Monarch caterpillars molt (shed their skin) five times throughout their larval stage. The "skin" that is left behind is actually the caterpillar's exoskeleton. An insect's skeleton is on the outside, as opposed to on the instead like mammals, fish, reptiles, amphibians and birds. Endoskeletons provide support inside the body in the form of bones and cartilage, whereas As caterpillars grow, their exoskeleton gets too tight, so they need to shed it in order to continue to grow. Each molt results in a new "instar" stage. Therefore, when the egg hatches, that tiny 2 mm caterpillar is in its first instar stage.
It will molt three more times and grow exponentially over a couple of weeks. Each stage lasts 3-5 days. The fifth and final molt is when the fifth instar caterpillar becomes a chrysalis.
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Monarch Butterfly ID & Gallery
Above is the anatomy of a monarch butterfly. The head has four parts: 1) Antennae 2) Compound eyes 3) Labial palps and 4) A proboscis. The Thorax has three parts broken into three segments. The first segment contains one set of small, not visible legs, the second segment contains the forewings and one set of legs, and the third segment contains the hindwings and the last set of legs. The abdomen is broken into 11 segments that are visible in the last three stages of the life cycle. You can see them in the caterpillar or larval stage as well as in the chrysalis, or pupal stage, where they are clearly visible at the top of the chrysalis near the cremaster and finally in the adult as seen above.
Above is the dorsal, or top view, of an adult male versus an adult female monarch butterfly. Note the two scent glands, also called alar glands, which are scales, on each hindwing of the male. They are harder to detect when the wings are closed, but very obvious when they are open. These glands, or lack thereof, are important when reporting monarchs for citizen science projects. Also note the thinner veins of the male compared to the female. Additionally, males have a pair of claspers on their abdomen which is used for mating, whereas females do not.
Above is the dorsal, or top view, of an adult male monarch butterfly. Note the two scent scales or alar glands on each hindwing. They are harder to detect when the wings are closed, but very obvious when they are open. Also note the thinner veins of the male compared to the female. The scent scales, or lack thereof, are important in correctly identifying monarchs, as well as the sex of a monarch, when participating in citizen science projects.
Above is the dorsal, or top view, of an adult female monarch butterfly. Note there are no scent scales on the hindwings like the male. Also note the thicker veins of the female compared to the male. The scent scales (alar glands), or lack thereof, are important in correctly identifying monarchs, as well as the sex of a monarch, when participating in citizen science projects.
Above is the ventral, or underside view, of an adult male monarch butterfly. Note the barely visible scent scale on the hindwing. Compare to the female below.
Above is the ventral, or underside view, of an adult female monarch butterfly. Note the lack of a scent scale on the hindwing and the thicker veins. Compare to the male above.
Adult Monarch Butterfly Gallery
