Ironomyiidae

McAlpine, David, 2011, Observations on Antennal Morphology in Diptera, with Particular Reference to the Articular Surfaces between Segments 2 and 3 in the Cyclorrhapha, Records of the Australian Museum 63 (2), pp. 113-166 : 128-129

The Ironomyiidae

I have already given some details of the antenna of Ironomyia nigromaculata White (D. McAlpine, 2008: figs 1, 2, 6, 7).

Segment 2 of I. nigromaculata has the basal body well developed, encircled by setulae and with prominent, flangelike rim surrounding its largely concave distal articular surface. The rim has a pair of opposed angular projections (dorsomedial and ventrolateral). The centre of the distal articular surface bears a large, distally swollen conus with dorsolateral button on its terminal surface. The conus is connected to each of the two angular projections of the rim by a separate bridge.

Segment 3 has a characteristic shape, as previously described. Study of more material shows that the number of sacculi in segment 3 can sometimes be more than two, as one male specimen shows two sacculi in the ventral gibbosity of the left antenna as well as one in the dorsal gibbosity, but, as the right antenna has an irregularly divided ventral sacculus, this may be regarded as an abnormal specimen. As this is the first occurrence of sacculi in the standard taxonomic sequence of higher Diptera (or Eremoneura), I have given it some attention.

The sacculus contains a large number of slender sensilla. These appear to be of several kinds but detail is at about the limit of resolution of available CLM. Sensilla on the floor and lower sides of the sacculus have the approximate form shown in Fig. 34 . These are almost circular in cross-section, without a finely filiform apex, and at least some show a division into two segments. Those on the wall nearer the orifice are closely packed and either more slender than the above or at least very slender and filiform apically. This rather limited information may be adequate to suggest a homology of the ironomyiid sacculus with that of Drosophila ( Drosophilidae ; Stocker, 2001), Delia ( Anthomyiidae ; Ross & Anderson, 1987 and 1991), Hippelates ( Chloropidae ; DuBose & Axtell, 1968), Calliphora ( Calliphoridae ; Fig. 33 reproduced here from Lowne, 1895), and other schizophorans. However, my data are insufficient to draw precise comparisons with the types of sensilla described in the sacculi of these flies.

I have previously summarizedevidence for a possible but uncertain close relationship between the Ironomyiidae and the Eumuscomorpha (D. McAlpine, 2008). Alternatively, Wiegmann et al. (2011) treated Ironomyiidae and Phoridae s.l. as sister groups, which separated c. 90 MYA. However, Euliphora grimaldii Arillo & Mostovski, 1999 , dating from c. 110 MYA (Early Cretaceous), showed to a significant extent the venational apomorphies of early but not basal phorids (see Brown, 2007), and apparently also (my interpretation of the illustrations by Arillo & Mostovski) the characteristic phorid reduction of antennal segment 2. The Ironomyiidae resemble the Phoridae in the partial fusion of the subcosta and vein 1, but otherwise possess none of these phorid apomorphies. Therefore, if the two families are sister groups, their initial divergence must have occurred at a much earlier time level, especially so if the sinolestine fossils (discussed by D. McAlpine, 2008) are close to true ironomyiids. The date of separation indicated by Grimaldi & Engel (2005: fig. 12.78)—between 130 and 140 MYA—is more credible. Arillo & Mostovski placed Euliphora , together with Prioriphora (Late Cretaceous) , in the phorid subfamily Prioriphorinae . It remains to be recorded whether these fossils have abdominal tergites 1 and 2 quite separate, as in Recent phorids (and apparently the Late Cretaceous Sciadophora ), or partly fused as in Ironomyia and most other cyclorrhaphans.

Contrary to the statement of J. McAlpine (1989: 1422), Ironomyia differs from almost all Phoridae in the holoptic condition of the males, the derived phorid genus Postoptica being an exception. It also differs from the Phoridae in the sexual dimorphism of the prelabrum (a condition met with in many taxa of Eumuscomorpha, see D. McAlpine, 2008: 22–23), and in the absence of barbed macrotrichia on the head, thorax, wing, and abdomen.

The Eumuscomorpha

Following Wada (1991) this apparently monophyletic group includes the Syrphidae , Pipunculidae , and the numerous families of Schizophora. Despite the great present diversity of the Eumuscomorpha, there appear to be very few undoubted fossils of the group from before the Tertiary (Cenozoic era). There is much diversity in structure of segment 2 in the Eumuscomorpha, but the groundplan conditions are probably those shared with certain syrphids and the less modified taxa in several superfamilies of Schizophora, e.g., the Sciomyzoidea. The accumulated antennal apomorphies of the basal Eumuscomorpha, probably absent in the groundplan of the Eremoneura, are as follows: segment 2 with rim extended as encircling flange; conus present, with button on its distal surface; annular ridge and distal foramen of segment 2 tilted dorsolaterally; segment 3 with one or more sacculi; arista arising dorsobasally from segment 3; arista threesegmented (doubtful apomorphy).

Figures

Figures 33, 34. Details of sacculus.(33) Calliphora vicina Robineau-Desvoidy (fam.Calliphoridae), longitudinal section of sacculus (after Lowne, 1895). (34) Ironomyia nigromaculata White (fam. Ironomyiidae), sensillum from lining of sacculus, approximate outline as seen with CLM; scale = 5 µm.