Their Classification and Diversity
There are now more than 1,360 species of Culicoides known worldwide. For the strong majority of individuals interested in biting midges, their concern is directed to this genus, which includes the species with the greatest medical and veterinary impact, along with important effects on tourism and recreation. As such, nearly all papers dealing with some aspect of Culicoides have the first paragraph of their introduction talking about how important they are to humans and their livestock. However, it is only a small percentage of the species in the genus which are responsible for these impacts.
A relatively minor number of these species are vectors of more than 83 different viruses, 49 protozoa and 29 filarial worms. In spite of the fact that they are important vectors, there are surprising and fundamental gaps in our knowledge of this genus, for several reasons, discussed below; also see Harrup et al. (2015).
First, there are certainly many more species of Culicoides yet to be discovered. There are numerous further unnamed species from nearly every area on the planet. Large swaths, especially in tropical areas, have never been collected in any organized manner (e.g. most of the Andes and upper Amazon basin, Borneo). Collecting methods for many areas have been very limited, often restricted to CO2 and/or light traps. Sweeping with an aerial net certainly produces more species and has the added advantage of often pointing to the habitat of the immatures. As new species are discovered, it is especially important to check types of previously described species from your own area as well as that of surrounding regions – we don’t need more confusion in this arena either. Most authors studying species differences do not bother looking at types and there is often a confusion of what name applies to which species.
Second, knowledge of the adults far outstrips knowledge of the immatures. The literature describing the immatures (eggs, larvae, pupae) was listed in a recent publication by Borkent (2014, Table 2). This publication also provides the first generic keys to pupae and the first means of recognizing the pupae of Culicoides as such. In addition, pupal morphology provides important information for interpreting phylogenetic relationships. Jamnbach (1965), Glukhova (1979) and Murphree and Mullen (1991) provide detailed comparisons and keys to the larvae of their areas. Currently, the larvae of 13% and the pupae of 17% of described species of Culicoides are known (but many poorly described). As a consequence, only a small percentage are known as immatures, which provides us with knowledge of the primary habitat of these species. All the others, known only as adults, are from unknown habitats. Culicoides are generally great dispersers, so where we collect them can be a substantial distance from where they generally live as larvae.
Third, there is a surprising lack of keys to actually identify species. A European team led by Bruno Mathieu has an excellent online key to the species of Culicoides in western Europe. Glukhova (2005) has a good key to the species of Russia but this is available only as a publication. Other areas, such as the Afrotropical, Oriental, Nearctic and Neotropical Regions do not have comprehensive keys. There is no key to the Culicoides of Canada, nor of the USA. Lots of work remains to be done!
Fourth, the present classification of species of Culicoides is, in my opinion, in terrible condition, such that for many years I simply listed all the species alphabetically in my world catalog published in 1997 and which was subsequently updated on a website. However the subgeneric and species group placements were presented in the most recent catalog in 2020 (Borkent and Dominiak, 2020; Borkent et al. 2022). Presently there are 33 subgenera, 38 unplaced species groups and more than 135 species placed in “miscellaneous”. The problems stem primarily from the lack of cladistic analysis – virtually all of the classification is currently based on overall similarity (phenetic similarity). I hope that the following will promote further study, as we start to recognize monophyletic lineages.
Phylogenetic relationships provide critically important information regarding the biology and distribution of species. In spite of the tremendous medical, veterinary and economic importance of so many species in the genus, there has been very little phylogenetic interpretation of the genus or of any group within the genus. For example, the species of Culicoides vectoring Bluetongue Virus do not form a monophyletic group but are scattered among six subgenera and one species group (the 11 species in C. (Avaritia) may be close but their relationships to others in this subgenus are unknown). This indicates that there is something shared between these vectors that is not being phylogenetically tracked.
There are few morphologically based cladistic studies (i.e. interpretations of derived character states based on outgroup comparisons). Shults (2015) provided the first cladistic analysis of any group of Culicoides, namely the species of C. (Monoculicoides). Otherwise, authors have repeatedly classified species according to their similarity to one or another taxon, sometimes noting that the antennal sensilla pattern is the same as one group, the wing another group and the male genitalia to yet another. It is clear that basing a classification on such similarities leads to phylogenetic chaos, and hence to continued instability in all future classifications. As such, there is a tremendous opportunity for future work to consider what might be derived within the genus (such as the elongate spicules on the operculum of the pupae of species of Avaritia, etc.). A good approach by an ambitious researcher would be to take the type species of every described genus, the characteristic species of each species group, and a goodly smattering of other species, compare these carefully in alcohol, cleared in glycerine, and then on slides (without crushing various parts), obtain the larvae and pupae of as many representative taxa as possible, determine derived character states (based on outgroup comparisons) and combine the results to synthesize a phylogeny which would then provide the basis for further interpretation of species. It would be a gift to our science!
At present it is uncertain whether most of the subgenera or species groups are monophyletic, although I believe some of them to be so, based on unpublished synapomorphies. Meiswinkel and Dyce (1989b) provided a rare and welcome overview of some character states of phylogenetic importance. Szadziewski et al. (2016) provided a synapomorphy for C. (Oecacta). Borkent (2014) and Shults et al. (2016) discuss synapomorphies of the genus as a whole. Shults and Borkent are nearing completion of a revision of all species of C. (Monoculicoides) including a cladistically based phylogeny.
