Homology of the Lateral Eyes of Scorpiones: A Six-Ocellus Model

Scorpions possess two types of visual organs, the median and lateral eyes. Both eyes consist of simple ocelli with biconvex lenses that differ in structure and function. There is little variation in the number of median ocelli across the order. Except for a few troglomorphic species in which the median ocelli are absent, all scorpions possess a single pair. In contrast, the number of pairs of lateral ocelli varies from zero to five across Scorpiones and may vary within species. No attempt has been made to homologize lateral ocelli across the order, and their utility in scorpion systematics has been questioned, due to the variation in number. A recent study examined the number of lateral ocelli among various Asian Buthidae C.L. Koch, 1837 and proposed a “five-eye model” for the family. This model has not been examined more broadly within Buthidae, however, nor compared with the patterns of variation observed among other scorpion families. An eyespot, referred to as an accessory lateral eye, situated ventral or posteroventral to the lateral ocelli, has also been reported in some scorpions. Analysis of its structure suggests it serves a nonvisual function. We present the first comparative study of variation in the lateral ocelli across the order Scorpiones, based on examination of a broad range of exemplar species, representing all families, 160 genera (78%), 196 species (9%), and up to 12 individuals per species. We propose a six-ocellus model for Recent scorpions with four accessory ocelli observed in various taxa, homologize the individual ocelli, and correct erroneous counts in the recent literature. We also investigate the presence of the eyespot across scorpions and discover that it is more widespread than previously recognized. Future work should investigate the genetic and developmental mechanisms underlying the formation of the lateral ocelli to test the hypotheses proposed here.


