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Flies are adapted for aerial movement, and typically have short and streamlined bodies. The first segment of the fly is the head and consists of ocelli, antennae, compound eyes, and the mouth. The second segment of the thorax, which bears the wings and contains the flight muscles, is greatly enlarged, with the other two segments being reduced to collar-like structures. The third segment bears the halteres, which help to balance the insect during flight. A further adaptation for flight is the reduction in number of the neural ganglia, and concentration of nerve tissue in the thorax, a feature that is most extreme in the highly derived Muscomorpha infraorder.
A scan of a house fly taken at 40 magnifications under a scanning electron microscope.
Flies have a mobile head with eyes, and, in most cases, have large compound eyes on the sides of the head, with five small ocelli on the top. The antennae take a variety of forms, but are often short, to reduce drag while flying.
Because no species of fly have teeth or any other organ or limb that allows them to eat solid foods, flies consume only liquid food, and their mouthparts and digestive tract show various modifications for this diet. The gut includes large diverticulae, allowing the insect to store small quantities of liquid after a meal.