Scorpion incidents, misidentification cases and possible implications for the final interpretation of results
© The Author(s). 2016
Received: 17 June 2016
Accepted: 17 June 2016
Published: 2 July 2016
The aim of this contribution is to bring general information on the classification and in particular on the specific identification of scorpion species dangerous to humans. Several generic groups are taken into consideration, but the Neotropical genus Tityus C. L. Koch, 1836 is used as a major example. The content of this paper is mostly addressed to non-specialists whose research embraces scorpions in several fields such as venom toxins and public health. Although efforts have been made in the last 20 years to create better links between ‘true scorpion experts’ and non-specialists who use scorpions in their research, such exchanges had never led to a consensus among those different branches of biological and medical research. Consequently, many cases of species misidentification and even more serious errors concerning scorpion classification/identification are often present in the specialized literature. In conclusion, it is suggested here that the frequent cases of misidentification observed in several reports may induce mistakes in the final interpretation of results, leading only to more inefficacity in the treatment of problems caused by infamous scorpion species.
KeywordsScorpion Dangerous species Misidentification Biased results Buthidae Tityus
Scorpions are a rather depauperate group within the class Arachnida with approximately 2,100 known species. They can be classified among the most ancestral arthropods both in origin and body morphology. They first appeared as aquatic organisms during the Silurian (approximately 450 million years ago – MYA) and underwent rather small morphological changes since . Their apparent conservative form led some authors to define them as ‘living fossils’. However, scorpions most certainly developed major biochemical, physiological, behavioral and ecological adaptations that have combined to ensure their continued success over the past 450 million years.
Even if for natural history experts scorpions can be considered fascinating animals, the interest shown by people in general is only connected with their negative reputation of ‘killers of men’. Nevertheless, only a limited number of species, probably less than 50, are actually responsible for serious or lethal incidents. It is true, however, that the interest on scorpion research in many distinct biological fields was generated by the fact that a number of species possess venoms with potent toxins, capable of killing humans. Most deadly species belong to the family Buthidae; though, species belonging to two other families, Scorpionidae and Hemiscorpiidae, also threaten humans.
The group first appeared as aquatic organisms. In their evolutionary history, scorpions almost certainly evolved from Eurypterida (‘water scorpions’) since both groups share several common morphological features. Marine and amphibious scorpions most certainly persisted well into the Carboniferous (359–299 MYA) and some species probably reached the Permian (299–251 MYA) and Triassic (251-200MYA) periods [2, 3]. The first unequivocally terrestrial (air-breathing) scorpion most certainly appeared on land during the late Devonian (416–359 MYA) or early Carboniferous [1, 4]. For a precise description of scorpion evolutionary history and toxin evolution, readers can refer to Lourenço .
From the beginning of scorpion studies in the late 18th century and during almost 150 years, researchers focused primarily on descriptive taxonomy, general anatomy and very rudimentary biogeography, followed by medical research on venom biochemistry. Since the 1970s, however, basic research on scorpions expanded greatly to encompass behavior, physiology, ecology evolutionary and reproductive biology. Moreover, a large amount of all the research was based on toxin studies and public health problems caused by dangerous species. New approaches were attempted in this last field of research with the use of more precise methods based on a better knowledge of life history strategies of these organisms [6–9].
In the general mass of recent publications devoted to venoms, toxins and public health problems, it is yet possible to observe major gaps between the most up-to-date information on scorpion identification/classification produced by true scorpion experts and the information diffused by researchers who use these organisms in their own research, but are not aware of their precise classification. For this reason, it is suggested here that the frequent cases of misidentification observed in many reports may induce important mistakes in the final interpretation of results, which can only lead to more inefficacy in the treatment of problems caused by dangerous scorpion species. In this article, I attempt to explain and elucidate a number of common problems with clarity, accuracy and unambiguous communication, hopping that in this way the matter may be accessible to a broad audience.
How to approach the identification of scorpions
Techniques for collection
Overturning stones, rocks and logs beneath which many species can be found. This method, although the most widely used, is time and energy consuming, and the results obtained are generally poor.
Pitfall traps may be placed in the soil, when fieldwork is expected to last at least for several days, and preferably for some weeks, in the same site. Results obtained by this method can be very satisfactory if the site chosen contains a large population of scorpions. This method is most successful during dry season, but the traps should be checked daily if adequate results are to be obtained.
Collection at night with the use of a portable ultraviolet light. Because of the presence of riboflavins in their exocuticles, scorpions become fluorescent under ultraviolet light (Fig. 1) and can be easily found . This method is most efficient in open areas such as deserts and savannas; however, it can also be used in rainforests. Generally, results are better in the absence of moonlight. This technique should be used with prudence, since snakes are not fluorescent and can represent a danger in dark nights.
Methods of extraction, such as Berlese, Winkler and Kempson may also be successfully employed to obtain very small species, or juveniles, that inhabit organic soil . These diminutive species, in general with less than 1.5 cm in total length, generally escape from the collection if the other already described methods are used.
Scorpions are often rather rare and the population densities of many species are low. For this reason, it is usually recommended that specimens should be brought alive into the laboratory for study. They can be maintained in boxes with saturated cotton wool in a dish. The bottom of the box must be covered with a layer of soil of about 2–3 cm in depth, preferably from the region where the scorpion was collected. Food such as spiders or insects (but not ants) should be offered once a week. In many cases, reproduction takes place, thereby providing additional material for study. Once specimens have died they should be preserved in 75 % alcohol .
