CATERPILLAR (LARVA) – GUSANO
The caterpillar (larva) emerges from an egg in approximately four days. After a tiny caterpillar emerges from an egg, it is hungry and needs to eat the egg shell because it is rich in protein. The larva eats the egg in a circular pattern. Then the caterpillar climbs to the top of a milkweed leaf and becomes a munching machine. If there are other holes it may be remains of other eggs. It could be that a spider, mite or even a monarch caterpillar ate the eggs.
Caterpillars have lively colors with bands of yellow, black and off-white striped skin. A young larva may roll up in a tight ball if handled. Perhaps the caterpillar is trying to protect itself from being attacked. A caterpillar will grow beyond its skin five times and each time the caterpillar needs to shed its skin. This is known as molting. Stages between molts are called Instars. A caterpillar remains soft and vulnerable until a new skin has a chance to harden and it is best not to handle caterpillars because of molting.
There are several threats to the caterpillar. Tachnid flies look for a host for its own larvae and may lay eggs on milkweed or on a caterpillar. The parasitic wasp is also a threat. Additionally, there are various bacteria and viruses that could be harmful. Notice how big the caterpillar is in the illustration? The larva appears to be in its fourth Instar.
A caterpillar has three body parts: Head, thorax and abdomen. There are two sets of tubercles, one at the head and another at the end of the abdomen. The antennae like tubercles may confuse predators because the larva mimics two heads. The front tubercles aid in sensing and smell.
The thorax segment has three pairs of true legs. All insects have six true legs. There are four sets of prolegs or false legs in the abdominal segment. Usually these are visible as in the illustration. They look like pads and have little hooks that help the caterpillar attach itself to a leaf.
There are prolegs at the end of the abdomen in the illustration; the anal prolegs are used by the caterpillar to spin a white silk button. When a caterpillar splits its exoskeleton for the last time, the larva hangs upside down and starts to molt first starting with the head. When the old skin is near the rear end of the abdomen, a cremaster is exposed. This is a strong dark appendage that is uses to attach to a silk button that a caterpillar made before it started to pupate. The newly formed chrysalis will harden and hang upside down on the underside of a leaf or twig until a monarch butterfly is born.