The jumping spider Portia labiata can complete detours in which it must move away from a goal (i.e. prey) before approaching it. This detouring behaviour can be divided into two phases: a scanning phase, during which Portia stays roughly in one spot and examines its environment using its principal eyes, and a locomotory phase, during which Portia performs the detour. Earlier experiments showed that when Portia is initially confined to a small, elevated platform from which it has an unobstructed view of the goal, it plans the initial stage of its detour by scanning the possible route and picking out an unbroken path from start to goal and then aiming at an initial objective along the detour. In the present experiments, I examined the detouring behaviour of P. labiata in an open arena where obstacles provided either indirect access or no access to the goal. Although scanning movements and detour decisions of spiders were similar to those of spiders initially confined to an elevated platform, spiders in the open arena not only scanned at the start but also along the route of the detour. Decision making thus occurred gradually, during both the scanning and the locomotory phases. Taken together, these results and those of previous studies suggest that detouring behaviour in Portia involves a form of vicarious trial and error in which the spider inspects possible routes and selects a series of intermediate goals during both the scanning and locomotory phases of a detour.