Taxonomic study and identification
The taxonomy of scorpions presents difficulties comparable with those encountered in most other groups of Arthropods. Before beginning the study of scorpions, it is necessary to be able to recognize all of the morphological features used in taxonomy. Recognition of males, females and juveniles is not always a simple task. For many species, males are bigger, with bulkier or longer pedipalps. This is not, however, a general rule, so another important character to be examined is the size and shape of pectines. Almost without exception, pectines in males are bigger, and overlap in their internal side. Recognition of juveniles is almost impossible for non-specialists. Only good samples can authorize the identification of different instars. It is not in the scope of this contribution to bring all the details about the procedures used in the taxonomic study of scorpions. For more details, readers can refer to Lourenço [5, 10, 13, 14].
Most relevant scorpion groups in toxin research and public health programs
The great majority of scorpion species related to public health problems belong to the family Buthidae C. L. Koch, 1837. In most cases, the same species are equally used in venom and toxin research. The reasons for this convergence are associated with both common tradition of studying the most infamous species, but also because these are in general the most readily available. Buthids represent the most important family within the order Scorpiones, comprising about 50 % of all the known species. This family contains also a remarkable number of generic groups, but only a few genera are well known by a broad audience (e.g. scholar doing research on venoms and toxins). In fact, many of the buthid genera are extremely rare. Many genera are monotypic or are composed of only 2 to 3 species that can be extremely rare. Consequently, these will never cross the scientific way of most biologists interested in scorpions as tools.
Most infamous species, largely used in venoms and toxins research, but also concerned by public health programs, are globally confined to a limited number of genera. Nevertheless, a great confusion in their classification can yet prevail and the cases of misidentification are rather common. In the next section, I attempt to bring some light over these groups.
Old World genera
Family Buthidae C. L. Koch, 1837
The genus Buthus Leach, 1815
It is quite clear, however, that even in present days the taxonomy of the genus Buthus remains complex and confusing. In his monograph about North African scorpions, Vachon  attempted to establish a better definition of this genus and transferred to other, recently created at that time, genera several species previously included in it. The classification proposed by Vachon  for the species of Buthus, and in particular for those belonging to the ‘Buthus occitanus’ complex of species, remained, however, unsatisfactory, mainly because of the existence of several poorly defined subspecies and even varieties.
Only after the preliminary revision attempted by Lourenço , a more precise definition of the species of Buthus belonging to the ‘Buthus occitanus’ complex was achieved. This situation was followed by the description of several new species and the promotion of subspecies to species rank. Other contributions have also attempted to clarify the taxonomic status of some of Buthus species associated with Buthus atlantis Pocock [18–20]. The studies on the genus Buthus are maintained in a rather positive pace and many other new species are continuously described [21, 22]. If in the monograph published by Vachon  only four species of Buthus were considered valid, today this number surpasses 30 species and will certainly increase yet.
The species of Buthus are distributed throughout Euro-Mediterranean countries, but most of them are widely found from North to Central Africa. Some species are also present in the nearby Middle East. Although the genus Buthus is not present in Asia, in many publications devoted to venoms and toxins, authors refer to Buthus in India or China. These are clearly cases of misidentifications with the genera Hottentotta Birula, 1908 and Mesobuthus Vachon, 1950 . Incidents caused by some African species of Buthus can be dangerous for young children, but do not reach the same scale of gravity as those caused by other genera of North African buthids.
The genus Androctonus Ehrenberg, 1828
The taxonomy of the genus Androctonus Ehrenberg has long remained complex and unclear, with species misplaced in other genera such as Buthus or Prionurus, 1828, genus now considered as a synonym of Androctonus. Vachon  in his contribution to the study of North African scorpions attempted to establish a better definition of the genus Androctonus and its species. His results were not globally satisfactory, but the number of known species in the genus, between 8 and 10, remained stable until the end of the 20th century, with, however, several poorly defined subspecific populations. After more than half a century of the monographic work of Vachon , Lourenço  attempted again to characterize the distinct populations of Androctonus. It is worth noting that during the entire period between the two contributions, no other authors proposed any major modification on the classification of the species of Androctonus. Contributions as the one by Levy and Amitai , in their Fauna Palaestina, maintained a very conservative position with only minor remarks on some species. With the contribution by Lourenço , a few species have been synonymised, some subspecies raised to the rank of species and new species described. After the publication of this first clarification on the taxonomy of Androctonus, more new species have been added to the genus [27–33]. Presently, the genus Androctonus contains about 20 species, some of which require reinvestigation.
The range of distribution of the genus Androctonus is extremely wide, extending, in Africa, from Senegal to East Africa over a line following the 10° N parallel. In Asia, the genus ranges from the nearby Middle East to India, including also countries such as Armenia . Androctonus includes large scorpion species with extremely toxic venoms to humans and mammals in general, being the first agent responsible for human incidents and scorpion public health threat in most North African and Middle East countries, causing hundreds of casualties every year.
Misidentifications at species levels are yet very frequent in general publications referring to scorpion incidents, but can equally occur in zoological studies. For example, Asian species are cited as African, or African species are cited as Asian. Besides, incorrect identifications are still often attributed to certain populations. Nevertheless, little by little a better understanding of the precise identity and range of distribution of each population is becoming possible thanks also to studies using molecular approaches (Lourenço, not published).
The genus Leiurus Ehrenberg, 1828
Because of its infamous reputation as a very dangerous scorpion, the toxins of L. quinquestriatus and L. hebraeus have been the subject of numerous biochemical studies [39, 40]. The sensory physiology and behavior of Leiurus quinquestriatus have been investigated by a number of authors, including Cloudsley-Thompson [35, 36, 38, 41]. Nevertheless, many aspects of the taxonomy of the genus Leiurus have long been controversial.
