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 very short antennae, mouthparts (upper lip, mandibles, and lower lip), and six pairs of very 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, which are sensory organs, help direct food into the larva's jaws.
Use the diagram on the left to identify the ocelli, mandibles and head capsule in the photo on the right.
Each thoracic segment has a pair of jointed, or true legs, while some of the abdominal segments have false legs, or prolegs. There are usually 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.