Male antennae
Antennal morphology
The male antenna of P. solenopsis is illustrated in Fig. 1A. It consists of ten segments grouped into three regions: scape (Fig. 1B), pedicel (Fig. 1B), and flagellum. The entire antenna length of the adult male is 844.65 ± 7.74 µm, (mean ± S.D.). The scape is the shortest segment of the antenna, roughly squared, with a mean length of 41.18 ± 2.37 μm and a width of 40.57 ± 1.32 μm. The pedicel is the broadest and most elongated antennal segment (45.25 ± 1.86 μm wide) and is longer (55.18 ± 1.84 μm) than the scape segment (Fig. 1B). By contrast, the flagellum (Fig. 1C–E) consists of eight morphologically different rod-shaped sections called flagellomeres. The flagellomeres are the thinnest and longest antennal segments, with a mean length of 92.14 ± 8.58 μm and a width of 24.07 ± 2.23 μm. The smallest segment is the second flagellomere with a mean length of 81.84 ± 1.66 µm and a width of 25.82 ± 1.09 µm. The fourth flagellomere is the longest segment of the flagellum, followed by the third flagellomere segment, with mean lengths of 107.66 ± 1.04 µm and 99.05 ± 1.60 µm and widths of 25.08 ± 0.68 µm and 23.28 ± 1.48 µm, respectively. The thinnest segment is the sixth flagellomere, with a width of 20.70 ± 1.21 µm, followed by the seventh and fifth flagellomeres with widths of 22.20 ± 0.72 µm and 22.40 ± 0.71 µm, respectively. By contrast, the broadest flagellar segment is the terminal flagellaromere (the eighth), followed by the first flagellomere with widths of 26.81 ± 0.85 µm and 26.30 ± 0.99 µm, respectively.
Types of sensilla
Eight sensillum types were identified on different antennal segments: trichoid sensilla (TS), three subtypes of chaetica sensilla (ChS1, ChS2, and ChS3), two subtypes of basiconic sensilla (BS1 and BS2), and two subtypes of campaniform sensilla (CaS1, CaS2) (Figs. 1 and 2). The TS are distinguished by medium–long, straight or slightly curved thin bristles characterized by a smooth wall with a sharp tip (Figs. 1B, E, 2A, C, F). The ChS are bristles characterized by a flexible circular membrane at the base and marked longitudinally by arranging grooves (Figs. 1B–E, 2B, C). The ChS3 is the longest one, followed by ChS1, whereas ChS2 is the shortest sensillum of the chaeticum type. The BS1 is a short and thick peg with an inflexible socket, characterized by a porous wall and uniform thickness with a distinctive blunt tip that has a typical cone shape (Figs. 1D, E, 2D, E). The BS2 is a very short, smooth-walled, cone-shaped sensillum distinguished by a widened and bifurcated tip (Fig. 2G–I). Two subtypes of CaS are found on the last two flagellomeres by two morphological appearances. Groups of aporous CaS1 appear as a smooth elliptical depression on the cuticle with a slightly raised, oval inner area (Fig. 2D, F). However, the other subtype of campaniform sensilla (CaS2) appeared as circular, dome-shaped organ surrounded by a cuticular fold (Fig. 2G).
Distribution and size of sensilla
The scape has four to six TS. Their lengths range from 25.45 to 33.37 µm. The pedicel bears two types of sensilla: TS and ChS subtype 1. No ChS2 were found on the pedicel. The ChS1 are the most abundant sensilla type (29–32) on the pedicel segment, with a mean length of 29.54 ± 1.87 μm (Fig. 1B). The TS (14–16 in number) are slightly shorter than the ChS1, with a mean length of 27.25 ± 2.44 μm.
The first seven flagellomeres carry various types of sensilla: TS, ChS subtype 1, and ChS subtype 2 (Fig. 1C). The TS (seven to nine per flagellomere) have a mean length of 30.76 ± 2.48 μm. The ChS1 and ChS2 are significantly different in length (Fig. 2A–C). The ChS1 are the most abundant sensilla type (32–36 in number), distributed on all flagellomeres and characterized by upright, slightly curved, and grooved wall bristles. The lengths of ChS1 range between 32.34 and 40.30 µm, with a mean length of 36.87 ± 2.54 µm. ChS1 are longer than ChS2 (14.02 ± 1.26 µm). The ChS2 are fewer (six to nine) per flagellomere than ChS1 for the first seven flagellomeres. The terminal flagellomere bears eight sensillum types (TS, three subtypes of ChS (1, 2, 3), two subtypes of BS (1, 2), and two subtypes of CaS (1, 2)) (Figs. 1D, E, and 2A–I). Fewer (two to three) TS were found on the terminal flagellomere than on other flagellomeres (six to nine), with a mean length of 23.51 ± 1.76 µm. Similarly, fourteen ChS1 were counted on the terminal flagellomere, with a mean length of 35.02 ± 4.56 µm, whereas four to five ChS2 were found on the terminal flagellomere, with a mean length of 16.67 ± 2.93 µm (Fig. 1D, E). A pair of large and sharp ChS (bristle-shape) with a grooved wall (ChS subtype 3) was found on the apical tip of the terminal flagellomere (Fig. 1D, E). The ChS3 on the terminal flagellomere are the longest and broadest ChS type on the antennal segments of male P. solenopsis. The mean length of ChS3 is 41.70 ± 0.79 µm, with a diameter of 3.00 ± 0.16 µm. The thickness of ChS3 is twice that of ChS1 (1.5 µm) as shown in Fig. 2G. Additionally, a basiconic sensillum subtype 1 (BS1) is found on the last two flagellar segments (seventh and eighth flagellomeres) as shown in Figs. 1D, E, 2D, E. On the last two flagellomeres, one and two BS1 were present on the seventh and the eighth flagellomere segment, respectively. For the eighth flagellomere, BS1 were present on the midline of the flagellomere, with a mean length of 15.18 µm (Figs. 1E and 2G). The BS subtype 2 was found only as a unique sensillum at the tip of the eighth flagellomere (Fig. 2G–I) and appeared extremely short in length (2.10 µm). The campaniform sensilla are also present on the last two flagellomeres (seventh and eighth) as shown in Fig. 2D–I. The CaS shows two morphological appearances (CaS subtype 1 and CaS subtype 2). Groups of aporous CaS1 are found on the base of the cuticle located among different sensillum types (Figs. 2D–F, H, I). Moreover, a slightly larger CaS1 is found at the extreme tip of the terminal flagellomere near BS2 (Fig. 2H, I). However, the second subtype (CaS2) was found only as a unique sensillum on the distal end of the terminal flagellomere (Fig. 2G).
