Wikipedia
Rat-tailed maggots are the larvae of certain species of hoverflies belonging to the tribes Eristalini and Sericomyiini. A characteristic feature of rat-tailed maggots is a tube-like, three-segmented, telescoping breathing siphon located at its posterior end. This acts like a snorkel, allowing the larva to breathe air while submerged. The siphon is usually about as long as the maggot's body (20 mm when mature), but can be extended as long as 150 mm. This organ gives the larva its common name.
The most commonly encountered rat-tailed maggots are the larvae of the drone fly, Eristalis tenax. It lives in stagnant, oxygen-deprived water, with a high organic content. It is fairly tolerant of pollution and can live in sewage lagoons and cesspools.