Using crab-like dummies, we have shown previously that fiddler crabs [Uca vomeris (McNeill)] defend their burrows against intruders in a burrow-centred frame of reference. The crabs respond whenever an intruder approaches to within a certain distance of the burrow entrance, and this distance is independent of the approach direction. We show here that the crabs combine information from the path integration system on the location of their invisible burrow and visual information on the retinal position of an intruder to make this allocentric judgement. Excluding all alternative visual cues, we propose that the crabs employ a small set of matched visual filters to determine the relationship between a crab-like object and the invisible burrow. To account for the constantly varying distance between the crabs and their burrows, the state of the path integrator may select the appropriate one of these retinal 'warning zones'. We have shown before that burrow-owning fiddler crabs are extremely responsive to potential burrow snatchers, which we simulated with crab-like dummies moving across the substratum towards the burrow of residents. The crab's decision to respond to these dummies depends mainly on the spatial arrangement between itself, its burrow and the approaching dummy. The most important factor predicting response probability is the dummy's distance from the crab's burrow: the crabs are more likely to respond the closer the dummy approaches the burrow. The dummy-burrow distance not only determines the overall response probability but also the timing of burrow defence responses (i.e. when the crabs decide to react). Most interestingly, this response distance is independent of the dummy's direction of approach to the burrow. In addition, the crabs respond earlier to a dummy approaching their burrow if they themselves are further away from it, indicating that knowledge of their own distance from the burrow has an influence on their decision to respond. These results raise a number of interesting issues, which are the focus of this paper, regarding the cues and the information used by the crabs in burrow surveillance.