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. The entire larval stage in monarchs lasts from nine to fourteen days under normal summer temperatures.

Larvae, just like all other insects, have three distinct body parts: the head, thorax, and 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 as it moves around, and the maxillary palps (sensory organs), help direct food into its jaws. The spinneret produces silk, which the caterpillar uses to anchor itself when needed and to create the silk pad it uses to hang from when it pupates.

Each 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 air tubes calledtracheae, which carry oxygen throughout the body.

As the caterpillar grows and 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 new 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.

More information is available for each instar, visit Guide to Monarch Instars.