Processionary behaviour is a distinctive feature of the dispersal of caterpillars of a number of Lepidoptera, including Ochrogaster lunifer Herrich-Schäffer (Lepidoptera: Notodontidae) from Australia. By cutting hairs on either the first or last three segments of the bodies of caterpillars and re-introducing them into processions of normal caterpillars, the present study provides strong evidence that thigmotaxis is the mechanism that maintains single-file, head-to-tail movement of groups of individuals. The inclusion of caterpillars whose posterior hairs have been cut results in the breakage of processions, whereas the inclusion of caterpillars whose anterior hairs have been cut rarely results in the breakage of processions. The breakage of processions occurs because caterpillars whose posterior hairs had been cut are difficult to follow and they will not stop when those behind became detached. The speed of movement of processions incorporating caterpillars whose hairs on the posterior segments of their bodies have been cut also has a significant effect on the likelihood of maintaining processionary movement; faster-moving processions are more likely to break than are more slowly-moving processions.