A historical account of the taxonomy of the genus Leiurus shows that when the original description of Androctonus (Leiurus) quinquestriatus Ehrenberg, 1828  was provided, this species was originally placed in the genus Androctonus and Leiurus was considered only as a subgenus of Androctonus. This attests the stage of indecision in the classification on the early 19th century. Several subsequent authors, such as Kraepelin , regarded Leiurus as a synonym of the genus Buthus Leach. Finally, Vachon  established Leiurus as a separate genus with only one species, Leiurus quinquestriatus. Vachon  was confident that this genus was monotypic, but refrained from revising its intraspecific structure. Two subspecies were considered to be valid by Vachon : Leiurus quinquestriatus quinquestriatus (Ehrenberg, 1828) and Leiurus quinquestriatus hebraeus (Birula, 1908). Subsequently, the systematic position of Leiurus quinquestriatus hebraeus (now recognized as a valid species) has been reviewed by Levy et al. , who presented tables that differentiate this subspecies from L. q. quinquestriatus.
The position of the two subspecies was considered again by Levy and Amitai . Leiurus quinquestriatus has been also the subject of regional investigations. In his study of the scorpions of Saudi Arabia, Vachon  mentioned Leiurus quinquestriatus without making any reference concerning the subspecies. He also commented on the subspecies Androctonus quinquestriatus brachycentrus (Ehrenberg, 1828), suggesting that more material would be necessary for the variability of coloration of metasomal segment five to be defined. Subsequently, Sissom  also cited Leiurus quinquestriatus in a study of the scorpions of Yemen, but decided not to assign the Yemen population to any particular subspecies.
It was only almost 170 years after the original description of the genus Leiurus that Lourenço et al.  described a new species, Leiurus jordanensis Lourenço, Modry & Amr, 2002 from Jordan. This species was later recorded from Saudi Arabia by Hendrixson . Only a few years after the description of L. jordanensis, Lourenço et al.  described another new species, but this time from Africa. Leiurus savanicola Lourenço, Qi & Cloudsley-Thompson, 2006 was collected is the transitional zone between the Sahel and savanna formations, consequently out of the typical desert formations where Leiurus species are commonly found. The new species was found in a burrow under a rock.
In recent years, some other new species have also been described by different authors. In a rather extensive paper, Lowe et al.  proposed a full revision of the genus Leiurus, but dealing mainly with the populations from the Arabian Peninsula. The status of some old species was revalidated, one recently described species was placed in synonymy, one subspecies was raised to species and four new species were described. This raised the total number of species in the genus Leiurus to ten. The characters used by these authors to define the species, as well as the proposed dichotomic key are rather difficult to be used. Nevertheless, one has to agree when these authors stated that ‘like many other scorpion genera, Leiurus is comprised of an assemblage of allopatric or parapatric species spread across different regions separated by physiographic barriers, each adapted to local environments and substrates’. In a very recent publication by Lourenço & Rossi , this assumption was once again confirmed, attesting that the African species of Leiurus are yet neglected and require intensive further studies. A new species from Somalia, Leiurus somalicus Lourenço & Rossi, 2016 was described, raising the number of Leiurus species to eleven.
The geographical distribution of the genus Leiurus can be summarized as follows: the range of distribution covers Algeria, Chad, Egypt, Ethiopia, Libya, Mali, Niger, Somalia, Sudan and Tunisia in Africa; and Sinai, Iraq, Israel, Jordan, Kuwait, Lebanon, Oman, Qatar, Saudi Arabia, Syria, Turkey, United Arab Emirates and Yemen in Asia. Most African populations certainly correspond to the species L. quinquestriatus, whereas those from Asia correspond to L. hebraeus, and in small portions to one of the several new species described in the last 15 years. The Isthmus of Suez apparently corresponds to the border between the African species of Leiurus and those distributed in the Middle East [26, 37]. The species Leiurus savanicola, only known from the Sahel in Cameroon, and Leiurus somalicus, found only in the south of Somalia, represent the most southern records on the distribution of the entire genus Leiurus.
Other genera also involved in incidents and misidentifications: Hottentotta Birula, 1908, Mesobuthus Vachon, 1950, Odontobuthus Vachon, 1950 and Parabuthus Pocock, 1890
The precise composition of several of these genera and in particular of the four previously cited, which are frequently misidentified with Buthus, is not completely clarified. Besides, the unclear position of both Hottentotta and Mesobuthus species distributed in Asia creates a great confusion in their identification. It seems clear that the genera Hottentotta and Mesobuthus are far from being uniform, and the used diagnostic characters are very often imprecise. Naturally, the exact clarification of most taxonomic situations is a matter for true experts, nevertheless it can retained that the genus Buthus has a distribution in Africa limited to the regions of the Sahara and Sahel. Its presence in the Middle East is rather discrete, with a distribution to East limited to Iraq . Any citation of Buthus to Central and Eastern Asia has to be considered as misidentification with the genera Mesobuthus or Hottentotta .
Family Scorpionidae Latreille, 1802
The genus Scorpio Linnaeus, 1758
The initial decision taken by Lourenço  of raising several forms or subspecies of Scorpio to the rank of species was received with scepticism by a number of authors (several reactions in litt.). Nevertheless, in recent publication Talal et al. , using more sophisticated molecular methods, followed the same analysis, previously started by Lourenço , and concluded that the raising of some Middle East subspecies of Scorpio to the rank of species was necessary. Previous analysis on some Scorpio species by Lourenço and Cloudsley-Thompson  suggested that those characterized by a pale coloration – e. g. Scorpio tunetanus Birula, 1910 (now Scorpio punicus Fet, 2000); Scorpio occidentalis Werner, 1936; Scorpio savanicola Lourenço, 2009; and Scorpio niger Lourenço & Cloudsley-Thompson, 2012 – most certainly corresponded to possible members of a single group that originated from a common ancestor, but today they occupy distinct regions of distribution. This position was corroborated by the recent discovery and description of S. tassili by Lourenço & Rossi . The range of distribution of S. punicus appears to be limited to the high plateaus of Tunisia and north of Algeria [16, 61], whereas the other species of this group are distributed much further to the south, in the Sahel region.
