The newly hatched monarch caterpillar starts out tiny (2-3mm long), but, over the next nine 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 eating, 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 growth stage called an instar. A monarch caterpillar has five instars. When it hatches out of its egg, it is a first instar, while the plump caterpillar that goes on to form the chrysalis is a fifth instar. Read on for more about the caterpillar at the different growth stages.
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.
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.
When it has moulted again, and the fifth instar is revealed, 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 some of its legs.
The fifth-instar caterpillar is an eating “machine”! It can gobble five or more large 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.