Fighter Fly

A Fighter Fly is a grinning pest that first appeared in the sewers of the original Mario Bros. arcade game. They are characterized by their ability to hop; an ability that allows them to cause trouble for Mario and Luigi.

Mario Bros. (Arcade 1983)
In Mario Bros., Fighter Flies were one of the myriad of pests that emerged from pipes to attack Mario and Luigi. They first appeared in Stage Six. After a Fighter Fly emerged, it would hop across the stage's platforms. As such, Mario had to precisely time his Jump when the Fighter Fly was on the ground to flip the creature over. If airborne, a Fighter Fly would not be flipped over - not even if Mario used the POW Block. Once flipped over, Mario had to run into the insect to clear it off the stage.

Super Mario Land (Game Boy)
During the events of Super Mario Land, Fighter Flies were seen in the Birabuto Kingdom of Sarasaland. In this game, they would once again hop towards Mario. Kumos from the Easton Kingdom attack in the same manner as Fighter Flies. When defeated, the Fighter Flies (just called "Flies" in this game) would award Mario with 400 points.

Mario & Luigi: Superstar Saga (Game Boy Advance 2003)
In the game Mario & Luigi: Superstar Saga, Fighter Flies lived in Stardust Fields, the border between the Mushroom Kingdom and the Beanbean Kingdom. To attack, a Fighter Fly will hop towards Mario or Luigi and bump into the plumber. The heroes could jump to avoid their attacks.

Mario and Luigi also encountered a stronger version of the Fighter Fly, the Super Fly, in the basement of Beanbean Castle.