The precise taxonomic situation of all Scorpio populations is not yet fully clarified. The species of this genus have a widespread distribution, ranging from Senegal to Sudan and Egypt in Africa and covering most regions of Middle East.
Family Hemiscorpiidae Pocock, 1893
The genus Hemiscorpius Peters, 1861
Besides the presence of the family Hemiscorpiidae in the Middle East, some species are also found in Eastern Africa, Somalia and Egypt [66, 67]. Nevertheless, the Iranian Hemiscorpius of the H. lepturus group are very divergent from the African species of this genus. The African species do not show the highly derived features presented by the Iranian one, such as a conspicuous sexual dimorphism and highly cytotoxic venom .
New World genera
Family Buthidae C. L. Koch, 1837
The genus Centruroides Marx 1890
The center of distribution of Centruroides is probably Central America . The species of this genus are distributed in a great array of distinct habitats, ranging from arid deserts to rainforests. Several species of Centruroides, in particular among those distributed in Mexico, possess venoms potentially lethal to humans and constitute a very important health problem. In fact, the number of incidents by scorpion stings in Mexico is from far the most important in the world. The total number of incidents is largely more important than those of any other region concerned by scorpion incidents in the world. A few fossil registrations from Miocene amber attest that this group was already present in the region since the middle of Cenozoic times. This group of scorpions deserves to be treated in detail, but this is out of the scope of the present contribution.
The genus Rhopalurus Thorell, 1876
The biogeographic pattern presented by the genus Rhopalurus constitutes one good example of a group showing a discontinuous distribution. This type of pattern can be observed in two different cases, among scorpions exclusively adapted to savannas or to rainforests. These examples have an important relationship with species endemic to islands of savanna in Amazonian and Guayanian enclaves , but also to those isolated in forest islands inside open vegetation formations (Caatinga, Cerrado and Campos). The endemic populations isolated inside savanna islands provide good evidence to support the hypothesis of past connections between the savannas of Central Brazil and the savanna enclaves in Amazon and Guayana regions. When forest cover was reduced, open vegetation formations probably coalesced during past dry periods which took place at the end of Cenozoic and during Pleistocene times [77, 78].
Scorpion patterns of distribution represent good examples that support this hypothesis. Rhopalurus most certainly exhibited a continuous distribution during Pleistocene dry periods and the current disrupted distribution is a possible consequence of the reestablishment of rainforest over the regions which previously served as corridors [70, 72, 79]. Evidence for this presumed palaeodistribution was provided with the discovery and description of the species Rhopalurus amazonicus Lourenço, 1986 endemic to a savanna enclave in the region of Alter do Chão (state of Pará, Brazil) and completely isolated within oriental Amazon forest . A careful analysis of Rhopalurus amazonicus placed it in a position close to Rhopalurus acromelas Lutz & Mello, 1922 that is found in central Brazil, but has a range of distribution nearby the area of transition between Cerrados and Amazon forest.
Since the revision of the genus by Lourenço , followed by the reanalysis of its distribution , very few new species have been described for South America. Nevertheless, all new discoveries confirmed the biogeographic pattern of distribution already observed for the genus.
The taxonomic status of the species distributed in the northern South American savanna formations always proved to be difficult to be defined. This situation is associated with the fact that these savannas are composed of several isolated fragments that probably coalesced during past dry periods. The most recent of these events can be dated only from 18.000 to 13.000 years before present (BP) . This very recent isolation led to a minor process of speciation and differentiation and, as consequence, the populations now found in several isolate fragments of savannas show very little morphological differences. In face of the observed patterns of distribution and differentiation, it becomes rather difficult to be sure about the true taxonomic status of some isolate populations. Consequently, one question can be addressed: are these populations true species, subspecies or simply local morphs belonging to large polymorphic populations?
The recent analysis of the species of Rhopalurus distributed in the savannas of northern South American clearly showed the existence of two distinct lineages . One represented by R. laticauda Thorell, 1876 with three associated forms (species or subspecies), and the other represented by R. pintoi Mello-Leitão, 1932 with possibly two associated forms. In both cases, the forms within each of the two lineages present only weak morphological differences, demonstrating a recent and only minor process of differentiation. This pattern of both distribution and differentiation is directly associated with more or less recent palaeoclimatic vicissitudes that took place in tropical South America during the Pleistocene [77, 78]. During dry periods, most of the South American savannas and/or open vegetation formations coalesced to form continuous areas of distribution for species exclusively adapted to these formations such as those belonging to the genus Rhopalurus. During wets periods, such as the present one, these same savanna formations undergo a process of fragmentation with a subsequent isolation of small populations.
The consequences on the process of speciation during the subsequent wet/dry/wet periods is difficult to measure, but probably was rather weak on groups such as scorpions with long term reproduction process and a low number of generations when compared to other zoological groups such as insects [79, 82]. It is therefore rather difficult to assign a precise status to the forms within the two well-established lineages. Are we in face of species, subspecies or only morphs of a large polymorphic species? If, at least for some of these forms, their specific condition could be demonstrated in association with a clear allopatric distribution of the populations, than the condition of superspecies sensu Mayr  could be applied to each of the two lineages. The species within each lineage would be represented by allopatric, parapatric our weakly sympatric groups, really or potentially intersterile in nature . Each of these species could than be defined as a prospecies in the sense of Birula .