Introduction
Scorpions possess two types of visual organs, the median and lateral eyes. The median eyes comprise a single pair of simple ocelli with biconvex lenses, situated dorsomedially on the carapace. All scorpions possess median ocelli, except for 26 troglomorphic species [1][2][3][4][5] (Table 1). The lateral eyes, also known as aggregate or schizochroal eyes, comprise multiple pairs of ocelli with biconvex lenses, located anterolaterally on either side of the carapace [6]. The number of pairs of lateral ocelli ranges from zero to five across the order [1-4, 7, 8]. Seventeen troglomorphic species lack lateral ocelli altogether [4,5,9,10] (Table 1) whereas many Buthidae C.L. Koch, 1837 possess up to five pairs [11]. The number of pairs may vary within some species [12,13].
The median and lateral eyes of scorpions differ structurally. The dioptric apparatus of the median eyes comprises a cuticular lens, preretinal membrane, hypodermis, vitreous body, retina, postretinal membrane, and a layer of pigment cells surrounding the retina, whereas the lateral eyes possess a cuticular lens, preretinal membrane, hypodermis, retina, layer of pigment cells and postretinal membrane, without a vitreous body and focusing lens [1,7,14,15]. Other differences exist at the cellular level. The retinula cells of the lateral eyes do not form discrete units as in the median eyes, and their rhabdomeres form a contiguous rhabdom suggesting continuous signaling among cells [1,7,15]. Assembly of the optic nerve occurs inside the retina of the median eye, whereas assembly occurs outside the retina of the lateral eye. The median eyes also possess numerous (three to six) neurosecretory fibers per retinula cell whereas the lateral eyes possess approximately one per retinula cell [15].
The structural differences between the median and lateral eyes imply different functionality. Whereas the median eyes are used for acuity and spatial discrimination, the lateral eyes have poor visual acuity [1,7,15,16]. Experimental studies on Androctonus australis (Linnaeus, 1758) suggest the lateral eyes function primarily as light detectors, perceiving differences in brightness at very low intensities, ten log units more than the median eyes [15,16]. Therefore, lateral eyes probably serve to detect Zeitgeber stimuli for regulation of the circadian rhythm [16][17][18].
The number of pairs of ocelli in the lateral eyes of scorpions has decreased since Paleozoic times [12]. Early fossil scorpions possessed compound or holochroal lateral eyes comprising many ommatidia sharing a common cornea [6]. Up to 1000 ommatidia were present in Proscorpius osborni (Whitfield, 1885) from the Silurian [19]. The compound lateral eyes of scorpions from later in the fossil record are intermediate between truly compound and aggregate forms. The Carboniferous Eoscorpius distinctus Petrunkevitch, 1949 possessed roughly 100 ommatidia, each separated from the others by sclerotization, which is characteristic of an aggregate eye [6]. More recent fossils, such as Compsoscorpius Petrunkevitch, 1999 and Palaeopisthacanthus Petrunkevitch, 1913 from the Carboniferous [12], possessed only three pairs of lateral ocelli [20][21][22] like living scorpions. Such variation among extinct scorpions suggests that compound lateral eyes evolved into the aggregate eyes observed in modern scorpions by progressive loss of ocelli [6,12,[23][24][25].
Embryological data suggest that the lateral eyes develop as the hypodermis thickens, from depressions which give rise to the ocelli. In one of few studies on the development of lateral eyes in scorpions, five depressions were described in the buthid, Centruroides Marx, 1890, four in a ventral plane and one in a dorsal plane [26]. The two medial depressions were the largest in the ventral plane, whereas the posterior-most depression was the smallest. The single depression in the dorsal plane was situated between the two large depressions of the ventral plane. All depressions except the posterior-most depression in the ventral plane, developed into ocelli.
