Comparative morphological study on the ultrastructure of antennal sensillae in two pollinating fig wasps

The surface of insect antennae is densely covered with various types of sensillae. The morphology, function, quantity, and distribution pattern of these sensillae reflect the evolutionary pressures borne by the antennae as signal-receiving organs. In the specialized pollinating insects of Ficus species, the antenna morphology of pollinating fig wasps is constrained not only by phylogenetic conservatism but also shaped by host traits.

In this study, we used scanning electron microscopy to compare the antennal sensillae and morphology of pollinating fig wasps from different genera: Platyscapa quadraticeps (pollinator of Ficus religiosa) and Blastophaga sp. (pollinator of Ficus ischnopoda).

The results indicated that the total antenna lengths of Blastophaga sp. and P. quadraticeps were (791.94 ± 3.04) μm and (672.83 ± 5.22) μm, respectively, with a significant difference. Both antennae were geniculate, consisting of the scape, pedicel, and flagellum. The scape was triconical, the pedicel was elliptical, and the flagellum was composed of the hook-shaped apical projection, a funicle, and a clava. On the antennae of both wasps, seven types of sensilla were identified: Böhm's bristles (BB), trichodea sensilla (TS), obscura sensilla (OS), chaetica sensilla (CHS), basiconic capitate peg sensilla (BCPS), basiconica sensilla (BS), and multiporous placoid sensilla (MPS). Among these, CHS-II and BCPS-II were present only on the antennae of Blastophaga sp., while the subtypes of MPS were more diverse on the antennae of P. quadraticeps. TS and MPS were the most abundant in both wasps, representing over 50% of the sensillae. MPS were olfactory sensillae, and they covered the largest surface area of the antennae of both wasp species.

In summary, although there were significant differences in antenna length between the two species of pollinating fig wasps, their overall morphology was similar. Both species possessed specialized structures such as the hook-shaped apical projection and obscura sensillae, which functioned to pry open the apical bracts of the fig and facilitate the wasp's entry into the fig syconium. This modified third antennal segment is a synapomorphy of Agaonidae. For both species, olfactory sensilla were the most numerous sensilla and covered. Clearly, odor perception is the primary function of fig wasp antennae. The comparison of antenna morphology between these two species supports the hypothesis of stabilizing selection, and hence phylogenetic conservatism on antenna morphology.