The systematic morphological difference between individuals of different sex in the same species.
This difference usually does not relate to the parts of the postabdomen, where male and female genitalia are always different. Often the differences between the sexes are not very great, for example, the distance between the compound eyes is different between male and female flies inmany groups (e.g., touching eyes in males of most Syrphidae, always separate in females); several midge groups have males with strongly plumose antennae while females have almost bare antennae.
Very striking differences may be in groups where males have developed seconday sexual characters, like ornamentation of legs (e.g., in many Dolichopodidae), head (e.g., some Tephritidae) or antennae (e.g., also in some Dolichipodidae); or in groups where females have developed characters to assist in choosing a mate (e.g., penate legs in Empididae) or specialised structures for oviposition (e.g., in Conopidae). In some extreme cases, mainly due to the life habits of species, the male and female may hardly resemble each other because one sex has a different life habit than the other (one sex living underground most of its life or on a certain host, the other free living). This may result in loss of wings and/or atrophiation of certain body parts in one of the sexes.
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