Midges often occur in huge swarms, usually in the evening. The larvae of midges occur in many types of aquatic habitats. Many of the aquatic forms live in tubes or cases composed of fine particles of the substrate cemented together with salivary secretion. The larvae of many midges are red, because hemoglobin is present in blood, and are known as bloodworms.

Midge larvae are often very abundant, and are an important item of food for trout. Unlike most aquatic insects, which adopt a horizontal attitude when going about their normal business, Chironomid pupae are suspended vertically in the water column. This is an important key to fish. A fly that rides horizontally can't look or act like a Chironomid pupa.