Milichiidae online

Three main shapes of basisternum were recognised by Speight (1969) in Acalyptratae. (1) The shape may be basiliform, which means that the area between the fore coxae is mostly sclerotised but the sclerite lies free in the cervical membrane. (2) It may be small ('reduced' in Speight), which means that the sclerite occupies only a small portion of the area between the fore coxae. Consequently this area is mostly membraneous. (3) A precoxal bridge is developed, which means that the area between the fore coxae is almost completely sclerotised, and the basisternum extends anteriorly and laterally to join the propleuron. Precoxal bridges are not necessarily homologous, because there are different ways in which the bridge may be constructed (see below).
In the Milichiidae all three main types of basisterna exist (Speight 1969). In my opinion, the basisternum is small in the stem-species pattern of the Milichiidae, because there are small basisterna in the stem-species pattern of the Acartophthalmidae and Carnidae. In the stem-species pattern of the Chloropidae there is a precoxal bridge. This is certainly autapomorphic because there cannot be a precoxal bridge in the stem-species pattern of the Milichiidae. This is because the two groups of the Milichiidae in which a precoxal bridge occurs, Litometopa and Desmometopa saguaro, belong to the Madizinae. In other Desmometopa species, in the stem-species pattern of the Madizinae, and in the stem-species pattern of its sister-group, the Milichiinae, there is a basiliform basisternum. The precoxal bridge therefore has to be a novelty in Litometopa and in Desmometopa saguaro (see below) and is convergent with the precoxal bridge present in the Chloropidae.
The stem-species pattern of the Milichiidae, with a small and roughly rectangular basisternum, is represented in Aldrichiomyza, Borneomyia, Phyllomyza, Stomosis, and Xenophyllomyza. In Neophyllomyza+Paramyia it is secondarily reduced to a narrow triangle. A large, V-shaped (basiliform) basisternum is found apomorphically in the Milichiinae+Madizinae. In Desmometopa saguaro there is secondarily a precoxal bridge, formed by a connection between basisternum and propleuron, but the sclerites are not fused. In Litometopa there is a precoxal bridge, too, but in this case basisternum and propleuron are fused.
The unfused precoxal bridge in Desmometopa saguaro could be an early stage of the fused bridge in Litometopa. This would mean that Litometopa is the sister-group of Desmometopa saguaro. The apomorphy for D. saguaro+Litometopa would be an unfused precoxal bridge, which secondarily fuses in Litometopa. There are in fact no apomorphic characters for Desmometopa, which would argue against this hypothesis. The development of a precoxal bridge strengthens the prothorax and could be an adaptation for predaceous behaviour in Litometopa and Desmometopa saguaro. The fore legs, which are raptorial in appearance, suggest that these species may be predaceous, but unfortunately nothing is known about their behaviour.
Apart from the strengthening of the prothorax, for example as an adaptation to a prey-catching behaviour, the function of different shapes of the basisternum in Milichiidae is, as yet, unclear. According to Speight (1969) the universal presence of the basisternum in Acalyptratae, whatever its shape, is presumably due to its function as the site for attachment of leg musculature. Secondly, the major characteristics of prosternal shape appear to depend primarily upon the role of the prosternum in strengthening and supporting the anterior part of the thorax.
Between basisternum and insertion of the fore coxae, there are secondary sclerites in species with a V-shaped basisternum and in Phyllomyza. Anterior to the basisternum there is the small triangular presternum. According to my observations, the presternum is weakly developed in Stomosis and Neophyllomyza, which is different from the results of Speight (1969), who mentions that the presternum is evanescent in the Milichiinae. A further difference between my observations and Speight's results is that he found a precoxal bridge not only in some Desmometopa species but also in Leptometopa and some Milichiella and Milichia species. With the exception of Desmometopa, I could not confirm these results. One explanation for our differing conclusions could be that Speight was using dry material in these cases. Even in the alcohol material that I studied, it was often very difficult to decide whether there is a precoxal bridge or not, because the posterior cervical sclerite, which articulates with the propleuron, is positioned close to the basisternum and forms a kind of functional precoxal bridge, which in my opinion is not homologous with a true precoxal bridge. In other cases, the sclerites of the basisternum and propleuron are joined in some specimens but there is a small cleft between them in other specimens. In these cases I decided against calling it a precoxal bridge. Another explanation for our different results could be that the species that I studied are not the same as those examined by Speight, who does not mention species names (ex Brake 2000)