A proboscis (plural proboscises or proboscides) is an item dropped by mosquitoes. 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.
You need to complete the Jungle Potion quest to access the minigame where you acquire proboscis yourself (Tai Bwo Wannai Cleanup), or you can buy it from the Grand Exchange.
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.