Vinegaroons are members of a guild of apex arthropod predators in the high desert grasslands of southeastern Arizona. Despite their importance as major predators in the ecosystem, almost nothing is known about their potential prey, predators, or competitors. We evaluated predator-prey relationships of vinegaroons and 30 species of potential prey, 27 species of potential predators, and the detailed interactions among three taxa of their apex predator guild. With few exceptions, vinegaroons overpowered and preyed on most potential prey within a suitable size range, were almost immune to predation, and appeared to be the dominant species in interactions with other predator guild members. Their most vulnerable life stages were the first two free-living instar stages, whereas adults and fourth instar individuals were not preyed on by any predators active in the same areas and times as vinegaroons. Third instar individuals were a crucial transition stage in which they had a few predators but also were large enough that they required capturing many prey items to grow sufficiently to molt to the fourth instar. In interactions among arthropod predators, the general observation was that when predation occurred, the larger individual usually prevailed irrespective of taxon. Cannibalism among adult and fourth instar vinegaroons does not occur under natural conditions in contrast to when they are placed together in artificial stressful situations. Cannibalism of the three smallest instars appears likely and might partially explain why they are solitary and spend minimal time foraging.

Group-living spiders are rare, and can be divided into multiple subcategories based on their tolerance of group mates. While social spiders are cooperative, colonial spiders are often antagonistic towards conspecifics. We examined colony dynamics in a colonial species, Philoponella republicana (Simon, 1891), focusing on aggressive behaviors to further understand this understudied species. We studied whether web region, sex ratio, web size, or spider size affected aggression. We also tested whether colony members discriminate against conspecific intruders, since this behavior, known as group closure, is prevalent in many other group-living animals but had not yet been tested in colonial spiders. Colony mates were often aggressive due to competition for limited resources, such as mates and orb webs, yet several characteristics of this species may reduce these competitive forces. First, female-biased secondary sex ratios appear to reduce male-male and female-male competition. Moreover, although some individuals defended orb webs, other areas in the communal web were not defended. Philoponella republicana also did not exhibit group closure. Our results further confirm that aggression between males decreases in colonies with more female-biased secondary sex ratios, and larger individuals correlate with a higher frequency of aggressive interactions. Moreover, we raise new questions concerning the evolutionary pressures that shape coloniality in spiders.

Supplemental Materials

Table S1.—Effects of inter-colony translocation treatment on responses of Philoponella republicana.  - Download file

Table S2.—Results of inter-colony translocation experiment. - Download file

Metabolism powers all of life's processes, making it fundamental to understanding organisms in nature. For many arachnids, however, we do not have direct measurements of either metabolic rate or the relationship between metabolic rate and body size (i.e., allometric scaling, an important determinant of metabolic rate). Here, we present the first measurements of metabolic rate and allometric scaling in a stridulating spined micrathena spider, Micrathena gracilis (Walckenaer, 1805). Since intraspecific variation in metabolic rate may provide insights into variation in energetically costly behaviors, we additionally explore the link between stridulation and metabolism in M. gracilis. Our data suggests a link between stridulatory behavior and allometric scaling, such that increased stridulation is associated with a weaker connection between body mass and metabolic rate. We discuss how links between energetics, expression of behavior, and body size inform our understanding of trait variation in these spiders.

Supplemental Materials

Figure S1.—Correlation between opisthosoma length and mass. Table S1.—Models for respiration ranked by AIC. - Download file

We describe two new Nesticus Thorell, 1869 from Walker County, Georgia, USA. Nesticus lula sp. nov. is known from two caves on the eastern edge of Lookout Mountain and N. cressleri sp. nov. is known from three caves on Pigeon Mountain. Morphological and molecular evidence indicates the distinctiveness of both species when compared to other Nesticus from the southern Appalachians. Nesticus lula has reduced eyes and N. cressleri is eyeless. Both species are of conservation concern, as they are known from only a handful of sites spanning extremely limited ranges. This work contributes to our understanding of cave biodiversity in Georgia and of the Nesticus radiation in the southern Appalachians.

