Manduca Life Cycle: the larval stage

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Larvae, also known as caterpillars, hatch from their tiny, pale green eggs three to five days after the eggs are laid. These "newborn" or first instar larvae are only a few millimeters long and very pale green, but that changes quickly. As the larvae get older and larger they become bright green with seven pairs of lateral white stripes bordered with black. At the hind end of the body is a horn or spinelike process that is usually red or reddish-brown. This horn is characteristic of most species in this family of moths, and serves as the basis of another common name: the tobacco hornworm. It is not clear what purpose the horn serves, although there is speculation that it may be useful in protective camouflage or mimicry. One thing it does not do however, is house a stinger, although it is a common myth that it does.

How many legs?
According to the picture, Manduca larvae appear to have eight pairs of legs. Aren't insects supposed to have just three pairs? The three pairs of legs on the thorax are the true legs. These will become the six legs of the adult moth. The larvae's stubby prolegs complete with claw-like "crockets", are part of the abdomen. They are used for crawling, clinging, and climbing ,and are changed into other structures during metamorphosis.

The name Manduca means "glutton" or "chewer". Looking at the larvae, it's easy to see where this name comes from. The larvae have mouthparts adapted for chewing, and they put these to use eating everything in sight. In the wild, they feed voraciously on such plants as tobacco, tomato, potato, pepper, and nightshade (Family: Solanaceae). Gardeners familiar with this insect know that a mature (5th instar) larva can consume an entire tomato or chili plant in a single night!

It's important to note that the larvae don't eat all the time. Right before each larval molt, the larvae stop eating and void their gut contents so that they're empty for the actual molt. During this phase, which can last 48 hours or more, the old hard head capsule slips forward to make room for the developing new one.

The larvae appear bubble-headed at this point and are informally referred to as head-caps. Eventually, the old skin (exoskeleton) is shed and is usually eaten by the larva in a perfect example of recycling. The molting process repeats itself four times, with each successive molt yielding a larger larva.

At the end of the fifth and final larval instar, something different happens. The huge larva climbs down from its host plant and begins a period of vigorous walking or wandering that typically lasts for about five days. During this stage, gut contents are again voided, body moisture is lost, and a dark bluish line or dorsal heart becomes visible along the larva's back. At this point, the larva Manduca bury themselves about 9 - 15 inches deep underground, in loose soil or leaf litter. This protects them from predators and the weather. After a few days, the buried larva molts a final time to form a light green pupa.

In research | Life cycle | The egg | The larva | The pupa | The adult