An eyespot, situated posteroventral or ventral to the lateral ocelli, was reported in the buthid, Parabuthus transvaalicus Purcell, 1899 [27], and seven species of chaerilid, Chaerilus Simon, 1877 [28][29][30]. A transmission electron micrograph analysis of the eyespot of juvenile P. transvaalicus indicated that it is structurally similar to the lateral ocelli, possessing photoreceptor cells, arhabdomeric cells and efferent neurosecretory fibers, but lacking a dioptric apparatus and pigment granules. Therefore, although the eyespot is sensitive to light, it appears to function primarily for nonvisual information processing [27]. Based on similar structure and position, the eyespot of scorpions appears to be homologous to the rudimentary 'accessory eye' of the horseshoe crab, Limulus polyphemus Linnaeus, 1758 [7], and, presumably for this reason, was also referred to as an accessory lateral eye [27,31]. We prefer the term 'eyespot' to 'accessory lateral eye', however, due to the differences in structure and function between this visual organ and the ocelli of scorpions. Below, we use 'accessory' strictly to refer to eyes in addition to the normal complement, that are rarely observed, following a tradition in use of this term in scorpion systematics, but see Melzer [31] for another opinion.
The number of pairs of lateral ocelli has long been used in scorpion systematics, to diagnose families and genera (Table 2), and more recently as characters in phylogenetic analyses [32,33], although the utility of this character has been questioned. For example, two pairs of lateral ocelli was long considered diagnostic for Chactidae Pocock, 1893, whereas three pairs was diagnostic for Vaejovidae Thorell, 1876, despite knowledge of chactids without two pairs of lateral ocelli and vaejovids without three pairs [12,13,[34][35][36][37]. The taxonomic distribution of the apomorphic states of this character prompted Stockwell [22] to dismiss it as ''nearly useless at the family level'' and a similar argument was advanced by Lourenço [38]. However, Prendini [39] demonstrated synapomorphies in the number of lateral ocelli at several levels within superfamily Scorpionoidea Latreille, 1802. Characters based on homology of the individual ocelli would be more informative than counts, however, which may conceal different combinations of the same number of units.
There have been few attempts to examine inter-and intraspecific variation in the number of lateral ocelli across the order Scorpiones and none to homologize the individual ocelli. A recent study examined the number of lateral ocelli among various Asian Buthidae and proposed a ''five-eye model'' for the family [11]. According to this model, the first three lateral ocelli are similar in size, and considerably larger than the fourth and fifth. The position of the fourth lateral ocellus is unstable, with three alternative positions: posterolateral, situated in the same plane as first three lateral ocelli (P1); posterior, situated on a ridge of the carapace (P2); posterodorsal, situated above a ridge of the carapace (P3). This model has not been examined more broadly within Buthidae, however, nor compared with the patterns of variation observed among other scorpion families.
We present the first comparative study of variation in lateral ocelli across the order, based on examination of a broad range of exemplar species, representing all families, 78% of the genera, 9% of the species, and up to 12 individuals per species. We propose a six-ocellus model for Recent scorpions with four accessory ocelli observed in various taxa, homologize the individual ocelli, correct erroneous counts in the literature, and investigate the presence of the eyespot across scorpions.

