where they come from
The Eye of Cthulhu makes these and nothing else does. There is no summon item and they never appear on their own, so you only meet them during that boss fight. They peel off while the Eye is in its opening form and stop arriving once it splits open and starts charging, which makes the swarm a first half problem.
clearing them
They emerge right at the pupil, so keeping your crosshair on the center of the boss kills most of them the moment they exist, and every shot that misses one lands on the Eye instead. Nothing you fire is wasted.
Do not write them off because they fold to one hit. They hit far harder than their size suggests, and they drift through terrain, so a walled arena keeps nothing out. Three of them clipping you at once is a genuine death on Expert.
trivia
- The sprite is the Suspicious Looking Eye turned upside down.
- They look like Demon Eyes but run the plain flying behavior rather than the Demon Eye pattern.
- Their contact damage is higher than their own total health, which only the Bee and the Meteor Head also manage.