Male preabdomen

In the stem-species pattern of the Milichiidae and of the Chloropidae family-group, tergites 2 to 5 (T2-5) are brownish microtomentose or shining in males and females. However, in males of most species of the genera Milichia, Milichiella, Pholeomyia, and Ulia, T2-5 are partly or completely silvery microtomentose. In these genera and in Eusiphona, the lateral margins of the tergites of the males are strongly bent under the abdomen, and in some Milichiella, Pholeomyia, and Ulia species there is even a crease in the tergites at the site of the bend. In this case, the dorsal side of the abdomen is flat, so that the silvery surface reflects light optimally. The light-reflecting abdomen could be an adaptation for swarming behaviour, so that males swarming in sunlight can be seen from a long distance. These swarms have been observed in Milichia fumicostata, the only species of the M. distinctipennis-group with silvery microtomentose abdomen (Deeming 1981), and in several Milichiella species (Mik 1887, Illingworth 1923 and 1929, Malloch 1934, Duda 1935b, Cuthbertson 1936, Hardy & Delfinado 1980, Deeming 1998). There is one swarming species, Milichiella lacteipennis, which does not have a silvery microtomentose abdomen. Instead, the abdomen is highly polished and shining, and also reflects the sunlight as I myself have observed.
The silvery microtomentum on the tergites could be an apomorphy for stem-species H (Milichiinae without Enigmilichia), in which case it must have been secondarily lost in several species, for example in Milichiella lacteipennis. Other possibilities are that the silvery microtomentum is an apomorphy for a smaller group of genera within the Milichiinae, or that it evolved independently within the Milichia distinctipennis-group, the Milichia speciosa-group, Milichiella, Pholeomyia, and Ulia. The latter hypothesis is quite improbable in my opinion. I believe that the silvery microtomentum evolved only once, either in stem-species H or within the Milichiinae after the branching off of Enigmilichia and Eusiphona, because in all the genera except for these two there are species with a silvery microtomentose abdomen.
Strongly bent tergites are either a novelty for the Milichiinae or they evolved after the branching off of Enigmilichia. From the description of Enigmilichia, I suppose that the tergites are not bent in this genus, but I have not studied males of Enigmilichia for myself.
In the genera Eusiphona, Milichia, Milichiella, Pholeomyia and Ulia, sternite 7/8 (S7/8, see below) and the male genitalia are more or less strongly folded ventrally under the abdomen, so that they are barely visible from the dorsal side. In some species of these genera, the membrane between T5 and the horse-shoe shaped S7/8 is partly or completely sclerotised or the lateral and distal margins of T5 are bent ventrally. In addition, S5 is considerably sclerotised and enlarged in these species, so that the genitalia are protected by a strong chitinous ring. At present it is not possible to decide whether these characters evolved once or several times.
The shape of the male abdomen, with silvery microtomentum, flat abdomen, ventroflexion of the genitalia, and maybe also the protection of the genitalia by a chinitous ring could be an adaptation to the swarming behaviour in sunlight. Females can recognise the swarm from a distance and visit it for finding a mate. Swarming is especially useful for species with scattered or thinly occupied larval habitats, for which mate finding is otherwise difficult (Downes 1969).
In the stem-species pattern of the Milichiidae and of the Chloropidae family-group, S5 is roughly rectangular and about as long as S4. In the Milichiinae and the Madizinae, S5 is generally as long as broad or longer than broad. In the Phyllomyzinae, S5 is generally slightly shorter than broad. In stem-species F [Stomosis+(Aldrichiomyza+Xenophyllomyza)], S5 is apomorphically strongly reduced in size so that it is much shorter than S4 or absent. This size reduction of S5 leaves a membraneous pocket in front of the genitalia, in which the distiphallus, which is very large in these genera, can be stored. In Stomosis, S5 is divided into a narrow anterior and a narrow posterior part. The anterior part is bent behind S4 and covered with setulae, which may be part of some kind of gland. (ex Brake 2000)