Why Hilara Is Not Amusing: the Problem of Open- Ended Taxa and the Limits of Taxonomic Knowledge

CHAptER TEN WHY HILARA IS NOT AMUSING: THE PROBLEM OF OpEN- ENDED TaXA AND THE LIMITS OF TaXONOMIC KNOWLEDGE Daniel Bickel Australian Museum, Sydney, Australia INTRODUCT I ON Recent concern over biodiversity loss has highlighted our ignorance about life on this planet. In particular, we do not even know how many species exist, or how many there are likely to be. Somewhere between 1.5–1.8 million plant and animal species have been described (this number is uncertain partially due to potential synonymy), but estimates of the actual number10 of living species range from 5–100 million (Stork 1997). Such estimates are based on varying methods of calculation, different species concepts, and trophic models, but a figure close to 10 million species has been used in many recent papers. Various projects have been proposed to electronically document all species in a unified format, the most recent being the Encyclopedia of Life (EOL), which plans a separate web page for each described species on the planet (Leslie 2007). Other projects, such as the defunct All Species Foun- dation, called for ‘the discovery, identification, and description of all remaining species on Earth within one human generation—25 years’ (Berg- er 2005). This unrealistically optimistic project, while acknowledging the small number of practicing taxonomists (6,000–7,000) and the rather low output of new species described each year (15,000), naively proposed that new tools and computer technology could overcome any problems. Previ- ously, E.O. Wilson (2000, 2003a) stated ‘to describe and classify all of the surviving species of the world deserves to be one of the great scientific goals of the new century.’ I do not question Wilson’s enthusiasm and his ground-breaking achievements in many areas of biology (including Wil- Diptera Diversity: Status, Challenges and Tools (eds T. Pape, D. Bickel & R. Meier). © 2009 Koninklijke Brill NV. 280 D. Bickel son 2003b, his magisterial monograph treating the New World ant genus Pheidole with 624 species, 337 newly described, this alone being far more than the individual output of most full-time taxonomists), but I doubt we can come even close to describing the Earth’s biota, and reaching even 3 million described species in next century would itself be a major accom- plishment. To begin with, the 1.5–1.8 million described species (those to be award- ed individual EOL web pages), include all the larger, prominent species (vascular plants and vertebrates), a general baseline of taxa (butterflies, mollusks, ants, algae, etc.), taxa of direct economic or medical interest (agricultural pests, mosquitoes, etc.), and random groups studied out of personal curiosity (for example, the 10,890 species & subspecies in the cranefly family Tipulidae described by C.P. Alexander; see Oosterbroek 2007). The remaining 5-100 million undescribed taxa comprise mostly invertebrates, fungi, and prokaryotes. Many of these are ‘orphan taxa’, groups that are totally neglected or lack current taxonomists (Janzen & Hallwachs 1994). For example, in many arthropod groups, a taxonomic ‘impediment’ exists at the level of genus, which means specimens cannot be identified much beyond family level. Quite simply, appropriate keys do not exist, old descriptions are unusable, and there are insufficient spe- cialist taxonomists in the world. This problem is not confined to rarely encountered or obscure taxa. In highly biodiverse regions of the world, some of the most abundant families taken in routine trapping cannot be identified much beyond family level. Further, many taxa are ‘open-ended’ by which I mean they are highly speciose, cosmopolitan or spanning several biogeographic regions, and that based on known local faunas, their true diversity cannot even be ac- curately estimated. Apart from being speciose, they often remain uncollected or unsorted from mass trapping residues, are unrecorded for vast regions where they undoubtedly occur, and often lack clear generic defini- tion. The total species numbers of such open-ended taxa are as uncertain as the estimates for the total world biota cited above. The effort needed to complete the taxonomic study of such groups is not a simple matter of ap- plying computer technology, as will be discussed later. This paper reviews examples of open-ended taxa in the Diptera drawn from a range of families and genera. This is by no means a complete conspectus, and specialists will be able to suggest additional examples. There are many large (>1,000 spp.), widely distributed and often para- Diptera Diversity: Status, Challenges and Tools (eds T. Pape, D. Bickel & R. Meier). © 2009 Koninklijke Brill NV. Why Hilara is not Amusing 281 phyletic genera in the Insecta alone, not to mention other classes. Simi- lar open-ended taxa occur in the Coleoptera, Hymenoptera, Lepidoptera, Hemiptera, Acari, Crustacea, Nematoda, etc. Also, the discussion below is confined to species based on morphology, as species based on molecular differences are another dimension, outside the scope of the traditional taxonomy discussed here. [For an example of the species richness that might be discerned using molecular techniques, see Condon et al. (2008). Here six sympatric cryptic species of Blepharoneura Loew (Tephritidae) were discovered in the floral calyces ofGurania spinulosa (Cucurbitaceae) in eastern Ecuador. They were ecologically and molecularly distinct, but diagnostic morphological characters to separate the species were not discovered.] 