The genus Tityus C. L. Koch, 1836
The genus Tityus was first created by C. L. Koch in 1836  having as type species, by monotypy, Scorpio bahiensis Perty, 1833 [= Tityus bahiensis (Perty, 1833)]. This means that when Perty , in 1833, described the species Scorpio bahiensis, he placed it in the old genus Scorpio Linnaeus, 1758. The type locality indicated by Perty  for Scorpio bahiensis [= Tityus bahiensis (Perty, 1833)] was “Habitat Prope Bahiam”, presumably in Brazil since the material he studied was originally collected during the early 19th century expedition performed in this country by J. B. von Spix and C. F. Ph. von Martius. This locality remains, however, extremely imprecise .
Since the creation of the genus Tityus, a large number of new described species were placed in it. Consequently, very early revisions concerning the status of the different species were required. A pioneer work is the one produced by Kraepelin  in 1899, in his Das Tierreich. His work was important because it was done in the context of a world revision of all scorpions known to that date. Although this contribution was extremely relevant at that time, covering all the 34 known species of Tityus, it could not be final. In fact, for several of these species Kraepelin  could only count on very limited samples what invalidated any analysis of the stability or variability of the taxonomic characters used by him.
Subsequently to Kraepelin’s  contribution, new species and subspecies were described in an important pace, leading to the need of new revisions. Besides, having into account the increasing complexity of the genus Tityus, several authors suggested the need to divide it in several groups of species, more or less ‘natural’. In 1911, Kraepelin  had already stated that the genus Tityus was the most complex among the American scorpions, because of its large number of species and the difficulties to find useful characters allowing the definition of natural groups. He attempted himself to create three groups of species using the pigmentation patterns: species of large size with blackish-brown pigmentation (group cambridgei), species of moderate size with dark longitudinal stripes (group bolivianus) and species of small size with a variegated pigmentation (group columbianus). Although he had observed a degree of variation in the pigmentation patterns and reached to the conclusion that this character was not entirely satisfactory, he stated, at the same time, that he could not find other characters!
Following the work of Kraepelin, other authors attempted to revise the genus Tityus. In 1924, Mello-Campos  proposed an identification key to the species, without, however, any division of the genus in natural groups of species. More solid attempts to divide the genus into groups of species finally came in the publications by Mello-Leitão [93, 94] and particularly in his monographic work on all South American scorpions published in 1945. In this study, Mello-Leitão  offered a synthesis of all his previous results and divided the genus Tityus into 15 ‘Formenkreise’ (what literally means ‘circles of morphs’ or natural groups). After 1945 and until 1975, a number of papers on the genus Tityus have been published, but no changes to this classification was proposed.
The matter concerning groups of species – or sometimes complexes – within Tityus restarted to be discussed by the end of the 1970s, in particular by Lourenço [96–100], Lourenço and Maury , and Lourenço and Eickestedt . Nevertheless, any attempt to proceed to a global revision of the genus proved to be extremely complicate, mainly because of the constant increase in the number of new species (e.g. Lourenço [103–105]). A major attention was than dedicated to the interpretation of the observed patterns of distribution and differentiation of several groups of species [106–109]. At the same time, the number of new described species was constantly growing (e.g. Gonzales-Sponga [110–114], Lourenço and Ythier ). Consequently, the number of described species of Tityus nowadays overpasses 220 and it is far from being final.
By the early 2000s, the issue of the division of the genus Tityus into groups of species was debated again. To a certain degree, Lourenço  restored the propositions made by Kraepelin  by retaining three major groups of species, based mainly on pigmentation patterns, but also some morphological characteristics. Each of these groups was named after the oldest species belonging to them, reaching the following results: species group Tityus clathratus, species group Tityus bahiensis and species group Tityus asthenes. Two other new insights were equally proposed in this period. The first was the description of a new genus Caribetityus Lourenço for the Tityus species found in the Greater Antilles ; a decision not always accepted by other authors . The second one was the proposition of a new species group for some particular species of Tityus found in the Amazon canopy .
In the face of all these novelties and polemics among several authors, a division of the genus Tityus in several subgenera was finally decided . The objective was to bring further stabilization to the classification of the group. The genus Tityus was than divided into five subgenera. Three of them were based on already available genus-group names. Two new subgenera were created: Archaeotityus Lourenço to accommodate the species previously associated to the Tityus clathratus group, and Brazilotityus Lourenço in which Amazonian canopy species were accommodated. The composition of the genus Tityus in five subgenera remains as follow: Archaeotityus Lourenço, 2006 comprising the Tityus clathratus group of species. Tityus C. L. Koch, 1836 comprising the Tityus bahiensis group of species. Atreus Gervais, 1843 comprising the Tityus asthenes group of species. Caribetityus Lourenço, 1999 comprising several Tityus species of the Greater Antilles. Brazilotityus Lourenço, 2006 comprising the Tityus adisi group of species. These decisions concerning the composition of the genus Tityus are still a matter of polemics among certain authors and are rejected by others. Nevertheless, in account of the complexity of this group of scorpions, this decision appeared as fundamental.
Among the enormous array of known Tityus species, only a minor number is generally the subject of attention from people managing problems of scorpionism. In the particular case of Brazil, only two/three species of Tityus call the attention of professionals working on public health problems. Many subjective arguments are often proposed to explain the causes and dimension of scorpionism; however, precise ecological aspects are frequently neglected. It is quite true that ecological studies on Brazilian and South American scorpions are extremely scarce. Ecological studies using scorpions as models only began in recent years, and most studies were carried out on species from North American deserts . Deserts are globally open formations and this makes scorpion observation rather easy during the night, with the use of ultra-violet lamps. Ecological observations in savannas and in particular rainforest formations, which are common in South America, are more complicate, limiting therefore the number of studies. The combination of these factors has contributed to the present situation regarding ecological research on South American Tityus.
