Hydrotaea armipes (Fallen, 1825)

General description of the antenna of Hydrotaea armipes View in CoL

Like those on other muscoids, especially H. chalcogaster ( Sukontason et al. 2004) , the antennae of H. armipes are composed of three segments: a proximal scape (Sc), pedicel (Pd), and distal funiculus (Fn) that bears a slender arista laterally ( Fig. 1A View FIGURE 1 ).

Scape is the shortest antennal segment of H. armipes , and the only sensilla on antennal scape are the socketed chaetic sensilla (Ch) ( Fig. 1B View FIGURE 1 ), which are arranged in a single row. The bases of the Ch are smooth, whereas the apices are grooved. Surrounding these and distributed evenly over this antennal segment, are microtrichiae (10.24± 1.54 µm in length), small hair-like structures with acuminate apices.

Pedicel is a segment surrounding and enclosing the base of antennal funiculus ( Fig. 1B View FIGURE 1 ). The antennal pedicel bears a thin layer of microtrichiae (10.90± 2.25 µm in length) while Ch are also found on this segment. The Ch on antennal pedicel are morphologically similar to those found on antennal scape, with exceptions of being more numerous and being variable in length ( Figs 1C–1D View FIGURE 1 ).

The antennal funiculus is the most prominent part of the fly antenna with clearly distinguishable anterior surface ( Fig. 1F View FIGURE 1 ) and posterior surface ( Fig. 1G View FIGURE 1 ). The antennal funiculus with a characteristic sensory pit on the middle posterior surface measures about 294–313 µm in length and 132–151 µm in width, while the arista measures 772–796 µm long. The entire surface of antennal funiculus is densely covered with microtrichiae (14.67± 3.30 µm in length) that taper to their acute apices. The antennal arista is comprised of three segments, two basal short ones and a distal long one. Short microtrichiae (17–29 µm in length) are located around the distal end of the second segment of arista and on the entire surface of the third segment ( Fig. 1E View FIGURE 1 ).

Antennal sensilla. The only sensilla distributed on the first two segments (scape and pedicel) are chaetic sensilla, while distributed on the entire antennal funiculus are several morphological types of sensilla: trichoid sensilla ( Fig. 2 View FIGURE 2 ), two subtypes of basiconic sensilla ( Fig. 3 View FIGURE 3 ), coeloconic sensilla ( Fig. 4 View FIGURE 4 ), and clavate sensilla ( Fig. 5 View FIGURE 5 ). The length and basal diameter of these sensilla on antennal funiculus are summarized in Table 2 View TABLE 2 .

Ba I = subtype I basiconic sensilla; Ba II = subtype II basiconic sensilla; Cl = clavate sensilla; Co = coeloconic sensilla; Tr = trichoid sensilla.

Chaetic sensilla. Chaetic sensilla (Ch) are observed on antennal scape and pedicel of H. armipes . 5 to 7 Ch are identified on antennal scape ( Fig. 1B View FIGURE 1 ), while on antennal pedicel, 13 to 15 Ch are located on the cuticle, with 10 or 11 shortish ones on the lateral side ( Fig. 1D View FIGURE 1 ), and 3 or 4 longish ones on the dorsal side ( Fig. 1C View FIGURE 1 ). The Ch on the dorsal side can be divided into two groups, five or six shorter ones and two or three longer ones. These sharp-tipped Ch with longitudinally grooved in their walls are surmounted by a bulbous seta and containing a tuft of miscrosetae ( Fig. 1C View FIGURE 1 ).

Trichoid sensilla. Trichoid sensilla (Tr) are the most numerous sensilla type found on antennal funiculus of H. armipes ( Fig. 2 View FIGURE 2 ). Each Tr arises from a conspicuous base, gradually tapers to a acute apex, extends above microtrichiae and occurs in parallel to the longitudinal axis of antennal funiculus ( Figs 2C–2D View FIGURE 2 ). The Tr on the anterior surface often have a characteristic middle curvature ( Fig. 2D View FIGURE 2 ), while those on the posterior surface are nearly straight or slightly curved ( Fig. 2C View FIGURE 2 ) and the whole number of these sensilla is greater on the anterior surface of antennal funiculus than on the posterior surface. Moreover, Tr not only become slightly smaller, but also display a conspicuous density gradient in that their density increasing from the proximal region to distal region.

Basiconic sensilla. Unlike Tr, the density of basiconic sensilla (Ba) is smaller on the anterior surface of antennal funiculus than on the posterior surface, and their length is much shorter than that of microtrichiae ( Fig. 3 View FIGURE 3 ). Two subtypes of basiconic sensilla, subtype I (Ba I) and subtype II (Ba II), can be distinguished according to shape and size. The Ba I appear as tall pegs that gradually taper to a sharp tip ( Fig. 3C View FIGURE 3 ), while Ba II are more slender and blunted tipped ( Fig. 3D View FIGURE 3 ). Both subtypes are evenly distributed along the main axis of antennal funiculus, but Ba II are in lower density than Ba I on the surface of antennal funiculus. The measurement of Ba I and Ba II, including length, width at the base are presented in Table 2 View TABLE 2 .

Coeloconic sensilla. Coeloconic sensilla (Co) are the shortest sensilla found on antennal funiculus of male H. armipes ( Table 2 View TABLE 2 ), and are characterized by their pegs that project from the floor of shallow depressions with deep longitudinal grooves on their walls ( Fig. 4 View FIGURE 4 ). They are scattered irregularly on the whole surface of antennal funiculus with less abundant than Tr and Ba.

Clavate Sensilla. Clavate sensilla (Cl) with a club-like shape are strictly found on the proximal part of antennal funiculus ( Fig. 5 View FIGURE 5 ), which are characterized by a distal swelling of about 1.88± 0.66 µm diameter. There are significantly less Cl compared to Ba and Tr.