Flies make up the fourth largest insect order, after the beetles, butterflies and moths, and bees and wasps. Physical Characteristics Flies have large compound eyes composed of thousands of individual lenses. The eyes are very sensitive to sudden movement. Most flies have highly developed mouthparts that are used for specialized feeding. Mosquitoes have piercing-sucking mouthparts and feed on blood and nectar.
Horse flies have scissorlike cutting blades.
Many advanced flies, such as the common house fly, have a soft proboscis with a two-lobed tip that sucks up fluids. The six legs of flies each have a clawed foot, and beneath the claws is an adhesive pad called a pulvillus, which allows the house fly to walk on surfaces at any angle, even up walls or across ceilings. Flies are the only major group of insects that have only one pair of wings. The rear wings are reduced to small knoblike structures known as halteres, which help the insect control its flight. Reproduction Flies go through a complete metamorphosis of egg, larva, pupa, and adult.