Horseflies are the largest of the blood-sucking flies, ranging in size from 8mm-25mm in length. Of the 3000+ species known worldwide, 30 occur in the British Isles.
Similar to many haematophagous flies, only the female horseflies feed on blood to obtain the necessary nutrients for the development of their eggs.
Males lack the piercing mouthparts necessary to bite and feed mainly on a diet of nectar. As a result of this many species are important pollinators of flowers. The sexes are easily differentiated by the size of their eyes: males have very large eyes which meet on the top of their head, while females have smaller eyes which are widely separated.
The deerflies of the genus Chrysops are very distinctive flies with their irredescent green eyes and strongly marked wings that are held in a delta shape when the fly is at rest.
In Britain, C. relictus is a common species in wet aluvial meadows south of a line from the Mersey and Humber rivers. It also occurs as far north as Sutherland, but is generally much scarcer.
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