However, there are a few exceptions. Ecological studies have been conducted on species inhabiting the Cerrado; one example is Tityus fasciolatus Pessôa, 1935, and some data is now available for species living in modified environments such as Tityus serrulatus Lutz & Mello, 1922 and Tityus bahiensis Perty . Results on the ecology of scorpions should always be interpreted in the light of evolutionary ecology. This approach can have an important impact on studies of biogeography, but also in the understanding of the evolution of the phenomenon of scorpionism. According to the model proposed by Pianka , scorpions should be one of the exceptions to the dichotomy proposed by him. Most of the traits of scorpions, with their K-selected strategies, resemble those of vertebrates rather than those of invertebrates . Even if some scorpions are opportunistic species, the majority can be defined as equilibrium species. Consequently, it is reasonable to accept the existence of different evolutionary gradients inside scorpion species in general.
Environmental conditions, stable and predictable or unstable and unpredictable, play a major role in evolution of life history strategies. Some precise cases of secondary succession among Tityus populations in South America and Brazil in particular can be suggested. These cases must be considered both in relation to historical factors, such as palaeoclimatic fluctuations, and to ecological factors, such as natural disasters or human activity on the environment. South America and Brazil are favourable regions for this type of discussions since several Tityus populations are presently well known and data on historical events that took place during Cenozoic and Pleistocene are rather abundant [79, 82, 107, 121].
Density of scorpion populations are extremely variable, but most species maintain populations with very low densities. Exceptions are found in some desert species, and among some populations that live in disturbed environments where the niche left vacant by equilibrium species has been entirely occupied by one or two opportunistic species . In this last case, populations may present demographic ‘explosions’ that are characteristic of species with density independent regulations. This phenomenon can be observed in some Tityus populations, in particular in Brazil. Tityus serrulatus and even Tityus bahiensis have invaded many towns and cities in the southeast and central regions of the country. In these regions, collection of several thousands of scorpions can be achieved in only a few days.
The patterns of distribution presented by the natural populations of Tityus species in South America are complex, but seem to be globally correlated to the major natural morphoclimatical regions of the continent. As already outlined by Lourenço , a correct biogeographical view of any region, especially at the continental scale, requires a precise knowledge of its morphoclimatical subdivisions, i.e. the sub-regions where specific conditions of climate and soil can be defined, having as consequence a particular type of vegetation. The heterogeneity of tropical South American vegetation can be described by major type formations based mainly on physiognomic and ecological conditions of climate and soil as discussed by Eiten  and Ab’Saber . In the particular case of the species of Tityus, three major vegetation formations are recognized. First, the rainforests or wet forests (e.g., the Amazon region, Guayana forest, Pacific Colombian-Ecuadorian forest, Brazilian Atlantic forest), where yearly rainfall varies from 1500 mm to 4000 mm as in Amazonia and Guayana, rising to 9000 mm in the Pacific Colombian-Ecuadorien forest. Secondly, open vegetations, such as dry formations in Chaco and Brazilian northeastern Caatinga, where the yearly average rainfall is 300 to 1000 mm with 7 to 10 months of strong dry season. Finally, non-xerophytic formations as Cerrados in central Brazil, Gran Sabana and Llanos, where the average temperature is 20-26 °C, with a few frosts in winter occurring at the southern edge, and with a average yearly rainfall of 750 to 2000 mm with a moderately strong dry season of 4 to 5 months. Transitional regions (e.g., Chaco/Amazon and Cerrados/Amazon) where intermediate climate conditions can be observed . These major types of vegetation are not uniform and several gradients may occur within each. In fact, the very important diversity observed in tropical South America can be explained, in a large proportion, by this very important heterogeneity.
Corridor A is typically transandine and extends from southern regions of South America (Chile and Argentina) up to the Panama in the north. A zone of transition (or rupture) of some species is observed on the boundary between Peru and Ecuador. This biogeographical barrier is probably associated with the transition from the Pacific coastal rainforest to the Pacific coastal desert. Most typical elements of the transandine region belong to non-buthid familles. The species of Tityus (Buthidae) are globally present in the northern part of this region [105, 125].
Corridor B is represented by open vegetation formations that occupy the diagonal from the Monte of Argentina to the Brazilian Caatingas. Typical elements belonging to buthids and non-buthids are present in this corridor. The center of origin for the elements of the genus Tityus could be in Brazilian Highlands since their possible centers of dispersion may correspond to the Brazilian shield .
Two species of Tityus, with phylogenetic relationship, can show quite different ecological requirements. These are the cases of Tityus fasciolatus from central Brazil and Tityus bahiensis distributed from southeastern Brazil to Paraguay and north of Argentina. The savannicolous scorpion Tityus fasciolatus is a typical equilibrium species that inhabits a stable and predictable environment. The savannas of the Brazilian Highlands are metaclimax formations that are both stable and predictable. These savanna formations are marked by two major seasons: the wet rainy season, from September to April, and a very dry season that lasts from May to August. Tityus fasciolatus is termitophilous, sheltering exclusively in the mounds of Armitermes euamignathus Silvestri. Its gestation period ranges from 2.5 to 3.5 months and, as in other Tityus species, females require only one insemination to produce multiple broods (1 to 5) during the more favourable wet season. As soon as the dry season begins, inseminated females develop reproductive diapause and gestation is blocked until the return of the next wet season, six months later. Diapause is characteristic of equilibrium species. By avoiding parturition during the dry, unfavourable season, immature forms are spared two selective ecological pressures: prey scarcity and the action of savannas fires that are quite common during the dry season. Unlike adults, which shelter exclusively in termite mounds, immature T. fasciolatus are errant and often hide under bark. Therefore, they are more exposed to fire [129, 130].