We analysed the dead-wood characteristics that determine the presence of saproxylic pseudoscorpion species in remnants of riparian cloud forest. We examined 98 dead-wood pieces (70 logs and 28 stumps), and recorded tree species, decaying wood stages, presence of the pseudoscorpion Lustrochernes grossus (Banks, 1893) (Chernetidae) and the Bess beetle Helicus tropicus. In these wood samples we found 24 L. grossus and one chela. We recorded the highest number of individuals in Clethra mexicana (11), followed by Quercus corrugata (6) and Liquidambar styraciflua (6). In Annona cherimola and Trema micrantha, one chela and one female were recorded, respectively. The presence of this pseudoscorpion is likely due to its relationship with the Bess beetle, which coexists in decaying wood. The distribution of L. grossus in dead wood may also be influenced by tree species and stage of decay. Forest fragmentation and the extraction of firewood from the remnant riparian fragments of cloud forest are factors that could jeopardize the saproxylic pseudoscorpion species and other arthropod diversity associated with decaying wood in this threatened ecosystem.

Ballus Koch, 1850 is a beetle-like jumping spider genus encountered in montane evergreen rainforests of the Central and Uva Provinces of Sri Lanka. The taxonomic literature documents three species of the genus for the island. However, neither the taxonomic validity nor the systematics of any of the three species have been previously examined. We used nuclear and mitochondrial DNA sequences (28S rRNA, H3, COI) as well as morphological characters to investigate the genetic and taxonomic diversity of Ballus populations in Sri Lanka, including specimens from type localities. No Ballus specimens were found outside of the central highlands. Results of molecular species delimitation and morphological analysis suggest the presence of only a single species of Ballus in Sri Lanka. We therefore propose B. sellatus Simon, 1900 to be a junior synonym of B. segmentatus Simon, 1900, while B. clathratus Simon, 1901 remains a nomen nudum. Further, we discuss the implications of our results for conservation planning.

Animals may build nests socially to minimize the energy required for nest construction. Paratemnoides spp. pseudoscorpions evolved sociality independently from all other social groups, and colonies create silken multi-chambered nests in which they molt and raise young, analogous in form to the nests of some wasps and bees. Here we describe these nests and examine pseudoscorpion construction efficiency. Silk is generally energetically expensive and as such, we hypothesized that P. elongatus build nests of a structure that minimizes silk use, thereby maximizing nest construction efficiency. We measured the number of nest chambers, their perimeter, and their area, for 31 nests, calculated several metrics of nest architecture, and developed five alternative mathematical models describing other possible nest geometries. We found that real social pseudoscorpion nests are constructed with high efficiency, measured as wall length per internal area, approaching that of mathematical optima. We also found that these nests use less silk per capita than if the same chambers were built separately, i.e., if they were solitary. This indicates a direct benefit to group members. We compared observed nest architecture with five mathematical models of nest geometry and found that pseudoscorpion construction efficiency outperformed all non-cooperative models and rivaled that of a cooperative one approximating the honeycomb conjecture - a mathematical proof describing the most efficient way possible to divide a 2-dimensional plane. In summary, social pseudoscorpions design group nests with multiple chambers in a way that minimizes wall length per internal area and approaches the efficiency of honey-bee-like hexagon constructcion.

“Primary” webs of uloborids have large numbers of very fine lines and usually lack sticky cribellum silk. This paper reviews their taxonomic distribution (19 species in 5 genera) and the ontogenetic stages in which primary webs are built (spiderlings newly emerged from the egg sac, older juveniles, mature males, and normal and senile females), expands the knowledge of construction behavior, and describes several previously unnoticed design details. Primary webs differ from typical uloborid orbs in several ways: large numbers of fine radial and non-radial lines; facultative hub removal and replacement; usually closely spaced temporary spiral loops; and lines beyond the frame lines. Construction of supplemental radii in primary webs is distinctive in several respects: break and reel construction; tendencies to lay successive radii either on opposite sides of the web or close together in the same sector; high frequencies of aborted trips from the hub to the frame; production of multiple lines during a single trip from the hub to the frame and back; long pauses during the production of single radii; and variation in the sequences in which radial lines are added to a given sector. Some aspects of primary web construction resemble araneoid rather than typical uloborid behavior. The relation between primary webs and the evolution of orb webs, and the mechanism that spiders use to produce abundant non-radial lines despite making only radial movements during web construction remain uncertain. We speculate that primary webs are favored when spiders are unable to afford the costs of producing cribellate silk for a typical orb.

