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Mastigoproctus Pocock, 1894, is the most speciose genus in the thelyphonid subfamily Mastigoproctinae Speijer, 1933, with eighteen described species distributed from the Southern United States to Colombia and Venezuela. Ten of these species occur in Mexico. In the present contribution, Mastigoproctus spinifemoratus, sp. nov., is described based on an adult male and two juveniles from Eastern Nuevo León and Southwestern Tamaulipas, Mexico. It differs from five other species of Mastigoproctus, in which spiniform tubercles are present on the retrolateral surface of the pedipalp femur, in the ventrally directed epistome of the carapace, and the absence of an accessory spine on the prodorsal margin of the pedipalp trochanter. The new species raises the number of Mastigoproctus species to nineteen and the number in Mexico to eleven. 

Due to their continuous growth, reclusive nature, and low vagility, the distributions and species limits of many whip spiders (Amblypygi Thorell, 1883) remain poorly understood, and much cryptic diversity remains unrecognized. Past attempts to separate the historical “forms” of Paraphrynus Moreno, 1940 into morphologically diagnosable species resulted, for example, in the division of Paraphrynus mexicanus (Bilimek, 1867) into three species—the nominotypical form, Paraphrynus cubensis Quintero, 1983, and Paraphrynus carolynae Armas, 2012. Nevertheless, the limitations of conservative morphology continue to hinder progress towards clarifying the diversity of Paraphrynus. One such example concerns P. carolynae, distributed from Arizona to central Mexico as currently defined. Through the acquisition of new, freshly collected material, the discovery of novel morphological characters, and molecular systematics analyses, it became apparent that P. carolynae comprises at least two morphologically diagnosable species. In this present contribution, the northernmost population of P. carolynae occurring in Arizona and California is described as a new species, Paraphrynus tokdod, sp. nov., raising the number of species in the genus to 22. This investigation also revealed more variation than expected in the secondary spine counts of the pedipalps and the trichobothrial counts of leg IV, previously used for species delimitation in Paraphrynus, suggesting that such characters should be used with caution. 

A new genus and species of short-tailed whipscorpion (Schizomida: Hubbardiidae Cook, 1899) is described based on specimens collected in the Venezuelan state of Amazonas. The new genus differs from other Neotropical genera in the presence of six setae on opisthosomal tergite II, the absence of seta Dm4 on the flagellum in both sexes, the female flagellum comprising four segments, and the median lobes of the spermatheca being four times longer than the lateral lobes. Jipai longevus gen. et sp. nov. increases the count of South American schizomid genera to fourteen and the count of species to 57. The type locality of the new taxon is situated in the Guiana region of Amazonia s. l., where three other hubbardiid genera have also been recorded. This discovery contributes to the understanding of Amazonian schizomid diversity and highlights the need for further sampling in this diverse, vulnerable and poorly explored area. 

Thelyphonida Blanchard, 1852, also known as vinegaroons or whip-scorpions, is a small arachnid order with 140 described species contained in a single family, Thelyphonidae Lucas, 1835. Despite being conspicuous and widely distributed across the tropics and subtropics on four continents, knowledge of the order has been slow to advance. Hypoctoninae Pocock, 1899, one of four subfamilies currently recognized and one of two represented in the New World, comprises five genera. Since its inception, Thelyphonellus Pocock, 1894 has remained the only hypoctonine genus occurring in South America, with only four species described prior to the present contribution. The first detailed morphological study and phylogenetic analysis of Thelyphonellus is presented herein. The morphological phylogenetic analysis—the first for Thelyphonida—includes all except one of the previously described species of Thelyphonellus in addition to two new species described herein; the species of Ravilops Víquez and Armas, 2005 (from the Caribbean island of Hispaniola); and the monotypic Old World genus Etienneus Heurtault, 1984 (from West Africa) scored for 45 morphological characters. A single, most parsimonious phylogenetic hypothesis revealed that Thelyphonellus is paraphyletic with respect to Ravilops. The New World Hypoctoninae comprises four clades with disjunct distributions and well supported by a combination of morphological characteristics, on the basis of which four genera, two of which are new, are recognized: Ravilops, with two species, endemic to Hispaniola; Thelyphonellus, herein restricted to Thelyphonellus amazonicus (Butler, 1872) and Thelyphonellus ruschii Weygoldt, 1979, occurring in Guyana, Suriname, French Guiana, and northern Brazil; Wounaan, gen. n., containing Wounaan vanegasae (Giupponi and Vasconcelos, 2008), comb. n. and Wounaan yarigui, sp. n. from Colombia; and Yekuana, gen. n., containing Yekuana venezolensis (Haupt, 2009), comb. n. and Yekuana wanadi, sp. n. from Venezuela. The two new species are described and illustrated. A key to the identification of the Neotropical genera of Hypoctoninae and a map plotting the known distribution of its species are also presented. 

