A small flying bug suddenly appeared at home. What bug is it?

This is gnat (the fourth sound of Rui)

Polygonum cuspidatum has two membranous wings, long articulated legs, big eyes and small antennae. A fragile body usually has a soft, hairy shell. The straw (or mouth) of Polygonum hydropiper is suitable for piercing and sucking. Larvae usually hatch in water.

Female insects can spread onchocerciasis, leading to blindness or subcutaneous nodules. After being bitten by gnats 1 min, people begin to feel pain, local redness, itching, and even inflammation and ulceration. In severe cases, anaphylactic shock may occur. In particular, a large number of stings can cause dermatitis, strong allergic reaction, secondary lymphadenitis, lymphangitis and "fever".

Having this kind of thing at home means that your garbage is rotting. First clean the trash can, spray the inside and outside of the trash can with pesticides, and then look for their nests, usually in dark and humid corners.