Horned spanworm moth (Nematocampa resistaria) is a small geometer moth. It occurs across the United States and southern Canada. In the U.S. it is common east of the Great Plains and in the northwest but is rare or absent elsewhere. Adults are found from early June to late September in deciduous and mixed forests and woodlands, in meadows, and in parks.
Female forewings are whitish or cream-colored with reddish-brown lines and veins, numerous short horizontal lines, and a purplish-brown patch on the inner half of the wingtip. The hindwing is similar, but the entire tip of the wing is dark. The male is similar but smaller, is usually yellowish, and there is a dark brown blotch at the tip of the wing.
The adult sometimes rests on the upper side of a leaf, where it resembles a dead leaf; on the underside of a leaf, where it resembles a dead patch; or on leaf litter on the ground, where it blends in with the background.
The caterpillar is up to ¾″ long and is instantly recognizable. The ground color varies from yellow to brown and is heavily mottled with brown. On each of the first and second abdominal segments there is a pair of curled, extendable, white-tipped tentacles (filaments).
The caterpillar often rests on an upper leaf surface with the body looped. It has been suggested that this mimics a fallen flower and its stamens. When alarmed, it inflates the filaments to twice their length.