Background/Objectives: Arachnids are a megadiverse arthropod group. The present study investigated the chromosomes of pedipalpid tetrapulmonates (orders Amblypygi, Thelyphonida, Schizomida) and two arachnid orders of uncertain phylogenetic placement, Ricinulei and Solifugae, to reconstruct their karyotype evolution. Except for amblypygids, the cytogenetics of these arachnid orders was almost unknown prior to the present study. Methods: Chromosomes were investigated using methods of standard (Giemsa-stained preparations, banding techniques) and molecular cytogenetics (fluorescence in situ hybridization, comparative genomic hybridization). Results and Conclusions: New data for 38 species, combined with previously published data, suggest that ancestral arachnids possessed low to moderate 2n (22–40), monocentric chromosomes, one nucleolus organizer region (NOR), low levels of heterochromatin and recombinations, and no or homomorphic sex chromosomes. Karyotypes of Pedipalpi and Solifugae diversified via centric fusions, pericentric inversions, and changes in the pattern of NORs and, in solifuges, also through tandem fusions. Some solifuges display an enormous amount of constitutive heterochromatin and high NOR number. It is hypothesized that the common ancestor of amblypygids, thelyphonids, and spiders exhibited a homomorphic XY system, and that telomeric heterochromatin and NORs were involved in the evolution of amblypygid sex chromosomes. The new findings support the Cephalosomata clade (acariforms, palpigrades, and solifuges). Hypotheses concerning the origin of acariform holocentric chromosomes are presented. Unlike current phylogenetic hypotheses, the results suggest a sister relationship between Schizomida and a clade comprising other tetrapulmonates as well as a polyploidization in the common ancestor of the clade comprising Araneae, Amblypygi, and Thelyphonida. 

Abstract Southeast Asia is a hotspot of karst systems in the tropics and many relictual taxa have been documented in caves across the region. The ancient, relictual scorpion family Pseudochactidae Gromov 1998 has a disjunct distribution and includes two hypogean subfamilies from caves in the Khammouan-Phong Nha-Kẻ Bàng Karst in the northern Annamite (Trường Sơn) Mountains of Laos and Vietnam, and one epigean subfamily from Central Asia. A recent revision identified six species in the family; however, how these taxa dispersed and diversified into Southeast Asian cave systems has not been tested. In the present contribution, the phylogeny of Pseudochactidae is reconstructed using three mitochondrial and three nuclear markers and 140 morphological characters, divergence time and ancestral range estimation analyses are conducted, and the evolution of troglomorphic characters is investigated. Results confirm a previous hypothesis that Pseudochactidae originated in Eurasia, most likely near the Tajik block in the Carboniferous, supporting the ‘Out of Eurasia’ hypothesis and contradicting the ‘Eurogondwana’ and ‘Out of India’ hypotheses for the origin of Southeast Asian scorpions. Pseudochactidae dispersed across Southeast Asia after the collision of the Cimmerian continent and Indochina with Eurasia in the Late Jurassic. Colonization of Southeast Asian caves began in the Late Cretaceous and was completed by the Miocene. The onset of aridification in Southeast Asia during the Late Miocene resulted in the extinction of epigean Pseudochactidae, whereas hypogean members of the family likely survived within caves in the limestone massifs of the Annamite Mountains, supporting the ‘Climate Relict’ hypothesis. 

Ricinulei Thorell, 1876 is an order of Arachnida currently represented in the New and Old Worlds by 103 living species. The order is also represented in the fossil record from the Carboniferous (ca. 305–319 Ma) and the Cretaceous (ca. 99 Ma) periods. In the present contribution, Hirsutisoma grimaldii sp. nov., a new extinct species of the suborder Primoricinulei Wunderlich, 2015, is described from a specimen preserved in Cretaceous Burmese amber. The specimen is a well-preserved adult male in which several taxonomically informative structures are visible, allowing the new species to be differentiated from Hirsutisoma bruckschi Wunderlich, 2017, the only other congener for which a male is known. This description raises the number of Cretaceous Ricinulei species to six. A comparative table documents morphological differences among the various species of this lineage. Hypotheses concerning the paleoecology and functional morphology of this species and, by extrapolation, other primoricinuleids, are presented. The evidence suggests that Primoricinulei were corticolous, scansorial predators.