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Larva and Caterpillar

Egg | Larva/Caterpillar | Pupa | Adult

The word larva refers to the growth stage of all insects with complete metamorphosis. Caterpillar refers only to a butterfly or moth in this stage. Either word is correct, but most scientists say larva. It is during this stage that monarchs do all of their growing; in fact this is just about all that they do. These "eating machines" take few breaks even for resting.

InstarsWhen the caterpillar becomes too large for its skin, it molts, or sheds its skin. The head capsule is the first part of the old skin to come off during the molting process. Then the old skin peels back from the front of the caterpillar. At first, the new skin is very soft, and provides little support or protection. This skin soon hardens and molds itself to the caterpillar.

The shed skin is often eaten before the caterpillar ingests more plant food! The intervals between molts are called instars.

Caterpillar AnatomyMonarchs go through five instars. The best way to tell the difference between the different instars is to compare the size of their heads — the rest of the body grows within each instar, but the head size stays constant. The entire larval stage in monarchs lasts from nine to fourteen days under normal summer temperatures. If you'd like to distinguish between the different larval instars, we have developed A Field Guide to Monarch Caterpillars. You can order this field guide from our store or use these illustrations by Kristen Kuda:

Egg | First instar | Second instar | Third instar | Fourth instar | Fifth instar

5 instars plus egg all together 

Color picture of all five instars together

Larvae have three distinct body parts. They have a head, and a body with a thorax and an abdomen. The head has a pair of short antennae, mouthparts (upper lip, mandibles, and lower lip), and six pairs of simple eyes, called ocelli. Even with all of these eyes, the caterpillar’s vision is poor. The antennae help to guide the weak-eyed caterpillar, and the maxillary palps (sensory organs), help direct food into its jaws.

Caterpillar headEach thoracic segment has a pair of jointed, or true legs, while some of the abdominal segments have false legs, or prolegs. Monarchs have five pairs of prolegs. The prolegs have tiny hooks on them that hold the larva onto its silk mat or leaf. The fleshy tentacles at the front and rear ends of monarch larvae are not antennae, but they do function as sense organs.

Like other insects, monarchs obtain oxygen through holes in the sides of their thorax and abdomen called spiracles. The spiracles are connected to a network of long airtubes called tracheae, which carry oxygen throughout the body.