Bamboo Heartworm.
Li Jun sat in the dark, Master Wu liked his house to be void of any light, even while he was reading or writing letters he wouldn't light any candles and he would leave his shutters down.
Master Wu tilted the glass jar and observed how the strange worm moved inside. He let out a deep breath before setting it down on the table in front of him.
"I'm not certain, but it seems to be a tracking bug." Master Wu said.
"What's a tracking bug?" Li Jun asked as he watched the man walk to a bookshelf and pull out a thick leather-bound tome. He brought it back to the table and then flicked through the pages.
"A bug used for tracking. Was that not obvious?" He asked sarcastically as he stopped on one of the book's many pages and pressed his finger against a charcoal illustration that looked similar to the worm Li Jun had coughed up.
"Here it is." Master Wu said. "It's a bamboo heartworm."