baby multiplies 3,000 times in size to that of an elephant, climbs up a tree, tears off its skin, and 10 days later flies away as a pterodactyl. Imagine for a moment that you have just welcomed a beautiful baby into the world.
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