Language Arts Activities

The Tobacco Hornworm

by

Cecelia Valenzuela -Davis Bilingual Magnet School -Tucson, Arizona

Scientific name: Manduca sexta

 

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Life Cycle

The name Manduca sexta has been given to this animal for two reasons:

1. Manduca means "Glutton", something or someone that eats a lot.
2. sexta means six in Latin. The moth has two rows of six bright yellow-orange spots on its abdomen.

The Manduca goes through a life cycle called 'complete metamorphosis'. Metamorphosis means change.Manducas go through several different stages until they are fully grown: egg, larva, pupa, and adult moth.

The Eggs

The larvae are green caterpillars with seven pairs of yellowish- white lateral stripes or bars (lateral means on the side). They have a horn on the hind end of their bodies. It is believed that they use it for defense, to appear fierce, or to camouflage themselves.

The larva changes size five times by molting. Between molts, it is called an "instar". Depending upon the number of molts the animal has experienced, it can be referred to as a 1st instar, 2nd instar, 3rd instar, 4th instar, or 5th instar. During the actual molt, which can take up to 48 hours or more, the former head capsule slips forward, creating a "bubble-headed" appearance, informally called the "head cap stage".

At each larval stage the caterpillars consume an ever increasing amount of food. An entire tomato or chile plant can be consumed by a single larva overnight. While they are molting, they do not eat.

At the end of the fifth instar, the dorsal aorta or heart becomes visible along the middle of the back. It is at this time that the caterpillar stops eating and wanders away from the plant, searching for a place underground to pupate. This change from feeding to "ambulatory" or wandering behavior is known as that: "wandering".

During the wandering stage, the contents of the digestive system are voided, (thrown out) and the soft larval skin begins to shrink and toughen. Once buried under the soil or leaf litter, the body continues to shrink. After a few days, the larva molts a final time to form a fragile, (delicate) green pupa.

Within hours, the pupa hardens and changes color to a rich, dark brown. The pupa shows something that looks like the stem of a twig curled up, and that is where the straw-like mouth or proboscis is going to grow. You can also see where the wings will develop by looking carefully at the pupa.

From the outside, the pupa looks like it is resting,but inside, incredible changes are happening. If you touch the pupa, you will see that it is capable of moving its tip hard enough to protect itself from ants and other predators. The pupal stage can last from 21 days to six months, depending on the latitude and the season of the year.

The adult moth is large and gray, with a wingspread of nearly four inches. They are active during the hours of dawn and dusk, and can often be found in backyards. Hovering from flower to flower, they feed on the only source of food they have: nectar. Since the nectar needs to be sucked out of theflower, they have a special mouth part called a proboscis that acts like a spiral straw.

After the adult moth emerges from the pupa, it needs to hold itself in a vertical position in order for the wings to straighten out properly. Manducas live for only a short time, seldom more than a week. During this time,they mate and lay eggs on the leaves of a tomato, chile, potato, eggplant, pepper, or tobacco plant, and the cycle starts over again.

 

 

 

 

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