Materials and Methods
Material was examined from the following collections (Appendix S1): In total, 519 individuals, representing all families, 160 genera (78%) and 196 exemplar species (9%) were examined and scored for the presence and relative size of the lateral ocelli (Table 3; Appendix S2). Scorpion suprageneric classification follows [33] with emendations [39][40][41][42][43]. Singletons were examined for 71 species, whereas up to 12 individuals were examined for 125 species. Ocelli were examined on both sides of the carapace to assess bilateral symmetry. Specimens were submerged in ethanol and/or examined under ultraviolet light to observe minor ocelli, which may be difficult to distinguish from granulation or infuscation when the surface of the carapace is dry. A selection of taxa, representing the patterns observed across scorpions, was imaged under visible or ultraviolet light using a MXL Microptics ML-1000 digital imaging system.
The following general assumptions were applied in assigning homology to the individual ocelli. It was assumed that the ocelli are bilaterally symmetric hence the same complement of ocelli is present on the sinistral and dextral sides of the carapace of an individual, regardless of minor differences in position. When an individual was observed to possess different numbers of ocelli on the sinistral and dextral sides of the carapace, the side with fewer ocelli was considered a subset of the side with more. In individuals with the same number of ocelli on the sinistral and dextral sides of the carapace, but with the minor ocelli on one side differing in position from the other, the ocelli on both sides were regarded as homologous, regardless of positional differences. The observation of positional variation in homologous ocelli on a single individual, in turn, permitted positional variation in homologous ocelli among conspecific and heterospecific individuals, and required homology assignment to be based on relative position and size, rather than absolute position on the carapace.

Six-ocellus Model
Based on the survey conducted, we propose a six-ocellus model for Scorpiones (white ocelli in Figure 1A) from which nine general patterns with fewer than six ocelli can be derived ( Figures 1B-J). The various ocelli were identified based on relative position and size (Figures 2-6), as follows. Major ocelli ALMa, MLMa and PLMa, arranged in a row, were similar in size and, with few exceptions, larger than minor ocelli ADMi, PDMi, and PLMi. ADMi was smaller than and dorsal, anterodorsal or posterodorsal to PLMa, and usually smaller than and anterodorsal to PDMi. PDMi was smaller than and posterior, posterodorsal or dorsal to PLMa. PLMi was smaller than and posterior to PLMa, smaller than and ventral to PDMi, and posteroventral to ADMi. Four accessory ocelli, observed in a few taxa, were not considered part of the six-ocellus model (black ocelli in Figure 1A): AALMa was smaller than and anterior to ALMa whereas APLMi 1-3 were smaller than and posteroventral to PDMi.

Bilateral Symmetry and Intraspecific Variation
Most of the individuals examined were bilaterally symmetric with respect to the number of lateral ocelli. Only 58 individuals (11% of the total examined) were observed with different numbers on the sinistral and dextral sides of the carapace and, in most cases, the side with fewer ocelli was a subset of the side with more (Appendix S2). Examples include a bothriurid, Lisposoma elegans Lawrence, 1928, with ALMa on the dextral side of the carapace only ( Figure 5Q) and a chaerilid, Chaerilus chapmani Vachon & Lourenço, 1995, with MLMa on the sinistral side only ( Figures 3D, 3E).
Some individuals with equal numbers of ocelli on the sinistral and dextral sides of the carapace exhibited positional variation in three minor ocelli (ADMi, PDMi, PLMi) on the sinistral and dextral sides. For example, in an iurid, Anuroctonus phaiodactylus (Wood, 1863), ADMi and PDMi were more anteriorly situated on the sinistral side of the carapace ( Figure 4M), resembling another iurid, Protoiurus kraepelini (Von Ubisch, 1922) ( Figure 3H), than on the dextral side ( Figure 4N), which resembled a vaejovid, Uroctonus mordax pluridens Hjelle, 1972 ( Figure 5E). Another example was an U. m. pluridens with four ocelli on both sides of the carapace, where the positions of ADMi and PDMi differed from sinistral to dextral ( Figures 5E, 5F). These observations confirm that the same complement of ocelli is usually present on the sinistral and dextral sides of the carapace of an individual, regardless of differences, and that positional variation in homologous ocelli may occur on a single individual, as well as among conspecific and heterospecific individuals. A notable exception was the scorpionid, O. jenseni, in which different combinations of minor ocelli were observed on the sinistral and dextral sides of the carapace (Figures 6N, 6O). Most of the species for which multiple individuals were examined, possessed constant numbers of ocelli. However, 42 species (21% of the total examined) in 38 genera were observed with polymorphic counts (Appendix S2). In most cases of intraspecific polymorphism observed, the pattern with fewer ocelli was a subset of the pattern with more. Examples include a chaerilid, Chaerilus variegatus Simon, 1877, in which the presence of ALMa and PLMi were polymorphic, leading to counts of two or three ocelli on different individuals ( Figures 3A-3C), and a vaejovid, U. m. pluridens, in which the presence of ADMi was polymorphic, leading to counts of three or four ocelli on different individuals ( Figures 5D-5F).

