I was just reading about EcoBot II, the fly eating robot. It's got 8 microbial fuel cells that flies get sucked into. The chitin exoskeleton gets broken down into glucose, the bacteria break up the glucose and generate electrons to power EcoBot II with electricity. On a full stomach (8 flies -- one in each fuel cell), it can go for 5 full days. But it takes 12 minutes to generate enough electricity for it to walk one 2cm step. That's 5 steps and hour, 120 steps a day, 600 steps in 5 days. It's probably not getting too far on a full stomach. But imagine if you could go for 5 days on a single meal..

The predecessor to EcoBot II was SlugBot, which hunted for slugs using its imaging systems. But its methane-based system took too long to power up. And EcoBot II draws its food to itself by reaking of sewage. Saves itself all that hunting and gathering time.