Traits

A huge membranous tail filled with liquid. The top of the tail always presents thicker patches of skin or opaque patterns. 

The liquid color varies, for example: It may be blood that's stored for future consumption, or a sweet smelling liquid the leech utilizes to attract prey. 
It's shown to be connected to the digestive tract, so any liquid stored inside must be ingested by the leech first, and processed or stored directly.