Nine General Patterns
Nine general patterns, representing mutually exclusive combinations of lateral ocelli, were consistently observed across multiple individuals in more than one genus. More than one combination of ocelli was rarely observed among conspecifics and was not considered a general pattern.
The nine patterns observed can be grouped by the number of ocelli, as follows (Figures 1B-J): five ocelli with one minor ocellus (PLMi) absent; four ocelli with either one major ocellus (ALMa) and one minor ocellus (ADMi or PLMi) absent or two minor ocelli (PDMi or PLMi) absent; three ocelli with one major ocellus (ALMa) and two minor ocelli (ADMi or PDMi, and PLMi) absent; two ocelli with one major ocellus (ALMa) and three minor ocelli (ADMi, PDMi and PLMi) absent or two major ocelli (ALMa and MLMa) and two minor ocelli (ADMi and PLMi) absent; one ocellus (all ocelli absent except PLMa).
Based on the survey presented here, major ocelli MLMa and PLMa are the most prevalent in scorpions and always large. ALMa is usually slightly smaller than MLMa. Among the major ocelli, ALMa is most commonly absent and PLMa most commonly present. ALMa is always coincident with MLMa and PLMa, and MLMa with PLMa.
Among the minor ocelli, PDMi is most commonly present whereas PLMi is uncommon. PLMi was observed only in Chaerilidae Pocock, 1893, Scorpiopidae Kraepelin, 1905 and the scorpionoid families Diplocentridae Karsch, 1880, Heteroscorpionidae Kraepelin, 1905, Hormuridae Laurie, 1896, and Scorpionidae Latreille, 1802. APLMi 1-3 were observed only in Diplocentridae, Heteroscorpionidae, and Scorpionidae. The taxonomic distributions of the patterns are discussed further below. This pattern, derived from Type 5 by the absence of PDMi, is characterized by a row of three large, similar major ocelli (ALMa, MLMa, PLMa) with one smaller minor ocellus (ADMi), situated anterodorsal or dorsal to PLMa ( Figures 1C,  2H, I). Type 4A is also restricted to Buthidae and was observed in 13 genera and 22% of the exemplar species (Table 4). Six of the genera presented the pattern on only one side of the carapace of a single individual. This pattern, derived from the six-ocellus model by the absence of ALMa and ADMi, is characterized by two large, similar major ocelli (MLMa, PLMa) and two smaller minor ocelli (PDMi, PLMi). PDMi is smaller and posterodorsal to PLMa, but larger and dorsal or rarely posterodorsal to PLMi, and PLMi is much smaller and posterior to PLMa (Figures 1D, 3P-R, 6D, J, P, Q). Type 4B was observed in all genera and exemplar species of Scorpionidae, and three genera and 80% of the exemplar species of Scorpiopidae (Table 4). It was also observed on one side of the carapace in three individuals in two genera of Hormuridae and a single individual of Diplocentridae. . ADMi is usually similar or smaller, rarely larger, and anterodorsal or anterior to PDMi. Type 4C is the most common pattern among the chactoid families and was observed in Superstitioniidae Stahnke, 1940 (monotypic), 9 genera and 81% of the exemplar species of Chactidae, two genera and 29% of the exemplar species of Euscorpiidae Laurie, 1896, and 8 genera and 82% of the exemplar species of Iuridae Thorell, 1876 (Table 4). Type 4C was observed in only 5 genera and 8% of the exemplar species of Buthidae, and two genera and 8% of the exemplar species (in one genus on one side of the carapace of a single individual) of Vaejovidae. It was also observed on one side of the carapace of a single individual in two genera of Bothriuridae Simon, 1880 and one genus each of Diplocentridae and Scorpionidae.
Type 3A: Three ocelli (MLMa, PLMa, PDMi), ALMa, ADMi, PLMi absent This pattern, derived from Type 4C by the absence of ADMi, is characterized by two large, similar major ocelli (MLMa, PLMa) and one smaller minor ocellus (PDMi), usually posterodorsal or dorsal, but rarely posterior, to PLMa ( Figures 1F, 2N, Q, 3N, 4F, L, R, 5B-D, G-N, R, 6A, E, H, I, L, R). Type 3A is the most common pattern in Bothriuridae, observed in 13 genera and 94% of the species, and among the scorpionoid families, observed in Hemiscorpiidae Pocock, 1893, 8 genera and 83% of the exemplar species of Diplocentridae, 10 genera and 93% of the exemplar species of Hormuridae, and three genera and 80% of the exemplar species of Scorpionidae (Table 4). Among the other scorpion families, Type 3A was observed in all genera and exemplar species of Vaejovidae, five genera and 31% of the exemplar species of Chactidae, three genera and 27% of the exemplar species of Iuridae, two genera and 3% of the exemplar species of Buthidae and one genus and 20% of the exemplar species of Scorpiopidae. Two individuals in two of the genera of Chactidae and single individuals of Buthidae and Iuridae presented the Type 3A pattern on one side of the carapace. This pattern, derived from Type 4C by the absence of PDMi, is characterized by two large, similar major ocelli (MLMa, PLMa) and one smaller minor ocellus (ADMi), dorsal to PLMa ( Figures 1G, 3K, L). It was observed in two genera and 43% of the exemplar species of Euscorpiidae (Table 4) and on one side of the carapace of a single individual in two genera of Buthidae and a single genus of Iuridae.
Type 2A: Two ocelli (MLMa, PLMa), ALMa, ADMi, PDMi, PLMi absent This pattern, derived from Type 3A or 3B by the absence of PDMi or ADMi, respectively, is characterized by two large major ocelli (MLMa, PLMa) and no minor ocelli (Figures 1H, 2L, 3C, D, O, 4H). Type 2A is the typical pattern of Chaerilidae, observed in all exemplar species of Chaerilus, one genus and 6% of the exemplar species of Chactidae, two genera and 43% of the exemplar species of Euscorpiidae, one genus and 20% of the exemplar species of Scorpiopidae ( Table 4). The Type 2A pattern was also observed on one side of the carapace in two individuals in two genera of Buthidae and single individual of Vaejovidae.

