Classification
Diptera (7)
Milichiidae (3)
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This site is moderated by Irina Brake. Content can be used in accordance with a CC Licence.
Eye
Eye
Edited by Irina Brake on Fri, 2008-07-11 11:20The eyes usually occupy most of the side of the head, but they may be greatly reduced or absent, especially in cavernicolous and some parasitic forms (from McAlpine 1981).
When the eyes are so large that they meet or almost meet on the median line the condition is referred to as holoptic, and when they are widely separate, as dichoptic. The tendency toward a holoptic condition is usually restricted to the male, where it is associated with swarming and aerial mating (from McAlpine 1981).
In the stem-species pattern of the Milichiidae and of the Chloropidae family-group, the eyes are more or less circular in lateral view. In the stem-species of the Milichiinae, the eyes are apomorphically enlarged in width and especially in height, and are thereby more than 1.5 times as high as wide. 
Only in Ulia must this enlargement have been secondarily lost. Correlated with the enlargement of the eyes, the genae are often narrow so that the eyes take up nearly the whole lateral side of the head. In addition there is a sexual dimorphism, because the eyes of the males are even larger than the eyes of the females. This further enlargement results in the postfrons being much narrower in males than in females, which is another apomorphy for the Milichiinae.
The enlargement of the eyes and narrowing of the frons probably is connected to the swarming behaviour in males, because the condition, in which the upper part of the eyes nearly meet in the mid-line, seems to be an adaptation to the recognition and capture of females in flight (Downes 1969). It is known from Milichia fumicostata and several Milichiella species that the males swarm in sunlight. The enlargement of the eyes in hovering or swarming males is known for some groups of Nematocera, many Orthorrhapha, Platypezidae, Pipunculidae, Phoridae, Syrphidae, and for Calyptratae (Downes 1969). Even in swarming species of Ephemeroptera, Hymenoptera, Lepidoptera, and Trichoptera the compound eyes are frequently larger in males than in females (J. F. McAlpine & Munroe 1968). 
A diagnostic character for Milichiella and Ulia is a notch in the middle of the posterior eye margin. Moreover, there are Pholeomyia species with a slight notch or an emargination in the lower half of the posterior eye margin, for example Pholeomyia nigricosta. This emargination was the reason why Becker (1907) separated the genus Pseudomilichia from Pholeomyia. However, there are transitional stages between notch, emargination, and a straight eye margin. This character alone is not useful for the separation of genera, and so Hendel (1932) synonymised Pseudomilichia with Pholeomyia (from Brake 2000).