A sibling species of T. fasciolatus, Tityus charreyroni Vellard, 1932 is equally a savannicolous element but both species occupy different and quite specific microhabitats in the savannas. T. fasciolatus is an exclusively termitophilous scorpion while T. charreyroni lives under stones in a region western of that occupied by T. fasciolatus. The area and range of distribution of each population depends upon the existence and distribution of their specific microhabitat [130, 131]. The construction of termite mounds is associated with particular grasses, which constitutes the specific food for termites . Variation in the soil gradient leads to variation of the covering vegetation. When grasses are no longer present, both termite mounds and T. fasciolatus disappear. Moreover, in areas where termite mounds are common, stones are rare, thus making the habitat unfavourable for T. charreyroni.
The ecological requirements of Tityus bahiensis are much less specific. This species most certainly also evolved in primary savanna formations, but these original savannas inhabited by T. bahiensis were destroyed or modified by human activities very early in the history of South America colonization. This species was, however, capable of a considerable degree of adaptation to such modified zones and is found today inside cities and small towns where it colonizes parks, gardens and cemeteries. In contrast, T. fasciolatus knew an important regression in the original area of distribution when this one also suffered with human activities. The great ecological plasticity and capacity of colonization of T. bahiensis is, though, less marked than that of other Tityus species such as Tityus serrulatus.
Tityus serrulatus Lutz and Mello, 1922 is well known as the most infamous Tityus species in Brazil and certainly also among all the known species of Tityus. The species was described only in 1922 by Lutz and Mello  in a short note devoid of any illustrations. The authors indicated only Belo Horizonte as the type locality and, by that time, it seems that the species was a common one in urban environments. Today, the Brazilian yellow scorpion is well known, even by a large public, and its geographic distribution has expanded considerably since its description. For further details on the history of this species, readers may refer to Lourenço . It is also of general consensus that this species poses exceptional risks to public health due to its rapid expansion in urban areas, rapid proliferation and great toxicity. In fact, all these ecological and biological parameters are linked to its unique modality of reproduction by parthenogenesis. Actually, one point remained enigmatic during several years: the absence of males from all known populations. This question was finally answered by Matthiesen , who first demonstrated that this species could reproduce by parthenogenesis. This phenomenon was later demonstrated in a few other species of scorpions, but remains rare . Tityus serrulatus was usually considered an obligate parthenogenetic species, but some isolated bisexual populations have finally been detected [136, 137]. However, bisexual populations appear to be extremely marginal within the present geographic distribution of the species, suggesting a possible elimination from most, if not all, of the unpredictable environments occupied by T. serrulatus [6, 9].
Several authors have noted that parthenogenetic animals have a tendency to appear in environments that are different from those inhabited by their bisexual relatives [138–142]. Parthenogenetic species also possess higher colonizing and dispersal potential than sexual species and Tityus serrulatus conforms to this prediction. Only a few hundreds years ago, this species probably occupied a restricted area in the state of Minas Gerais, but is now widely distributed over a large region of south-eastern and central Brazil and can also reach Argentina and Bolivia via Paraguay and the Brazilian state of Rondonia [7, 9]. Its geographical expansion is undoubtedly due to its introduction into newly created cities and towns by human agency. Newly established human communities may, therefore, be invaded within a few years after their foundation, although the surrounding natural areas (savannas/Cerrados) may be virtually devoid of such species.
The creation of new habitats suitable for colonization by T. serrulatus may be compared with natural clearings in dense primary forest . The Cerrados of the Brazilian Highlands may represent such forests and the new towns may be comparable to the clearings. In both cases, the new environments represent insular-type habitats that are now known to favor the establishment of parthenogenetic populations . The new towns are, in many cases, separated by several hundred kilometres. Consequently, the area between them remains almost pristine, representing a formidable barrier to colonization. When parthenogenetic scorpions are transported by anthropogenic agents, such as road or rail, the process of colonization is greatly accelerated. It is greatly facilitated by the higher prolificacy and superior colonizing ability inherent in the parthenogenetic mode of reproduction, which favors the colonization of remote and unoccupied territory . However, such territory does not need to be different, or a disclimax, as is currently believed by many authors; it should be merely unoccupied . In the case of parthenogenetic cave crickets, Hubble and Norton  found that the environments of the sexual and unisexual forms are equable, humid, undisturbed and highly predictable. The essential factor for the establishment of parthenogenesis in this case is that remote and uninhabited caves must be colonized by sweepstake dispersal in which the chances for success are very small for any individual and overwhelmingly against the simultaneous arrival of both sexes.
Polymorphic species within the genus Tityus
The already discussed species Tityus bahiensis provides a good example of a clinal polymorphic species with a very regular gradient. This was first interpreted as a polytypic situation by Lourenço , with populations of species and subspecies. Such situation also illustrates the difficulties of clearly defining the biogeographic state of a given population. T. bahiensis is very common in the southeastern region of Brazil and represents an opportunistic species often living in unpredictable disclimax environments. Differences in the patterns of body pigmentation of individuals belonging to what appeared to be two different allopatric populations permitted the recognition of two different species : T. bahiensis with a meridional distribution and T. eickstedtae Lourenço, 1982 with a septentrional distribution. Subsequently, additional data from the zone of contact between the two supposed distinct populations indicated what could be interpreted as hybrid forms. More detailed studies, however, suggested the existence of a single polytypic species with two subspecies (and two subpopulations), one in the south (T. bahiensis) and the second in the north (T. bahiensis eickstedtae) . Finally, studies performed on material from additional localities showed that T. bahiensis is clearly a polymorphic species, as are several others in the genus Tityus.
