The twospotted spider mite Tetranychus urticae Koch (Acari: Tetranychidae) aggregates on its host plant until mated females collectively disperse by walking to a new colony. Some females disperse singly and found new colonies. I experimentally investigated the role of trails in collective and solitary behaviors and found that dispersing females simply follow the trails left by preceding females. This behavior seemed to induce an amplification process, resulting in aggregation at a new colony. A solitary female readily joined a trail made by other females if she came upon one. Otherwise, a female should encounter her own trail at a new feeding site and automatically establish a colony, presumably by following her own trail. Thus, the collective and solitary behaviors that constitute the basic structure of the T. urticae colony life cycle seem to be induced by one rule: follow a trail.