Species of the genus Tabanus Linnaeus (horse flies) are significant pests and vectors transmitting pathogens to economically important animals including cattle. Therefore, knowledge about genetic diversity; a necessity for understanding disease epidemiology, is of paramount importance. However, such information is limited in Thailand, particularly in the northeastern region despite it being a major cattle production area. We examined genetic diversity of three Tabanus species, T. striatus Fabricius and T. megalops Walker, and T. rubidus Wiedemann based on mitochondrial cytochrome oxidase I (COI) sequences. There was relatively high genetic diversity (max. 2.14%) within populations of T. rubidus from northeastern Thailand but no indication of genetic structuring. Phylogenetic relationships between Thai populations and those recorded from Malaysia, India and Bangladesh involved three genetically divergent lineages which possibly represent different but morphologically similar species. High levels of genetic diversity (max. 4.43% and 5.20%) were found within northeastern populations of T. megalops and T. striatus due to the existence of three genetically divergent lineages within these species. However, there was no signal of geographical association of lineages suggesting panmixia within populations. Phylogenetic relationships between specimens obtained in the present study and those recorded from other countries revealed cryptic genetic diversity with three divergent clades. Further study is required to solve this taxonomic complexity of the T. striatus-complex.

Culicoides oxystoma Kieffer is a vector of viruses, filarial nematodes and protozoa of the genus Leishmania transmitted to humans and other animals. Understanding genetic diversity, genetic structure and genetic relationships among geographically widespread populations will provide important information related to disease epidemiology. In this study, genetic diversity, genetic structure and genetic relationships between Thai C. oxystoma and those reported from other countries were inferred based on mitochondrial cytochrome oxidase I (COI) and nuclear internal transcribed spacer 1 (ITS-1) sequences. A high level of genetic diversity was found in C. oxystoma from Thailand. The maximum K2P intraspecific genetic divergence for COI gene and ITS-1 sequences were 4.29% and 6.55%, respectively. Despite high genetic diversity, no significant genetic differentiation was found within the 13 Thai populations. This could be a result of unspecialized habitat requirement of the larval habitat, abundance and continuous distribution of host blood sources, potential for long distance movement with host via trading. Mitochondrial genealogy analysis of the global population of C. oxystoma revealed three (A, B and C) genetically divergent lineages. Specimens from Thailand were included in the main lineage (A) with those from all other countries except those from Senegal that formed lineage B and those of Lineage C that was exclusively found in Bangladesh. The nuclear (ITS-1) genetic markers genealogy indicated that Thai C. oxystoma belong to the same genetic lineage with those from East, South and Southeast Asia which presumably the true C. oxystoma.

Cytoform C, one of three cytoforms in the Simulium angulistylum Takaoka & Davies complex from a high mountain in northeastern Thailand was morphologically and molecularly investigated. All known life stages (larva, pupa, adult male and female except egg) were morphologically similar to, but distinguishable from S. angulistylum s. str. and S. isanense Takaoka, Srisuka & Saeung in the adults by the relative length of the fore and hind basitarsi and relative length of the tooth to the claw. It is also morphologically distinct from other species of the S. epistum species-group. Here, it is formally described as a new species, S. prayooki. Molecular genetic data based on mitochondrial cytochrome c oxidase subunit I (COI) also supported the morphological similarity between the new species and the two related known species (S. angulistylum s. str. and S. isanense) as phylogenetic analysis retrieved them all from a single clade and with a low level of interspecific genetic divergence (1.74%). This might possibly have resulted from incomplete lineage sorting as they are likely to share a recent common ancestor. Despite limitation of molecular genetic differentiation, the new species was distinctly different from two other cytoforms of S. angulistylum complex based on polytene chromosome banding patterns and ecology of the immature stages. Thus, this study highlights the necessity of using an integrated approach for fully understanding black fly biodiversity.

Simulium triglobus Takaoka & Kuvangkadilok from Thailand, in the Simulium (Simulium) multistriatum species-group, is unique among species in the family Simuliidae in having the female terminalia with three spermathecae (rather than one spermatheca). This species was described from Nan province, northern Thailand based on larvae, pupae and females but its male has remained unknown. In this study, the male of S. triglobus is described for the first time based on adult males reared from pupae collected from the type locality. The most distinctive characteristic of the male of S. triglobus is the shape of the ventral plate, which is hexagonal when viewed ventrally. No other members of S. multistriatum species group known thus far have such a unique ventral plate. In addition, the number of upper-eye (large) facets and color patterns of the legs can be used to differentiate this species in the male from other members of the S. multistriatum species-group. Mitochondrial cytochrome c oxidase I sequences enabled association of adult male specimens of S. triglobus with previously known life stages. Phylogenetic analysis based on these sequences revealed that specimens of S. triglobus formed a strongly supported monophyletic clade, being genetically distinct from other members of S. multistriatum species-group in Thailand.