Although caterpillars of Thaumetopoea pityocamnpa may mark their pathways with silk, this study shows that the material is essential to neither the elicitation nor maintenance of trail-following or processionary behavior. Trail following is dependent upon a pheromone the caterpillars deposit by brushing the ventral surfaces of the tips of their abdomens against the substate. Earlier instars are strongly bound to their trail system; in the laboratory, caterpillars followed circular trails continuously for as long as 12 hr before breaking away from them. The trail pheromone is long-lived and soluble in nonpolar solvents, but its volatilization or degradation allows the caterpillars to distinguish new from aged trails. In contrast to trail following, processionary behavior, the head-to-tail, single-file movement of the caterpillars is dependent on neither silk nor the trail pheromone. Stimuli associated with setae found on the tip of the abdomen of the precedent caterpillar serve to hold processions together, and such stimuli take priority over those associated with either the trail pheromone or silk. Although the caterpillars discern trail strength and choose stronger over weaker trails, the trail marking system of the processionary caterpillar appears less sophisticated than those of other, previously studied species of social caterpillars, and colonies are relatively inefficient in abandoning exhausted feeding sites in favor of new food finds. In laboratory studies, females were more likely to lead processions than males, and leaders, regardless of gender, expended more energy in locomotion than followers.