For a few years, a British company called Inscentinel has been developing chemical-detecting honeybees for security and sanitation purposes. Bees are trained to respond to certain smells and then are loaded into cartridges that suck in air from an object. When they alert, the user knows that the chemical is present in the sample. From the company website:
Our "sniffer bees" are honeybees trained to recognise a specific odour. They are trained using a well known Classical Pavlovian conditioning protocol - a simple association of a smell with a food reward. The insect is exposed to the odour in controlled pulses and simultaneously rewarded with sugar syrup. After three to five presentations and rewards the bee is trained. When the bee detects the odour it expects a food reward and extends its tongue (proboscis). This response is a reflex action (Proboscis extension Reflex, PER) and is not consciously controlled by the bee. A "panel" of bees can be trained in as little as a few hours to remember a particular odour for several days.
Although there are a variety of newspaper articles about this invention, I haven't found the company's claims supported by respected scientific periodicals.
Link via CrunchGear