A proboscis (plural proboscises or proboscides) is an item dropped by mosquitoes encountered during the Tai Bwo Wannai Cleanup minigame. It is used, alongside a fungicide spray, to inject sulliuscep mushrooms in the Tar Swamp, causing them to die and respawn in another location. This is most often done when the sulliuscep is found in a location where the player is easily exposed to Tar Monsters.
It can also be one of the randomly selected items that the player must take to Filliman Tarlock in order to make the magic secateurs used in Fairytale I - Growing Pains.
Completion of the Jungle Potion quest is required to access the Tai Bwo Wannai Cleanup minigame and obtain this item. Alternatively, it can be purchased from the Grand Exchange.
Proboscides can also be obtained by killing mosquitoes spawned by other players playing the minigame (also works as an ironman), bypassing the requirement to complete Jungle Potion.
For an exhaustive list of all known sources for this item, see here
Examine was changed from "A giant mosquito's proboscis: aerodynamic, sharp and not very useful!" to "A giant mosquito's proboscis, aerodynamic and sharp!" after the release of Fossil Island.