A hornworm is a caterpillar. Hornworms are the larval stage of a specific group of moths within the order Lepidoptera. The term “hornworm” is a common name given to the juveniles of the Manduca genus, most notably the Tomato Hornworm and the Tobacco Hornworm. This life form is solely focused on consuming plant matter to prepare for metamorphosis.
Defining the Caterpillar Stage
A caterpillar is the common name for the larva of any species belonging to the insect order Lepidoptera. This stage is one of four distinct phases in the process known as complete metamorphosis (holometaboly). Complete metamorphosis involves the four stages of egg, larva, pupa, and adult. The larva and adult forms often have vastly different diets and functions, minimizing competition between the young and the adults for resources.
The larval phase is primarily focused on feeding and growth to accumulate energy. Caterpillars possess specialized features, including six true legs near the head and several pairs of fleshy prolegs along the abdomen, which help them grip host plants. During this stage, the hornworm will shed its exoskeleton four to five times in a process called molting. The period between molts is called an instar.
Distinctive Features of the Hornworm
Hornworms are large, cylindrical caterpillars, often reaching lengths of three to four inches at full maturity. The two most common species, the Tomato Hornworm (Manduca quinquemaculata) and the Tobacco Hornworm (Manduca sexta), are known for their voracious appetites, feeding primarily on plants in the nightshade family. Their bright green color provides effective camouflage, allowing them to blend seamlessly with the foliage of host plants like tomato and pepper.
The most defining characteristic is the caudal “horn,” a pointed, fleshy projection on the last abdominal segment. This structure is harmless and may serve to deter predators through bluff. The two species are distinguished by their markings: the Tomato Hornworm has a black horn and white V-shaped markings, while the Tobacco Hornworm has a reddish horn and diagonal white stripes.
The Adult Moth Form
Once the hornworm reaches its full size, it burrows into the soil to pupate. It emerges as a large, heavy-bodied Sphinx moth, also commonly called a Hawk moth. These moths are powerful, agile fliers with wingspans that can reach four to five inches.
The adult moths are gray or brown and are often mistaken for hummingbirds as they hover in front of flowers to feed on nectar around dusk. The Tomato Hornworm becomes the Five-spotted Hawk moth, which has five pairs of orange-yellow spots on its abdomen. The Tobacco Hornworm becomes the Carolina Sphinx moth, which has six pairs of spots. The adult’s primary function is reproduction, laying eggs to begin the life cycle anew.