Female antennae
Antennal morphology
The female antennae consist of nine segments grouped into three regions: scape, pedicel, and flagellum (Fig. 3A). The entire length of the female antenna is 546.04 ± 9.20 µm (mean ± S.D.). The antennal segments are illustrated in Figs. 3 and 4. The scape appears as a short segment with a mean length of 58.24 ± 2.56 μm and is the broadest antennal segment (65.08 ± 3.38 μm wide). The pedicel is a slender structure and the longest antennal segment with a mean length of 90.67 ± 2.75 μm and a width of 37.79 ± 1.76 μm (Fig. 3B, C). The flagellum consists of seven flagellomeres with lengths varying between 38.66 and 77.14 μm. The seventh flagellomere is the longest segment, with a mean length of 76.39 ± 2.07 μm and a width of 29.16 ± 1.32 μm. The first flagellomere is the broadest flagellum segment, with a width of 32.57 ± 1.45 μm, and the second longest flagellomere, with a mean length of 65.46 ± 5.91 μm. The sixth flagellomere is the smallest antennal segment (38.66 ± 2.06 µm in length and 27.22 ± 0.74 µm in width), followed by the fifth flagellomere, with a mean length of 48.86 ± 2.76 μm and a width of 28.35 ± 1.06 μm. The third flagellomere is the third longest, with a mean length of 61.98 ± 6.96 μm and a width of 28.33 ± 1.55 μm. The second and fourth flagellomeres have intermediate size, with mean lengths of 58.26 ± 2.06 μm and 52.22 ± 3.19 μm and widths of 31.03 ± 2.20 μm and 28.94 ± 1.22 μm, respectively.
Types of female sensilla
Two subtypes of sensilla chaetica (ChS1 and ChS3), two subtypes of basiconic sensilla (BS1 and BS2), trichoid sensilla (TS), and sensillum campaniformium subtype 2 (CaS2) were identified on different antennal segments of P. solenopsis females (Figs. 3 and 4). The ChS1 are distinguished as upright bristles with slightly curved and grooved walls and flexible circular membranes at the base (Fig. 3D–G). The wax pores (TWP) and filaments (TWF) of P. solenopsis were illustrated in Fig. 3C, D respectively. These hydrophobic wax filaments are secreted by various dermal pores upon invasion by microorganisms and parasites. The TWF serve as a protectant, preventing physical and chemical damage at various developmental stages. A pair of ChS3 appears only on the terminal flagellomere as sharp bristle-shaped sensilla with longitudinally arranged grooves (Fig. 4A–E). The TS sensilla are distinguished by straight thin bristles characterized by a smooth wall with a sharp tip (Fig. 4A, B, E). The BS1 are characterized by a thick wall, with a flexible socket and a distinctive blunt tip (Fig. 4A, D). By contrast, the BS2 were appeared as a short smooth peg characterized by a conical shape and a rigid socket (Fig. 4B, C, F). The CaS2 were appeared as a circular, dome-shaped organs surrounded by a cuticular fold (Fig. 4A, B).
Distribution and size of sensilla
The ChS1 are the most abundant sensillum type on all antennal segments in females, with lengths ranging between 31.35 and 60.54 µm. The scape has three to four ChS1 that are distributed on the tip of the segment, with a mean length of 40.56 ± 3.01 µm (Fig. 3B, C). The pedicel bears eight ChS1 distributed throughout the segment surface, with a mean length of 54.14 ± 3.74 µm (Fig. 3D). The first five flagellomeres of the flagellum carry ChS1. Seven to nine ChS1 are present on the first flagellomere and fewer number (five to six) on other flagellomeres except the terminal one (Fig. 3E–G). Four to five ChS1 are present on the sixth flagellomere and 10–12 on the terminal seventh flagellomere (Fig. 4G). The ChS1 are differentially distributed among the flagellomeres. On the first flagellomere, ChS1 are distributed on the base and top of the flagellomere (Fig. 3A, E), whereas it is distributed in the midline of the second to sixth flagellar segments (Fig. 3A, G).
Unlike the male antennae, the ChS2 are not found on the female antennae. The ChS3 and TS are only found on the seventh flagellomere (Fig. 4A–E). The ChS3 are slightly shorter than ChS1 (35.10 ± 0.86 µm in length) as shown in Fig. 4A. The BS1 are only found on the last two terminal flagellomeres (Fig. 4A, D). Similar to the male antennae, a unique BS2 is observed at the tip of the terminal flagellomere in female antennae (Fig. 4A–C, F). The mean lengths of the BS1 and BS2 are 35.01 ± 0.73 µm and 3.71 ± 1.11 µm, respectively. One CaS2 is found on the distal end of the seventh flagellomere (Fig. 4B).