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

Abstract

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.

Trypanothacus gen. n., a new genus of burrowing scorpion from the Arabian Peninsula (Scorpiones: Buthidae)

“…Diagnosis. Medium-sized buthids (Kovařík, 2009;Sissom, 1990), adults 40-60 mm long; carapace trapezoidal with straight anterior margin, without downward-sloped preocular area, surfaces with sparse to moderately dense, fine and coarse granulation interspersed with smooth areas, only anterior median carinae developed, other carinae obsolete, anterior area glossy in females; 5 lateral eyes in 'type 5' pattern (Loria & Prendini, 2014); chelicerae with typical buthid dentition (Vachon, 1963), ventral aspect of fixed finger with two denticles; tergites I-VI with three short, granulose posterior carinae, not projecting beyond posterior margins; tergites I-II with lateral carinae inconspicuous; sternite VII with 2 pairs of weak to moderate carinae; pectines with fulcra, marginal and middle lamellae with dense fine reddish setae; female pectines reach or extend beyond distal end of coxa IV, pectinal tooth counts, ♂ 22-26, ♀ 15-22; hemispermatophore flagelliform, capsule in 3+1-lobe configuration, with 3 sperm hemiduct lobes well separated from flagellum, and small, knob-like basal lobe; sternum subtriangular; metasomal segments moderately robust, segment II as wide as others, segments I-III with 10 carinae, median lateral carinae complete on I, incomplete on II-III; ventromedian carinae on segments II-III well developed with strong denticles that are larger in females, segment V with enlarged lobate dentition on ventrolateral carinae; dorsal carinae of metasoma with sparse medium to long macrosetae in both sexes; telson vesicle bulbous, without subaculear tubercle, aculeus shorter than vesicle; pedipalps short relative to body, femur and patella as long as or shorter than carapace, chelae small with carinae reduced or obsolete, movable finger not more than 1.6 times ventral length of manus, dentate margin of movable finger armed with 7-9 non-imbricated linear subrows of primary denticles or granules, each subrow flanked by internal and external accessory denticles, 3-6 subterminal denticles; males without recess or scalloping of dentate margins at base of pedipalp fingers, chela manus broader than in females; trichobothrial pattern orthobothriotaxic type A (Vachon, 1974); femur with dorsal trichobothria in ß-configuration (Vachon, 1975), petite trichobothrium d 2 located on dorsal surface, e 2 distal to d 5 ; patella with 7 external, 5 dorsal, and 1 internal trichobothrium, patella d 2 present, d 3 positioned internal to dorsomedian carina; manus with V 1 -V 2 axis nearly collinear with long axis of chela, fixed finger with db on proximal part of finger between esb and est, it located near tip; legs moderately robust, tibial spurs present on legs III-IV, but spurs may be reduced or absent on leg III; basitarsi I-III compressed, equipped with retrosuperior bristle-comb of macrosetae about as wide as segment, soles of telotarsi with ca. 5-10 unpaired macrosetae near mid-ventral axis, tarsal ungues of moderate to short length, strongly curved and tapered.…”

A new Sky Island species of Vaejovis C. L. Koch, 1836 from Sonora, Mexico (Scorpiones, Vaejovidae)

“…Nomenclature and mensuration follows Stahnke (1970), except for trichobothrial terminology after Vachon (1974), cheliceral dentition after Vachon (1963); metasomal, pedipalpal carination, as well as the hemispermatophore terminology after González-Santillán and Prendini (2013); metasomal setae counts, modified from Santibáñez-López and Sissom (2010); telotarsal spination and setal counts following Contreras-Félix et al (2015); terminology for the lateral eyes follows Loria and Prendini (2014); laterobasal aculear serrations (= LAS) terminology follows Fet et al (2006); hemispermatophores were dissected following Vachon (1952) , and cleared using the technique of Álvarez-Padilla and Hormiga (2008). Higher level taxonomy of scorpions follows Sharma et al (2015) and Prendini and Wheeler (2005).…”

“…2a): Anterior margin slightly concave, almost straight; anteromedian longitudinal sulcus shallow; surface of carapace minutely granular on area surrounding the median ocelli, rest of surface granular. Ocular tubercle with superciliary carinae lower than medial ocelli; lateral ocelli type 3A (Loria and Prendini 2014). …”

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“…In describing the first 'Grosphus' group species, Scorpio (Androctonus) madagascariensis, Gervais (1844: pl. XI, fig. 3) illustrated the carapace showing forward placement of the median eyes, and also accurately depicted five pairs of lateral eyes, now recognized to be the prevalent buthid configuration (Loria & Prendini, 2014;Yang et al, 2013). In spite of this, Fage (1929) incorrectly declared that Grosphus (sensu lato) only bore 3 pairs of lateral eyes, and Lourenço (1996b) cited only 3 -4 pairs.…”

“…We predict that other 'Grosphus' group species will also comply with the 5-eye pattern. Although undercounting of lateral eyes is perhaps attributable to overlooking of the smaller posterior and upper ocelli, 10 of the published 3-eye counts post-date introduction of the 5-eye model by Yang et al (2013, coauthor Lourenço) and Loria & Prendini (2014). Paradoxically, Lourenço et al (2007a) claimed 3 lateral eyes in boilerplate descriptions of G. hirtus and G. polskyi, yet their figures clearly depict all 5 lateral eyes as being present in both species.…”