Apystomyiidae

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 : 124-125

The Apystomyiidae

The Nearctic genus Apystomyia , originally placed in the Bombyliidae , was elevated to a separate monotypic family, Apystomyiidae , in the Asiloidea by Nagatomi & Liu (1994). The placement of Apystomyiidae alone as the sister-group to the Cyclorrhapha by Trautwein et al. (2010) and Wiegmann et al. (2011) has brought this taxon into prominence. Shaun Winterton has generously supplied me with specimens of A. elinguis Melander for antennal study from the type locality, Sheep Creek Canyon, Bernadino Co., California.

Nagatomi & Liu (1994) gave a general account of the morphology of Apystomyia , including the gross features of the antenna. As in many of the less advanced brachycerans, the general form of the antenna approximates to radial symmetry, except for the bilateral compression of segment 3.

Segment 1 is small, collar-like, with many microtrichia, not evenly distributed, and few dorsal setulae.

Segment 2 ( Fig. 174 ) is considerably larger than segment 1, rotund, slightly bilaterally compressed, slightly higher than long, without trace of a rim or conus, with the usual covering of microtrichia and a small but variable number of large dorsal and ventral bristles, not forming an encircling series. The annular ridge is scarcely raised above the distal surface of the segment, but bears numerous moderately short, inwardly directed microtrichia. The foraminal ring is slightly raised and finely crenulated. There are two dorsal to dorsolateral buttons (in both sexes) with their peripheral convex rings in contact or partly fused ( Fig. 175 ).

Segment 3 ( Fig. 176 ) is ovoid, slightly bilaterally compressed, with small basal stem fitting into the annular ridge of segment 2. The surface is rugose, more coarsely and irregularly so distally, with many small microtrichia and larger projections, probably trichoid sensilla, but typical macrotrichia are absent. There are no sensory pits, nor anything resembling the sacculi or pedicellar pouches which are present in various cyclorrhaphans.

The slender distal part of the antenna, here termed stylus (though with some doubt regarding its homology with the eremoneuran stylus or arista), arises terminally and symmetrically on segment 3. It consists of two sections (? segments). The more basal section is subcylindrical with its surface broken by many deep grooves into a series of transverse microtrichose plates or sclerites. The terminal surface of the basal section has, ventrally to the base of the terminal section, the structure here termed the stylar goblet ( Fig. 177 , go). The goblet consists in both sexes of a smooth hemispherical cuticular prominence of c. 4.1 µm diameter, with, at its summit, a subcircular aperture of c. 2.1 µm diameter. The aperture clearly leads into a capacious cavity, but further details are not visible because of the condition of the specimens and the minute size of the goblet. On further examination it may be possible to identify the goblet with a particular category of arthropod sense organ. The smaller terminal section of the stylus is articulated with the basal section and has a smooth outer surface (surface contaminated as seen in Fig. 177 ). Examination with CLM shows it to be hollow, with the inner surface of its cuticle densely micropustulose.

The Apystomyiidae are not morphologically typical of the Cyclorrhapha, because the male postabdomen has a full complement of symmetrical tergites and sternites (which suggests that there is no circumversion of the genital segment), and abdominal plaques can be detected under SEM (which suggests that there is no puparium). I am aware of no other brachyceran with two buttons on each pedicel, but the pedicellar distal articular surface has been examined for very few asiloid flies. I am not aware of any structure resembling the stylar goblet in any other family of Diptera , but this is perhaps due to its minute size and the lack of SEM study of most lower brachycerans. At present I regard the antennal morphology of Apystomyia as an ambiguous indicator of relationships, as it resembles that of some more reduced asiloid taxa as well as that of certain more or less basal eremoneurans. The articulation between antennal segments 2 and 3 is unlike that of the Chimeromyiidae and probably more plesiomorphic. If the Apystomyiidae constitute a plesiomorphic sister-group to the Cyclorrhapha, then this would seem to confirm the basal conditions for the eremoneuran antenna indicated in Fig. 23.

Figures

Figures 174–177. Antenna of Apystomyia elinguis Melander (fam. Apystomyiidae), male. (174) Both antennae, segs 3 removed. (175) Buttons of left seg. 2. (176) Right flagellum, medial view. (177) Distal part of right stylus, distomedial view; scale = 4 µm. ar, annular ridge; bu, pedicellar buttons; go, stylar goblet.