Systematic studies of Anopheles (Cellia) kochi (Diptera: Culicidae): Morphology, cytogenetics, cross-mating experiments, molecular evidence and susceptibility level to infection with nocturnally subperiodic Brugia malayi

Morphological characterization of antennal sensilla in Toxorhynchites theobaldi, Toxorhynchites violaceus, and Lutzia bigoti adults: a comparative study using scanning electron microscopy

Some mosquitoes, including species of the genus Toxorhynchites, are known for actively preying on other mosquito larvae, making these predators valuable allies in the fight against vector-borne diseases. A comprehensive understanding of the anatomy and physiology of these potential biological control agents is helpful for the development of effective strategies for controlling vector populations. This includes the antennae, a crucial component in the search for hosts, mating, and selection of oviposition sites. This study utilized scanning electron microscopy to characterize the sensilla on the antennae of adult mosquitoes from two species that are exclusively phytophagous, including Toxorhynchites theobaldi and Toxorhynchites violaceus, as well as Lutzia bigoti, which females are allegedly hematophagous. The types of sensilla in each species were compared, and five basic types of antennal sensilla were identified: trichoid, chaetic, coeloconic, basiconic, and ampullacea. The analysis also found that they were morphologically similar across the three species, regardless of feeding habits or sex. The identification and characterization of basic types of antennal sensilla in T. theobaldi, T. violaceus, and L. bigoti suggest that these structures, which play a crucial role in the behavior and ecology, have common functions across different mosquito species, despite differences in feeding habits or sex.

Antennal morphology and sensilla ultrastructure of the malaria vectors, Anopheles maculatus and An. sawadwongporni (Diptera: Culicidae)

Mosquitoes rely mainly on the olfactory system to track hosts. Sensilla contain olfactory neuron receptors that perceive different kinds of odorants and transfer crucial information regarding the surrounding environment. Anopheles maculatus and An. sawadwongporni, members of the Maculatus Group, are regarded as vectors of malaria in Thailand. The fine structure of their sensilla has yet to be identified. Herein, scanning electron microscopy is used to examine the sensilla located on the antennae of adults An. maculatus and An. sawadwongporni, collected from the Thai-Myanmar border. Four major types of antennal sensilla are discovered in both species: chaetica, coeloconica, basiconica (grooved pegs) and trichodea. The antennae of female An. maculatus have longer lengths (μm, mean ± SE) in the long sharp-tipped trichodea (40.62 ± 0.35 > 38.20 ± 0.36), blunt-tipped trichodea (20.39 ± 0.62 > 18.62 ± 0.35), and basiconica (7.84 ± 0.15 > 7.41 ± 0.12) than those of An. sawadwongporni. Using light microscopy, it is found that the mean numbers of large sensilla coeloconica (lco) on both flagella in An. maculatus (left: 32.97 ± 0.48; right: 33.27 ± 0.65) are also greater when compared to An. sawadwongporni (left: 30.40 ± 0.62; right: 29.97 ± 0.49). The mean counts of lco located on flagellomeres 1-3, 6, and 9 in An. maculatus are significantly higher than those of An. sawadwongporni. The data in this study indicate that two closely related Anopheles species exhibit similar morphology of sensilla types, but show variations in length, and likewise in the number of large sensilla coeloconica between them, suggesting they might be causative factors that affect their behaviors driven by the sense of smell.

Ultrastructure of sensilla on the antennae and maxillary palpi of the human-biting black flies, Simulium nigrogilvum and Simulium umphangense, (Diptera: Simuliidae) in Thailand

Antennae and maxillary palpi are the most important sensory organs involved in the behaviors of black flies. The ultrastructure of sensilla on these sensory appendages of two human-biting black fly species, Similium nigrogilvum and Simulium umphangense, was studied for the first time. Wild adult females of both species were collected in Umphang District, Tak Province, western Thailand. The morphology and distribution of sensilla were examined using scanning electron microscopy. Overall, the morphology of the antennae and maxillary palpi and distribution of sensilla are similar in the two species. Four major types of sensilla were found on the antennae of both species: sensilla basiconica (three subtypes), coeloconica, chaetica (four subtypes), and trichodea. However, sensilla basiconica subtype IV are only present on the antennal surface of S. nigrogilvum. Sensilla trichodea are the most abundant among the four types of sensilla that occur on the antennae of both species. Significant differences in the length of the antennae (scape and flagellomere IX), length of the maxillary palpi (whole and palpal segments I, III, IV and V), and the length and basal width of four sensilla types (trichodea, chaetica, basiconica, and coeloconica) were found. In addition, two types of sensilla were observed on the maxillary palpi: sensilla chaetica (three subtypes) and bulb-shaped sensilla. Differences were observed in the numbers of bulb-shaped sensilla in the sensory vesicles of S. nigrogilvum and S. umphangense. The findings are compared with the sensilla of other insects, and the probable functions of each sensillum type are discussed. The anatomical data on sensory organs derived from this study will help to better understand black fly behavior.

Morphological and molecular identification reveals a high diversity of Anopheles species in the forest region of the Cambodia-Laos border

Background

To develop an effective malaria vector intervention method in forested international border regions within the Greater Mekong Subregion (GMS), more in-depth studies should be conducted on local Anopheles species composition and bionomic features. There is a paucity of comprehensive surveys of biodiversity integrating morphological and molecular species identification conducted within the border of Laos and Cambodia.

Methods

A total of 2394 adult mosquitoes were trapped in the Cambodia–Laos border region. We first performed morphological identification of Anopheles mosquitoes and subsequently performed molecular identification using 412 recombinant DNA–internal transcribed spacer 2 (rDNA-ITS2) and 391 mitochondrial DNA–cytochrome c oxidase subunit 2 (mtDNA-COII) sequences. The molecular and morphological identification results were compared, and phylogenetic analysis of rDNA-ITS2 and mtDNA-COII was conducted for the sequence divergence among species.

Results

Thirteen distinct species of Anopheles were molecularly identified in a 26,415 km² border region in Siem Pang (Cambodia) and Pathoomphone (Laos). According to the comparisons of morphological and molecular identity, the interpretation of local species composition for dominant species in the Cambodia–Laos border (An. dirus, An. maculatus, An. philippinensis, An. kochi and An. sinensis) achieved the highest accuracy of morphological identification, from 98.37 to 100%. In contrast, the other species which were molecularly identified were less frequently identified correctly (0–58.3%) by morphological methods. The average rDNA-ITS2 and mtDNA-COII interspecific divergence was respectively 318 times and 15 times higher than their average intraspecific divergence. The barcoding gap ranged from 0.042 to 0.193 for rDNA-ITS2, and from 0.033 to 0.047 for mtDNA-COII.

Conclusions

The Cambodia–Laos border hosts a high diversity of Anopheles species. The morphological identification of Anopheles species provides higher accuracy for dominant species than for other species. Molecular methods combined with morphological analysis to determine species composition, population dynamics and bionomic characteristics can facilitate a better understanding of the factors driving malaria transmission and the effects of interventions, and can aid in achieving the goal of eliminating malaria.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05167-0.