The term inchworm is the common name given to the larval stage of moths belonging to the large insect family Geometridae. This name refers to the unique way the caterpillar moves, suggesting it is measuring the surface it crosses. The family name Geometridae is derived from the Greek words for “earth measuring,” emphasizing this distinct locomotion. Inchworms are found across the globe and represent one of the largest families of moths, with an estimated 23,000 species worldwide. The larval stage is dedicated almost entirely to feeding and growth before the insect transforms into its adult moth form.
The Mechanics of Looping Movement
The inchworm’s characteristic looping gait, which also earns them the nickname “looper” or “spanworm,” is a direct result of their unusual anatomy. Unlike most caterpillars that possess five pairs of fleshy abdominal prolegs to support their midsection, inchworms have only two or three pairs located near the posterior end of their body. This absence of support in the middle segments forces the distinctive movement pattern.
To move, the larva first extends its body forward, anchoring itself with the three pairs of true legs located on the thorax. It then draws its hind end forward until the posterior prolegs meet the true legs, causing the legless midsection to arch upward into a high loop. Once the prolegs anchor the rear, the inchworm releases its front grip and extends its forward body again, effectively “spanning” the distance to the next anchor point.
Primary Role as Herbivores
The principal activity of the inchworm larva is feeding, which fuels its rapid growth and subsequent metamorphosis. As herbivores, they consume plant foliage, including the leaves, buds, and flowers of trees and shrubs. Their diet includes both deciduous and coniferous species, such as oak, maple, elm, and various fruit trees.
Inchworms are often considered agricultural or forest pests, as large populations can cause damage, sometimes leading to the complete defoliation of trees in a matter of days. To evade predators while feeding, the larvae employ effective camouflage. Their slender bodies and color—often green, brown, or gray—allow them to mimic twigs or stems, especially when they are disturbed and stand erect and motionless on their prolegs.
The Journey Through Metamorphosis
The inchworm stage, which can last from a few weeks to several months, culminates in the pupal stage before adulthood. When ready, the larva uses silk threads to lower itself from its feeding location to the ground. Pupation typically occurs beneath the surface, where the inchworm burrows into the soil or leaf litter to form a protective cocoon.
The pupa remains dormant for a period, often lasting through the winter depending on the species and climate, as it transforms into a moth. The adult Geometrid moth emerges primarily to complete the life cycle through reproduction and dispersal. The adult stage is short-lived and generally non-feeding or minimally feeding on nectar, unlike the larval inchworm.