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What Do Monarch Larvae Do?

Monarch caterpillars eat voraciously. Find the two larvae in the photo. One is 2,000 times larger than the other. A monarch larva can grow this much in about two weeks, depending on temperatures.

Monarch caterpillars grow through five stages as a larva. Each stage is called an instar. The smallest stage is the 1st instar. As the larva grows it goes through the 2nd, 3rd, 4th and 5th instars. After that, the larva becomes a chrysalis.

This monarch larva just shed its exoskeleton and is now eating the remaining portion, called the exuvia. A monarch must shed its exoskeleton to make room for growth. The process of shedding is called molting and happens between each instar. The most likely reason for eating the exuvia is to recycle the nutrients it contains, especially hard-to-get nutrients like nitrogen.

Monarch caterpillars have tiny, oval-shaped holes on the sides of their bodies. Dr. Karen Oberhauser explains what they are and how they work: "Monarchs 'breathe' through tiny openings on the sides of the abdomen and thorax called spiracles. The spiracles are in their cuticle, like our skin. I put quotes around the word 'breathe' because this is very different than the kind of breathing you do. Monarchs don't have lungs. The air goes into their spiracles and through a whole series of tubes in the body called trachea. The trachea carry oxygen throughout the monarch's body."

Walk and Grip
Monarch larvae have two different kinds of legs, true and false. Like all insects, the caterpillars have six true legs. They are attached to the thorax. The false legs, or prolegs, are attached to the abdomen and are only present during the caterpillar stage. False legs have pads at the ends with sticky hooks called crochets. When you hold a caterpillar, you can feel the grip of the crochets. Crawling along a paper-thin leaf, or eating while hanging upside down, monarch caterpillars defy gravity.