curious caterpillars of Pennsylvania

A Monarch butterfly perches in the palm of a child’s hands.

By Tianna Godsey, events coordinator

The word caterpillar comes from the Old French words chatepelose meaning “hairy cat” and piller which means “pillager,” essentially translating to “hairy destroyer.” Caterpillars are the larval/juvenile stage of butterflies and moths. They have long, segmented bodies and typically possess three pairs of true legs along with additional fleshy prolegs. Caterpillars vary greatly in color, texture, and size… from the “inchworm” (yes, he’s a caterpillar), to the hot-dog sized hornworm. Their entire purpose in life is to eat and grow, from egg to chrysalis.

Caterpillars are extremely important members of food webs. It is estimated that about 95% of birds raise their young on insects, many of which are caterpillars. Foxes and bears are also known to eat caterpillars. One caterpillar equals the same nutrition as about 200 aphids. One nest of chickadees can go through 4,000 to 9,000 caterpillars feeding their young.

The vast majority of caterpillars are folivores (specialize in eating leaves). Approximately 1% of caterpillars are predators (and some are cannibals). To help caterpillars, plant natives as they make up about 75% of caterpillar food source and are essential for habitat.

Metamorphosis: From Caterpillar to Winged Insect

The absence of the juvenile hormone triggers a caterpillar to begin spinning a chrysalis or cocoon. While sometimes these are used interchangeably, a cocoon and chrysalis are not the same.

A cocoon is made from silk and natural materials. This is separate from the pupa’s body, which is encased inside. The chrysalis is made from chitin, and the caterpillar’s body is within the chrysalis. Inside the cocooned chrysalis, a remarkable transformation takes place.

During metamorphosis, the caterpillar’s original tissues begin to break down–essentially turning to goo. Specialized groups of cells known as imaginal discs are already present in the caterpillar from birth. These cells multiply rapidly and eventually form the structures of the adult insect, including wings, legs, antennae, eyes, trachea, and other body parts.

Scientists have observed that some adult butterflies can retain vague memories of experiences from their caterpillar stage.

Notable Caterpillars and Moths of Pennsylvania

Inchworms and Geometer Moths:

“Inchworm” is a common name for the caterpillars of geometer moths. Most inchworms are caterpillars of geometer moths, sometimes called “earth measurers” because of the way they move, appearing to measure the ground as they travel.

Unlike many caterpillars, inchworms have only two pairs of prolegs near the back of the body. Because they lack some of the middle prolegs found on other caterpillars, they move in a distinctive looping or measuring motion, bringing the rear of the body forward and then extending outward again. This unique movement gives rise to the name “inchworm.”

As small forest dwellers often less than an inch in length, they are masters of crypsis, often resembling twigs, small sticks, or leaves when resting.

Tiger Moths (Arctiidae):

Tiger moth caterpillars are primarily ground-dwelling species and are often covered in dense hairs, giving many of them a fuzzy appearance. These hairs have microscopic barbs containing histamines and alkalines. Many tiger moth caterpillars accumulate defensive chemicals from the plants they eat, helping protect them from predators.

Some of these caterpillars are specialists, and a handful of them (lithosiines) feed on blue-green algae and lichen.

Notable examples include:

• Isabella Tiger Moth (Pyrrharctia isabella), whose caterpillar is commonly known as the “woolly bear”
• Milkweed Tussock Moth (Euchaetes egle)
• Banded Tussock Moth (Halysidota tessellaris)
• Giant Leopard Moth (Hypercompe scribonia)

Swallowtails (Papilionidae):

Pennsylvania is home to approximately seven species of swallowtail butterflies.

Their caterpillars are covered in minute hairs but don’t appear hairy. They possess a defensive organ called an osmeterium, located just behind the head. When disturbed, this forked structure can be everted and emits defensive chemicals that help deter predators.

Many species resemble bird droppings or snakes during early growth stages, an effective form of camouflage.

Brush-footed Butterflies (Nymphalidae):

The brush-footed butterflies comprise one of the largest butterfly families (over 75 species) in eastern North America. One in every three butterflies is a brushfoot.

Adults effectively walk using only four legs while the reduced front pair remains tucked close to the body.

Many species in this family display bright colors and striking wing patterns: Monarchs, Viceroys, Checkerspots, Fritillaries, Crescents, and Mourning Cloaks.

Hawk Moths or Sphinx Moths (Sphingidae):

Hawk moths, also called sphinx moths, are powerful fliers and include some of the fastest-flying moth species. They also have the most acute vision of any animal.

Roughly 70 species occur in the eastern United States. Many species hover while feeding, resembling hummingbirds.

The larvae of many sphinx moths are known as hornworms because of a prominent horn-like structure found at the rear of the body.

Examples include:

• Hummingbird Clearwing Moth (Hemaris thysbe)
• Tobacco Hornworm (Manduca sexta), known as Carolina Sphinx Moth in its adult form

Giant Silkworm and Royal Moths (Saturniidae):

The giant silkworm moths are among the largest and most spectacular moths found in Pennsylvania. Approximately 28 species occur in North America.

The caterpillars are often large and may bear colorful knobs, bumps, or spines.

Examples include:

• Polyphemus Moth
• Cecropia Moth
• Luna Moth
• Hickory Horned Devil (the caterpillar of the Regal Moth)

Whites and Sulphurs:

The evolutionary base between butterflies and moths. Many of these caterpillars are cannibalistic.
Often considered garden “pests” by farmers because they love legumes and vegetables.