Orbital plate

In J. F. McAlpine (1981), an orbital plate is called 'fronto-orbital plate' if it extends to the anterior margin of the frons, and 'orbital plate' if it is restricted to the upper half of the frons. However, these plates are homologous structures and so I am using only the name 'orbital plate'.
According to Hennig (1958), the orbital plate convergently became smaller along different evolutionary lines of the Schizophora, so that the orbital plate with the orbital setae is restricted to the upper half of the postfrons. In the lower half of the postfrons, the frontal vitta extends from eye margin to eye margin (holometopy), and only a narrow strip along the lower eye margin remains more strongly sclerotised. This strip may widen secondarily and the lateral row of frontal hairs may develop into strong medioclinate setae, which are called frontal setae and look exactly like orbital setae. This secondarily widened plate is called the frontal plate. This situation (schizometopy) evolved independently in the Calyptratae, Tephritidae, and Milichiidae, and can be recognised by a clear separation between the orbital and frontal plates in some species. It must be differentiated from the ancestral shape, in which the orbital plate reaches the anterior margin of the frons. In some of these ancestral shapes (present in the Agromyzidae, Carnidae, Odiniidae), the anterior orbital setae are medioclinate and are called frontal setae by some authors, though they are true orbital setae (from Brake 2000)