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.

Use the diagram on the left to identify the ocelli, mandibles and head capsule in the photo on the right.

A butterfly's relatively enormous compound eyes are made up of thousands of ommatidia [SEM photo (21 K)] each of which senses light and images. The two antennae and the two palpi, which are densely covered with scales, sense molecules in the air and gives butterflies a sense of smell. The straw-like proboscis is the butterfly's tongue, through which it sucks nectar and water for nourishment. When not in use, the butterfly curls up its proboscis [SEM photo (13 K)]

Three segments make up the thorax. Each segment has a pair of legs attached to it, while the second and third segments each have a pair of wings attached as well. The legs consist of six segments. They end in tarsi (singular, tarsus [SEM photo (13 K)]), which grip vegetation and flowers when the butterfly lands on a plant. Organs on the back of the tarsus "taste" sweet liquids. Monarchs (and other nymphalid butterflies) look like they only have four legs because the two front legs are tiny and curl up next to the thorax.

All butterflies and moths have four wings, two hindwings and two forewings. Small structures attach the wings to the thorax, and muscles attached to these structures move the wings. The butterfly can also move its wings by changing the shape of its thorax. Wing veins, tubes with thickened walls, contain trachea, nerves, and space for hemolymph to move through. Veins give the wings structure, strength, and support.

The abdomen consists of eleven segments, the last two or three of which are joined. On male Monarch butterflies you can see a pair of claspers on the end of the abdomen. These appendages grasp the female during mating.