Web-building spiders are formidable predators of insects, and their venom and silk also afford them powerful defence. Nevertheless, some insects specialize on hunting web-building spiders. Stenolemus giraffa is a very large araneophagic assassin bug with an unusually elongated pronotum ('neck'). This araneophage routinely stalks web-building spiders either by walking onto the web or by leaning into the web to grasp spiders from vantage points on adjacent surfaces. Silk transmits even minute vibrations to resident spiders, and we investigated how S. giraffa circumvents this highly sensitive early warning system to approach spiders undetected. To investigate hunting decisions, and their relation to detection risk, we manipulated the predatory routes available to S. giraffa in pursuits of two spider species that build very different webs. For each prey spider species, S. giraffa tended to take routes that minimized contact with the web and its support lines. Stenolemus giraffa usually resorted to stepping onto the web to pursue spiders only once alternative routes had been explored and probably found inadequate, but even then they avoided the web capture area. For both prey spider species, route preferences of S. giraffa closely reflected risks of detection, indicating a predatory strategy that is finely linked to the perceptual world of the prey. Together with its subtle predatory strategy, S. giraffa's unusually elongated morphology probably functions to increase its ability to reach spiders from vantage points close to the web without stepping onto the web itself.