This page will help you learn to identify Midges so when you encounter them you will be able to identify the insect and then match the hatch with the proper fly. Check out Jason Neuswanger aquatic insect site from Cornell University on Aquatic Insect guide to Entomology for Trout Fishing
Chironomid is the scientific name for a midge. They are small (1-10 mm length), delicate insects that are somewhat mosquito-like in appearance, but they lack scales on the wings, and do not have a long proboscis (they do not bite).
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.