Aside from the species placed to subgenus, there are numbers placed in separate species groups of uncertain affiliation and a further list of miscellaneous species which no one knows what to do with. Aside from the theoretical problems noted above, this is also a reflection of various scientists working on their local fauna and constructing classifications as best they can on their local and inherently limited knowledge. There is a great need to study Culicoides on a worldwide basis, using exemplars of the various groups from each region.
Readers are also reminded that members of the genus Culicoides are known from the mid Cretaceous (99 million years ago) (Szadziewski et al., 2019). The genus is clearly ancient and was certainly diverse even before the Cenozoic.
Numbers of papers over the past 20 years have used genomic methods to interpret phylogenetic relationships between select species of Culicoides (e.g. Aguilar-Vega et al., 2021; Ander et al., 2013; Augot et al., 2013; Bakhoum et al., 2013; Carvalho et al., 2022; Duan et al. 2022; Jin et al., 2022; Lassen et al., 2012; Linton et al., 2002; Morag et al., 2012; Schwenkenbecher et al., 2008; Talavera et al., 2017; Tay et al., 2016). Most use COI (or a couple of genes) as a measure but it is highly doubtful as to whether these genes (and especially COI on its own) can be used to interpret phylogenetic relationships. These results are single gene evolutionarily reconstructions using phenetic (= genetic distance) rather than individual character-based data and are generated by a simplistic algorithm. They do not portray phylogenetic relationships although they are almost universally labeled as such.
Barcoding is becoming an increasing source of valuable information regarding species, their identification and their distribution. Not only can they help to distinguish morphologically similar species, it also provides a powerful tool to study their field collected immatures[MOU2] , once a reasonable number of adults have been sequenced in a given area.
It should be noted that too few barcoding papers indicate where their voucher specimens are housed – indeed, many do not even keep vouchers, preventing future studies from being able to check published identifications. As such many of these will become throw-away studies because of uncertain species identification. This poor practice should be rectified immediately so that future researchers can reexamine those vouchers and interpret discrepancies. Photographs of specimens in BOLD are most often not identifiable.
In spite of the cataloging of worldwide species, generally reflecting the literature, there remains some serious nomenclatorial issues. For example, the placement by Yu et al. (2005) of C. dendrophilus (type species of C. (Amossovia)), C. segnis (type species of C. (Wirthomyia)) and C. fascipennis (type species of C. (Silvaticulicoides)) in the subgenus C. (Oecacta)actually results in all the aforementioned subgenera becoming junior synonyms of the subgenus C. (Oecacta); these subgeneric placements (without action on the subgeneric synonymy by Yu et al., 2005) have not been followed. In relation to this, Szadziewski et al. (2016) have recently redefined C. (Oecacta) with a synapomorphy and relegated a number of species previously placed in this subgenus or considered as miscellaneous to C. (Sensiculicoides). Nandi and Mazumdar (2014) suggested that the clavipalpis species group be placed within C. (Diphaomyia) but the reasons for doing so are based on phenetic similarities that continue to plague a logical, cladistic basis for the classification of this genus.
The classification of African species is particularly messy and one need only compare the the following to see major discrepancies in the placement of many species: Khamala and Kettle (1971), Boorman and Dipeolu (1979), Itoua et al. (1987) and Glick (1990).
As it stands now, readers of the available catalogs will see that there is a particularly long list of miscellaneous species, not placed in any group, representing 13% of the world fauna. A number of these, especially those in old publications, are likely nomina dubia. Unless types can be found, they need to be relegated to the dust bin by taxonomists – we already know that most of Kieffer’s types are destroyed and most of his remaining names are therefore likely nomina dubia. Some workers have placed single species as the sole representative of a species group (e.g. Khamala and Kettle, 1971) and these are simply included in the list of miscellaneous species in the most recent catalog.
There is a further important gap in our knowledge and application of some names of species of Culicoides because types of numbers of species need to be reexamined and studied carefully. A case in point is the current acceptance of Culicoides lupicaris as a synonym of C. delta in the catalog (which should only reflect the the present literature). Although evidence indicates that the two are actually valid and distinct species (Lassen et al., 2012; Meiswinkel et al., 2004; Ramilo et al., 2012, 2013), no one has formally made the change in a publication. The types should be reexamined to confirm the differences and ensure the names are properly applied.
One of the more serious problems with taxonomic studies of Culicoides is the formulatic manner of their descriptions. In general, only the following are studied: pigmentation pattern (thorax, wing, legs), mandible, palpus, ommatidia, flagellum (proportions of flagellomeres and distribution of sensilla coeloconica), wing pattern, number of hind tibial spines, male genitalia and female spermathecae. In fact, there are many more features available with specimens in glycerine and specimens on slides in which the head and thorax are not crushed by the coverslip (a standard problem with most collections – the Canadian National Collection is a rare exception). In fact, there are numerous other features of the head, thorax and female genitalia that vary between species of Culicoides that are either entirely unstudied or very poorly known
As such, there is a tremendous amount of systematic work that needs to be done. As the saying goes, “Use the classification at your own risk!”
References:
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