Insect evolution toward aquatic habitats; reassessment of antennal sensilla in the water bug families Ochteridae, Gelastocoridae and Aphelocheiridae (Hemiptera: Heteroptera: Nepomorpha)

Abstract

The first comparative morphology study on antennal sensilla of Ochteridae, Gelastocoridae and Aphelocheiridae, carried out with the use of a scanning electron microscope, is provided. Our research hypothesis was: the antennal sensilla of the studied families reflect their different adaptations for use in aquatic habitats, while maintaining a common set of sensilla similar to other water bugs (Nepomorpha). Therefore, the number and placement of antennal sensilla of several species in the mentioned families were studied using scanning electron microscopy. Nine main types of mechano- chemo- and thermo-hygroreceptive sensilla were confirmed on their surface, including sensilla trichodea, chaetica, club-like, campaniformia, basiconica, coeloconica, plate-like, ampullacea and sensilla placodea multilobated. While seven of these were already documented in other species, two of them (sensilla plate-like and placodea multilobated) were yet to be documented on the antennae of Nepomorpha. All families display differences in the shape and length of antennae as well as among sensilla types. These findings support our hypothesis regarding differences in sensillar structures among families adapted differently to suitable niches. Differences between these families and previously studied nepomorphan taxa (Nepoidea) were also documented. However, the general set of sensilla observed on the antennae of the studied species is very similar to the one documented in Nepoidea. Therefore, we confirmed our assumptions regarding similarities in antennal sensilla between the studied families and other nepomorphan insects.

The variability of antennal sensilla in Naucoridae (Heteroptera: Nepomorpha)

“…and Limnocoris asper . Similarly, sensilla ampullacea and coeloconica have not been proven in all the studied species from previous studies on nepomorphans 24 , 33 , 34 .…”

“…Nepoidea's sensillum ST3 and Corixioidea’s sensilla ST3 and ST4 are straight, thick structures with a rounded or pointed tip and ribbed surface 33 – 35 , and are different than the sensilla described in the present study as ST3 and ST4: Cheirochelinae species show a long, straight sensilla, flattened and with almost the same width through the whole length. On the other hand, ST1 and ST2 sensilla as described in Ochteridae, Gelastocoridae and Aphelocheiridae are identical to naucorid species 34 . The five different types of trichoid mechanosensilla from the studied species of Naucoridae represent different methods of gathering mechanical information from the environment.…”

“…Sensilla basiconica SB2 are present in some of the studied naucorids and are like other nepomorphans’ (Nepoidea, Ochteridea, Gelastocoridae and Aphelocheiridae) 33 , 34 . Sensilla basiconica SB3 probably derived from the type SB2, with a slight modification (they are longer and more acute distally) but are dominant in most species except Cryphocricos montei .…”

“…Sensilla trichodea were also observed in all pentatomid species 37 and are like the ST1 in naucorids; and ST1 and ST2 sensilla found in Tingidae's 38 are identical in naucorid species. Mechanosensilla chaetica types are absent in Naucoridae, although they are present in many other Heteroptera 33 , 34 , 37 , 39 , 40 .…”

“…Another type, sensillum SB4, is represented by grooved and porous sensilla. It was found in most naucorid species, Ochteridae and Gelastocoridae as the same type 34 and in Corixoidea was identified as SB because, in that taxon, only one type, the olfactory sensilla, was distinguished 35 . Nevertheless, SB4 corresponds to the SBsh (sensilla basiconica short with longitudinal grooves and wall-pores) in pentatomid species 37 , in some species of Apoidea 43 and in most other species of insects 44 .…”