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A fifth instar caterpillar of the monarch butterfly eating a swan plant leaf, in close-up

The newly hatched monarch caterpillar starts out tiny (2-3mm long), but, over the next 9 to 20 days, it will grow to many times that size – up to about 45mm long. By weight, that is 7,000 times its size on hatching! To grow so much, it has to eat – and, as you’ll know if you’ve ever seen a big caterpillar on a plant, it sure can eat!

As the caterpillar eats and grows, it gets too big for its skin. Every few days, it moults (sheds its skin) to reveal a new skin underneath. Each time it moults, it starts a new stage called an instar. A monarch caterpillar has five instars. When it hatches out of its egg, it is a first instar, while the fifth instar is the plump caterpillar that goes on to form the chrysalis.

A first instar caterpillar of the monarch butterfly, with a black head and pale brown stripes on its grey body, on a leaf

When the first instar caterpillar emerges from its egg, it is 2-3mm long and is a greyish-brown colour. After about a day, pale brown stripes have become visible on its body. The first instar has a black head and tiny knobs behind its head where its front tentacles will be.

The second instar is about 6mm long when it first sheds its skin. It now has the characteristic yellow triangle on its “face” and brown, yellow, and whitish stripes along its body. Its front tentacles are small but distinct, and its back tentacles are also visible.

A third instar caterpillar of the monarch butterfly on a leaf, its black, white, and yellow stripes reflected in a metal ruler

After moulting again, the third instar is about 9-10mm long. Its colouring now looks more like a typical monarch caterpillar, with more distinct black, yellow, and white stripes and longer tentacles at the front and rear.

The fourth instar is about 13-14mm long when it has just moulted. It looks much like the third instar, except that its tentacles are almost twice as long. The caterpillar grows a lot in this stage, especially in length.

A fifth instar caterpillar of the monarch butterfly alongside a white measuring tape

When it has moulted again, and the fifth instar emerges, it is about 25mm long. Its stripes are often bright and clear in this stage, and the black stripes may look velvety. Its tentacles have again doubled in length (to about 11mm in the front and 4mm at the back). There are clear white dots on its prolegs (the five sets of legs with suckers on the abdomen and rear end of the caterpillar; the three sets of legs at the front of the caterpillar are its true legs).

The fifth-instar caterpillar is an eating “machine”! It can gobble five or more swan plant leaves a day. If you have a lot of fifth-instar caterpillars on your swan plants, your plants can get eaten very fast. The caterpillar stays in this instar longer than the previous instars and grows very plump. It eats a lot to store up energy for turning into a butterfly.

[Coming soon] Click on these links for more detailed information about:

  • moulting
  • the distinguishing features of each instar
  • the anatomy of a caterpillar.