The Order Diptera are the True flies, with around 100,000 known species worldwide. They are sometimes known as 'Two-winged flies', because the forewings are fully formed and used for flight, whereas the hind wings are reduced to small knobs on short stems known as halteres. These are used for stability in flight, acting in a similar way to a gyroscope, balancing the fly while it flies. There are some exceptions to this description of Diptera wings, as a few species such as the Louse-flies (Hippoboscidae) are parasites with very reduced wings.

Diptera wings are usually clear with many veins. The wing vienation is extremely variable and is often an important identification feature.

The antennae are also important as identification features, at the sub-order level of classification and also at the species level. The number, size and shape of the segments and whether there is an arista (bristle) at the tip are all used to identify flies.

The Diptera have compound eyes (many facets or lenses to the eye). These are often so large that they meet at the top of the head, allowing them to detect very small movements over a very large area. The meeting of the eyes is often an indication that the fly is a male, although in some groups the females eyes are like this.

The mouth-parts of flies are also variable. With a few exceptions adult flies feed on liquids. Many species have mouth-parts consisting of pads used to mop up their food. They regurgitate digestive fluids to help with this process, which is part of the reason why house-flies are such important carriers of disease.