The adults ofStreptogonopus phipsoni(Pocock), an Indian polydesmoid millipede, emerge from the soil at the onset of the monsoon and mate. Eggs are laid in the soil and the adults die. The larvae leave the soil at stadium III and, apart from moulting periods, remain on the surface until the end of the monsoon. The larvae are aggregated in swarms, usually consisting of several hundred individuals. They are active on open ground, including metalled roads, during daylight, but usually spend the night in an inactive state under cover of stones or vegetation. If a swarm is disturbed, the individuals scatter but later reaggregate.Laboratory experiments show that the larvae are sensitive to light even though they lack eyes, and if given a choice spend more time in the light than the dark. Just before and after moulting however they spend more time in the dark than in the light.Experiments were carried out on the reactions of the larvae to aquatic suspensions of benzaldeyde, since this substance is related to components of the exudate of the repugnatorial glands; strong concentrations (10‐1) repelled the larvae, whilst weak concentrations (10‐6) attracted them. It is suggested that swarms may be dispersed or reaggregated by the effects of variations in concentration of components of the exudates. The benefits to the larvae which may result from swarming are discus