Tityus costatus was described by Karsch , based on specimens collected in the state of Rio de Janeiro. This species has not an infamous reputation, although it may be responsible for a number of incidents in the coastal regions of Brazil. These incidents are certainly misidentified in several cases and attributed to T. bahiensis or T. serrulatus. Even though not much is known about the venom or toxins produced by this species, Bücherl and Pucca , in an already old publication, suggested that it could have a strong activity on mice.
Tityus costatus is an autochtonous element of the Brazilian Atlantic forest, but yet rather poorly known by non-experts. For a long time the species remained poorly characterized. Lourenço  stated that it was difficult to distinguish T. costatus from Tityus trivittatus dorsomaculatus Lutz & Mello. Studies of further material from several localities in the Atlantic forest, from the states of Espírito Santo to Rio Grande do Sul, established the existence of a mosaic pattern of polymorphism in Tityus costatus . The two nominal species are only different morphs of one single species. T. costatus is defined as a maculata form and T. dorsomaculatus as a trifasciata form. The considerable variability observed for T. costatus concerns mainly the pigmentation patterns observed. Two major patterns are observed and related to the maculata and trifasciata forms with, however, an important gradient of intermediate forms between these two extremes. Research on the variability of differentiation patterns showed that no correlations could be drawn between latitudinal gradient and the pattern of pigmentation . This type of complex pattern has previously been described by Mello  for a freshwater crab, Trichodactylus fluviatilis Latreille.
Further analysis of environmental factors and variable pigmentation patterns suggests that the maculata form of T. costatus occurs in sites ranging from sea level (very light form) to 1000 m (much darker). The trifasciata form was found at sites above 1000 m. The relief of this part of the Atlantic forest is very irregular; changes from sea level to more than 2000 m can occur over distances of less than 30 to 40 km. Consequently, it was suggested that T. costatus should be defined as a mosaic type polymorphic species whose pigmentation pattern changes with altitude. The actual environmental factor responsible for this variability seems to be the temperature. A similar example has been described in the lizard Amphisbaena alba .
The most complexes models of polymorphism observed for species of Tityus concerned those distributed in the Amazon and Guayana regions. Most certainly, other examples are present also for other morphoclimatical regions in South America, but remain not studied. Nevertheless, most of the well-studied populations of Amazonian scorpions are monomorphic species. Examples include Tityus obscurus (Gervais, 1843); T. metuendus Pocock, 1897; Tityus dinizi Lourenço, 1997; Tityus tucurui Lourenço, 1988; and Tityus neblina Lourenço, 2008. Monomorphic species either exhibit average ranges of distribution or local endemism [106, 123].
Prance  postulates that the reduction of forest to small patches stimulated many changes in plant and animal populations in the refugia, ranging from extinction to an increased rate of speciation. More recent evidence for the refuge theory comes from biogeographical studies of scorpions in tropical South America [79, 106, 107]. These investigations led to the postulation of about 25 refugia or areas of high endemism (Fig. 28). The number, size and location of these refuges correlate well with those described by Prance  from his studies of woody plant biogeography, and studies by Haffer [156, 157] and Brown  on birds and butterflies, respectively, to identify regions of high endemism. Together, these studies provide a basis to identify and measure the most important centers of species diversity in South American tropical rainforests (Fig. 28).
The first objective of this article was to bring general information about scorpions of medical importance, addressed to non-specialist people whose research embraces this group. The idea is to demonstrate that the group diversity and patterns of distribution are much more complex than it seems at first sight, in particular to those having access to a limited number of species. If the group diversity is important, the same should be applied to the diversity of toxins. Today, only a very limited number of distinct taxa retain the attention of toxin experts, but maybe this overview about scorpions may encourage the interest of researchers on biochemistry and molecular biology of venom toxins to expand their research to a broader array of scorpion groups, in particular for those that can be informative on the evolution of complex venoms.
It seems obvious that the taxonomy and classification of scorpions is far from being a simple task. In fact, this classification is permanently changing and this situation brings an unconfortable situation for people who use these organisms in their research. For instance, certain toxins are presently associated with species names that no longer exist in the zoological literature; one example is the ‘Tityus cambridgei toxin’, which has no longer a sense since the correct name for this species is now Tityus obscurus .
Nevertheless, people using scorpions in their research should be aware of more precise identifications of the species they are using in their research. In the previous sections, I exposed the quite many situations of mix-up that can be found in an important number of scientific publications. The progress observed in the taxonomy of scorpions, specially the discovery and description of a remarkable number of new species, in the last 15 to 20 created a totally new scorpiological panorama. These new descriptions concern all the known groups of scorpions, but have a conspicuous incidence on the groups presenting medical importance, such as the genera Androctonus, Buthus, Centruroides, Leiurus and Tityus.
In face of all these taxonomic difficulties, the best avenues should be more exchanges between people using scorpions in their research and true experts. Naturally, the number of true experts is yet very limited, but more and more young people are now being trained in scorpion taxonomy. Some previous examples of cooperation do exist in the literature and have proven their efficacity .
I am most grateful to the staff of JVATiT for their invitation to contribute to the journal. Thanks are also due to Lucienne Wilmé (Missouri Botanical Garden, Madagascar) and to Elise-Anne Leguin, Museum (Paris) for the preparation of several maps; and to Tiago J. Porto (Universidade Federal da Bahia, Salvador), Eric Ythier (SyntechResearch, France) and Philippe Geniez (France) for the permission to use some of their scorpion photos.
The author declares that there are no competing interests.
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