Lateral Ocelli Absent
The absence of ocelli is derived from the Type 1 pattern by the absence of PLMa. Lateral ocelli are absent in 9 genera and 17 species of the following troglomorphic taxa (Table 1)

Taxonomic Distribution of Lateral Ocelli
Among the major ocelli, PLMa is present in all scorpions with lateral ocelli whereas ALMa is absent in all families except Buthidae and, rarely, Bothriuridae ( Figures 3B, 5Q), Chactidae and Chaerilidae. Among the minor ocelli, PDMi is present in most scorpion families, whereas PLMi is absent in all except Scorpiopidae, Scorpionidae and some Chaerilidae, Diplocentridae, Heteroscorpionidae and Hormuridae. Below, we summarize the patterns observed in the major scorpion clades (Table 4).

Buthidae
The number of pairs of lateral ocelli in Buthidae varies from two to five, with the broadest range of patterns among Recent scorpions, including Type 2A, 3A, 3B, 4A, 4C and 5. All buthids possess MLMa and PLMa, with MLMa usually larger than or similar to PLMa. The position and size of PDMi varies among buthids with five pairs of lateral ocelli (Type 5). In most Buthinae, PDMi is usually posterior to MLMa and larger than ADMi. In other buthid genera with five pairs of ocelli, PDMi is posterodorsal or rarely dorsal to MLMa and smaller than ADMi. Among buthids with fewer than five pairs of lateral ocelli, ALMa, ADMi and PDMi are commonly absent (Figure 2). Many buthids with fewer than five ocelli are small (total adult length less than 30 mm) suggesting the loss of ocelli is associated with reduction in body size. The Type 4A pattern is observed in all buthid subfamilies (Table 4) whereas the Type 4C pattern is restricted to Buthinae, Rhopalurusinae, and Uroplectinae ( Figure 2). The Type 3A pattern occurs in Isometrinae and Uroplectinae ( Figures 2N, 2Q). The Type 2A and 3B patterns are uncommon and observed on only one side of the carapace in three genera. The presence of ALMa, ADMi and PDMi is polymorphic in various buthids (Appendix S2), e.g., ALMa in Pseudolychas ochraceus (Hirst, 1911), ADMi in Butheoloides milloti Vachon, 1948, and PDMi in Lychas burdoi (Simon, 1882).

Basal Non-buthid Families
Except for the eyeless genus Vietbocap, all species of Pseudochactidae (i.e., two species of Pseudochactas Gromov, 1998 and the monotypic Troglokhammouanus Lourenço, 2007) are unique among scorpions in exhibiting the Type 1 pattern, a single pair of ocelli ( Figure 3F). All species of Chaerilidae except the eyeless Chaerilus sabinae Lourenço, 1995 (Table 1) exhibit the Type 2A pattern, with two pairs of similar major ocelli (MLMa, PLMa; Figures 3C, D). However, one or three pairs are rarely observed due to the absence of MLMa in some troglomorphic species, e.g., C. chapmani ( Figure 3E), or the presence of ALMa or PLMi, e.g., in C. variegatus (Figures 3A, B).

Chactoid Families
Except for twelve eyeless species of Akravidae, Typhlochactidae and Belisarius Simon, 1879 (Table 1), chactoid taxa exhibit two, three or four pairs of lateral ocelli, with a broad range of patterns including Type 2A, 2B, 3A, 3B, 4B and 4C. Only PLMa is consistently present and, among the chactoids which also possess MLMa, usually smaller (22 vaejovid genera, nine iurid genera, four euscorpiid genera, three chactid genera, three scorpiopid genera, one superstioniid genus), but may be similar (ten chactid genera, six vaejovid genera, two euscorpiid genera, two scorpiopid genera, one iurid genus) or slightly larger (three chactid genera, two iurid genera) than the latter.  Figures 3P-3R). The Type 3A pattern, with PDMi smaller than and posterodorsal to PLMa, occurs in all vaejovid genera, five chactid genera, three iurid genera, and some individuals of Parascorpiops (Scorpiopidae). The Type 3B, 2A and 2B patterns are evident among relatively few chactoid taxa. Type 3B is observed in two euscorpiid genera and on one side of the carapace in an iurid genus. Type 2A is observed in one chactid genus, one euscorpiid genus, some individuals of Parascorpiops, and on one side of the carapace in a vaejovid genus (Figures 3O and 4H). Troglotayosicus Lourenço, 1981 (Troglotayosicidae) is unique among chactoids in presenting the Type 2B pattern ( Figure 4I).

Bothriuridae
Most Bothriuridae exhibit the Type 3A pattern, with MLMa similar to or larger than PLMa, and PDMi similar to or slightly smaller, but rarely larger than PLMa ( Figures 5M, N, R). The monotypic Vachonia Abalos, 1954 exhibits the Type 2B pattern, with MLMa absent ( Figure 5O). Three individuals were observed with four pairs of ocelli on one side of the carapace: ALMa in one L. elegans ( Figure 5Q) and ADMi in one Centromachetes Lønnberg, 1897 ( Figure 5P) and one Phoniocercus Pocock, 1893.

