Cryptochetidae, Brues & Melander, 1932

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 : 137-138

The Cryptochetidae

This Old World family includes the polytypic genus Cryptochetum , also doubtfully the Australian Librella and the fossil Phanerochaetum . I have examined the antenna of all these genera, but the material has been inadequate for a detailed study.

Antennal structure in Cryptochetum is distinct from that of any other acalyptrate taxon ( Figs 68–70 ). Segment 2 is completely encircled by the prominent, sharp-edged rim without an incision, notch or marked sinuosity. The distal articular surface is slightly concave, with large, elongate, erect, central conus. Because the conus is fragile and deeply embedded in segment 3, I am unable at present to describe it in detail, and the precise positions of the button and distal foramen have not been observed. Segment 3 is large, elongate, bilaterally compressed, with minute, unsegmented, subterminal arista or none and with the sacculus opening by a small pore on the ventrolateral surface before mid-length; the lateral and medial surfaces each support a number (9 to 12 or more) of shallow pits. These sensory pits each contain several club-like sensilla with thick, rounded apices ( Fig. 69 ), probably to be classed as basiconic sensilla. Though such pits are of uncommon occurrence in acalyptrate schizophorans, similar ones are present on segment 3 of Maorimyia bipunctata (family Helcomyzidae ), but they are differently distributed.

In Cryptochetum the elongate, centrally based conus, deeply inserted into the basal hollow of segment 3, resembles that of some conopids, some diopsids, and some milichiids (but probably not the more plesiomorphic taxa among the latter). Also, in the lower cyclorrhaphous family Phoridae the conus is deeply embedded in the centre of segment 3. The only claim of Cryptochetum to relationship to any of these families is to the Milichiidae , particularly in view of the interpretation of the venation of the anal region of Cryptochetum given by J. McAlpine (1989: 1476–1477). However, my thorough and prolonged investigation of the venation leaves no doubt that J. McAlpine was mistaken (see D. McAlpine, 2002), and that the venation of Cryptochetum (like that of Librella ) approximates to that of the Ephydroidea (including Curtonotidae and Drosophilidae ), not to that of the Milichiidae and Canacidae . Undoubtedly this development of the conus has evolved in a number of separate eremoneuran lineages, and is not always a reliable indicator of relationships.

The antennal structure of Librella is very different from that of Cryptochetum . Segment 2 is of a much more primitive form ( Fig. 71 ). The rim expands on each side forming a broad lateral and a medial lobe, but these lobes are not approximated dorsally to produce the cucullate or cup-like condition as in most Ephydroidea (including Drosophilidae ), but diverge from their origins on each side of the dorsal notch. The conus is stout, deep, and asymmetrical, slightly displaced on to the medial lobe of the rim, with the foramen facing dorsolaterally. Segment 3 ( Fig. 72 ) is deep and bilaterally compressed, with medium-sized basal hollow and the foramen on a broad prominence near the margin of the hollow. The arista is three-segmented and of normal length.

I have examined the holotype (in amber) of Phanerochaetum tuxeni Hennig (ZMUC) , and the most significant addition I have to add to the original description ( Hennig, 1965) regards the relation of the free anal vein (vein 6, CuA+A1, or cu1b+1a) to the cell cup (or anal cell). Interpretation is difficult because there is partial separation of the wing surface from the amber, but reflected light off the wing surface gives an apparently more accurate representation than in Hennig’s fig. 263, and indicates that the anal cell is more like that of Cryptochetum and some ephydroid taxa, with anal crossvein strongly inclined distad and the vein delimiting the anal cell posteriorly little sclerotized. There is no trace of the basal crossvein, though Hennig (1969: fig. 43) found this to be distinct in a further specimen of Phanerochaetum (perhaps a second species).

Figures

Figures 68–69. Antenna of Cryptochetum sp. (fam. Cryptochetidae). (68) Left seg. 2, dorsolateral view, conus broken. (69) Sensilla in sensory pit of seg. 3; scale = 2 µm.

Figures 70–72.Antennae of Cryptochetidae.(70) Cryptochetum sp., seg. 3 of left antenna, medial view.(71) Librella sp., left seg.2, lateral view. (72) The same, left seg. 3, medial view. bf, basal foramen; bh, basal hollow; pi, sensory pits; sc, sacculus.