1. Examples of Open-Ended Taxa in the Diptera 1.1 Family Cecidomyiidae Adult Cecidomyiidae are fragile flies that are often abundant in trap sam- ples (their numerous body fragments might even be regarded as a major thickening agent in the Malaise trap ‘soup’). Although commonly called ‘gall midges,’ cecidomyiids have a wide range of larval habits. Much of the taxonomic attention given to the family is related to pest management, although the vast majority of species have no economic impact. From a total of 5,451 described species, Gagné (2007) listed the number of Ce- cidomyiidae known to occur in each zoogeographical realm: Palaearctic 3,057, Nearctic 1,169, Neotropical 533, Oriental 335, Australasian 247, Af- rotropical 165, and Oceanian 32. The geographical numbers show a com- mon pattern among many poorly known Diptera families, with the Palae- arctic region having most species, reflecting a long history of research in a settled and accessible region. By itself, Germany has 838 cecidomyiids (Jong 2004), more than any other zoogeographic realm except the Nearc- tic. If the more than 800 species from a recently glaciated Central Euro- pean country is a glimpse of potential cecidomyiid diversity, then the true world total, considering the rich floras, varied habitats and ancient land- scapes of the tropics and southern hemisphere, must be at least 1–2 orders of magnitude greater. This family is truly immense and open-ended, but unlikely to attract casual entomologists, not the least because species are so small and fragile. Further, as Gagné noted, much of the named fauna is Diptera Diversity: Status, Challenges and Tools (eds T. Pape, D. Bickel & R. Meier). © 2009 Koninklijke Brill NV. 282 D. Bickel poorly described and needs revision in a wider context to have any mean- ing. For further discussion, see Espíritu-Santo & Fernandes (2007) on the diversity of gall-inducing insects in general based on ecological consider- ations. 1.2 Family Ceratopogonidae Culicoides Latreille. Culicoides is a large and difficult genus of some 1,200 described species (Borkent & Wirth 1997), and it includes many species of medical and veterinary importance. The bite of some species causes irritation and allergic reactions in humans, and more importantly, Culi- coides spp. are proven vectors of arboviruses, protozoa and filarial worms, including Bluetongue virus, a serious disease of sheep and cattle. Despite its economic importance, this genus is poorly known. For example, Dyce et al. (2007) published a photographic atlas showing the diagnostic wing patterns for all the Australasian Culicoides species, of which 145 were described and 120 undescribed. This means that 40% of the known Australasian species are represented only by code names and will probably remain in this informal taxonomy for some time. Neverthe- less they are identifiable and of practical use to medical entomologists. Although considerable funding has been available to study and describe potential disease vectors, large and complex genera such as Culi- coides can still present major problems. Dyce et al. (2007) suggested that the publication of their atlas would serve as a framework on which to fine- tune species identification using molecular techniques. Possibly so, but this serves to emphasize the importance of basic morphological identification before molecular work can proceed. 1.3 Family Corethrellidae Corethrella Coquillett. The Corethrellidae, or frog-biting midges, comprise a single genus, Corethrella, with both extant and fossil species. The family is predominantly tropical, and females are known to feed on the blood of male frogs, being attracted to them by their mating calls. The en- tire family was treated by Borkent (2008), and of the 97 extant species, 52 were newly described. Of these, 38 species are known from Costa Rica, a country that has been intensively collected, by both general Malaise sam- pling and with frog-call traps. Based on the species collected and regions not visited within Costa Rica, Borkent suggests 35–50 additional species will be found in that small Central American country. Clearly a large un- Diptera Diversity: Status, Challenges and Tools (eds T. Pape, D. Bickel & R. Meier). © 2009 Koninklijke Brill NV. Why Hilara is not Amusing 283 described/ uncollected fauna exists elsewhere, not only in the Neotropics but in the even more poorly known Old World tropics. Here is an example of a fauna whose true diversity cannot even be estimated without a spe- cific collecting technique that targets an aspect of its feeding behaviour.

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