Assassin bugs from the genus Stenolemus (Heteroptera, Reduviidae) are predators of web-building spiders. However, despite their fascinating lifestyle, little is known about how these insects hunt and catch their dangerous prey. Here we characterise in detail the behaviour adopted by Stenolemus bituberus (Stål) during encounters with web-building spiders, this being an important step toward understanding this species' predatory strategy. These bugs employed two distinct predatory tactics, "stalking" and "luring". When stalking their prey, bugs slowly approached the prey spider until within striking range, severing and stretching threads of silk that were in the way. When luring their prey, bugs attracted the resident spider by plucking and stretching the silk with their legs, generating vibrations in the web. Spiders approached the luring bug and were attacked when within range. The luring tactic of S. bituberus appears to exploit the tendency of spiders to approach the source of vibrations in the web, such as might be generated by struggling prey.