Reexamination of the ''Five-eye Model'' for Buthidae
According to the ''five-eye model'' [11], the position of the fourth lateral ocellus (PDMi) of Buthidae is unstable, with three alternative positions: posterolateral, situated in the same plane as the first three lateral ocelli (P1); posterior, situated on a ridge of the carapace (P2); posterodorsal, situated above a ridge of the carapace (P3). The data presented here confirm this observation and further demonstrate that PDMi is usually situated in the posterolateral position (P1) in Buthinae whereas PDMi is situated in the posterior (P2) or posterodorsal (P3) positions in the other subfamilies.
The presence or absence of the eyespot was usually unambiguous in taxa with dark integument and/or infuscation, but its presence or absence was impossible to verify in some taxa with pale and immaculate integument, in which it was therefore recorded as unknown (Appendix S2). The number of taxa in which the eyespot is present is thus probably an underestimate. A white to yellow, glabrous eyespot, situated ventral to the lateral ocelli, was identified in 27 genera and 30 species of Buthidae ( Figure 2). The eyespot of Buthidae is uniquely different from those of other families, being rather elongated and situated posteroventral and ventral to the lateral ocelli. All species of Chaerilus examined also possessed an eyespot (Figure 3), situated posteroventral to PLMa, usually yellow in color and with a distinct, rounded shape differing from the eyespot of Buthidae. The eyespot of Chaerilidae was previously illustrated [28][29][30]. A yellow, glabrous eyespot was also identified in the iurid genera  Figure 5). The eyespot of Troglotayosicus was previously described as a reduced ocellus [48][49][50] but closer examination revealed that it does not possess a lens ( Figure 4I).
An eyespot was absent in Pseudochactidae; the chactoid families Akravidae, Euscorpiidae, Scorpiopidae, Superstitioniidae, Typhlochactidae, all except one chactid genus, all except four iurid genera, and at least six vaejovid genera; Bothriuridae; and the scorpionoid families Diplocentridae, Hemiscorpiidae, Heteroscorpionidae, Hormuridae, Scorpionidae and Urodacidae. It also did not appear to be present in nine buthid genera ( Table 5). The presence or absence of an eyespot was ambiguous in Belisarius (Troglotayosicidae) and the remaining genera of Buthidae and Vaejovidae surveyed.
The eyespot is apparently more common in scorpions than previously recognized, with considerable variation in shape and size. Little research has been conducted on this visual organ to date, and future research should investigate its structure and function across the order.

Development of Lateral Ocelli
The limited embryological data available suggest that the lateral eyes develop as the hypodermis thickens, from depressions which give rise to the ocelli. Five depressions form in Centruroides, four in a ventral plane and one in a dorsal plane, all of which develop into ocelli except for the posterior-most depression in the ventral plane, which disappears [26]. Centruroides, like most buthids, exhibits the Type 5 pattern, with a row of three major ocelli (ALMa, MLMa, PLMa) and two minor ocelli (ADMi, PDMi), respectively situated dorsal and posterodorsal to PLMa. The dorsal depression observed by Parker [26] appears to correspond to ADMi whereas the anterior three ocelli in the ventral plane correspond to ALMa, MLMa, and PLMa. The small posterior-most depression in the ventral plane appears to correspond to PDMi, which is typically reduced in many New World buthid genera. It is likely that Parker [26] did not observe PDMi in the adult specimens as this ocellus may be difficult to see in many New World buthid Table 5. Recent scorpion families and genera in which an eyespot is present, including observations from the literature [27][28][29][30]. genera because of its small size. More research is needed to determine the developmental pathways underlying the formation of the lateral ocelli.

Systematic Utility of Lateral Ocelli
Based on the survey presented here (Appendix S2), the literature is replete with incorrect counts of lateral ocelli in scorpions ( Table 2). We attribute this error to the greatly reduced size of the minor ocelli in many taxa, which are often almost indistinguishable from granulation and/or infuscation when the surface of the carapace is dry. The minor ocelli can often be identified only under a highpowered microscope when the specimen is submerged under ethanol, and it remains to be seen whether the minor ocelli of some small buthid taxa, measuring less than 20 mm in total adult length, are absent or reduced to the point of being invisible without scanning electron microscopy. Other researchers have recommended the use of ultraviolet light for detecting minor ocelli [11]. Despite controversy concerning the systematic utility of lateral ocelli [12,13,22,[33][34][35][36][37][38][39], we recommend their continued use in scorpion systematics. Although limited intraspecific variation is observed in the number of lateral ocelli in some taxa, it is clear that many of the patterns presented here are both phylogenetically informative and diagnostic for genera, families and even major clades of scorpions.