Web designs have long been used to characterize spider taxa and to deduce the relations between them; but systematic documentation of the amount of variation in webs within and between taxonomic groups is rare. This study, based on previously published observations and new observations of 15 species in the family Uloboridae, including two genera, Octonoba Opell, 1979 and Siratoba Opell, 1979, whose webs were previously undocumented, reviews the taxonomic distribution and variation in 22 orb web traits in at least 43 species in 11 genera in uloborids. These traits appear to occur in all orb-weaving genera in which reasonable samples are available, though only small samples are available for many species. Larger samples of the webs of three species of Uloborus Latreille, 1806, two of Hyptiotes Walckenaer, 1837, and one each of Zosis Walckenaer, 1841, Siratoba, Octonoba, Waitkera Opell, 1979 and Philoponella Mello-Leitão, 1917, revealed greater intra-specific consistencies in some traits than others. Hub traits were especially consistent. Variations in three traits may represent adjustments to the size of the space in which the orb is built. “Primary” webs, which combine orb and sheet-web traits, are built by spiderlings newly emerged from the egg sac and by adult males in at least five genera of orb-weaving uloborids and may be unique to this family. Preliminary comparisons between uloborid and araneoid orbs suggest that uloborid orbs may also differ from araneoid orbs in combining several other traits.

According to a recent global literature survey, a total of 39 out of the 129 known spider families (∼30%) contain species capable of capturing vertebrate prey. The finding that the percentage of spider families engaged in vertebrate predation is so high is novel. Two groups of vertebrate-eating spiders are distinguished: “habitual vertebrate-eaters” vs. “occasional vertebrate-eaters”. The habitual vertebrate-eaters comprise ten spider families (Araneidae, Atracidae, Ctenidae, Lycosidae, Nephilidae, Pisauridae, Theraphosidae, Theridiidae, Trechaleidae, and Sparassidae) to which can be attributed 91% of all reported vertebrate predation incidents. The habitual vertebrate-eaters have evolved prey-capture adaptations such as (1) sufficient physical strength coupled with large body size, (2) the use of potent venoms, and (3) the use of highly efficient prey-catching webs. By contrast, unexpected feeding on vertebrates by the occasional vertebrate-eaters (i.e., Actinopodidae, Agelenidae, Amaurobiidae, Anyphaenidae, Barychelidae, Clubionidae, Corinnidae, Ctenizidae, Cyrtaucheniidae, Deinopidae, Desidae, Dipluridae, Eresidae, Filistatidae, Gnaphosidae, Haplonoproctidae, Linyphiidae, Liocranidae, Miturgidae, Oxyopidae, Pholcidae, Porrhothelidae, Salticidae, Selenopidae, Sicariidae, Sparassidae, Tetragnathidae, and Thomisidae) might be considered as chance events that took place when a tiny vertebrate crossed the path of an opportunistic spider. For a few families (e.g., Idiopidae) their status as habitual or occasional vertebrate predators is still unclear. In conclusion, our survey unveiled a large number of spider taxa previously not anticipated to feed on vertebrate prey. These findings improve our general understanding of spider feeding ecology and provide a first assessment of the significance of vertebrate prey as a food source for spiders.

South Korea is a country with more than 1000 caves that support a high diversity of invertebrate species. Here we review the pseudoscorpion genus Spelaeochthonius Morikawa, 1954 (family Pseudotyrannochthoniidae) that is exclusive to caves in Japan and South Korea and reinstated here as a valid taxon. Based on recent field collections, three species in this genus are described from South Korea: S. dentifer (Morikawa, 1970), comb. nov., S. cheonsooi sp. nov., and S. seungsookae sp. nov. Based on present-day knowledge about range sizes and morphological variability in this fauna, three Japanese taxa currently classified as subspecies are elevated to species rank: Pseudotyrannochthonius kobayashii akiyoshiensis (Morikawa, 1956) = Spelaeochthonius akiyoshiensis Morikawa, 1956, stat. nov., Pseudotyrannochthonius kobayashii dorogawanesis (Morikawa, 1956) = Spelaeochthonius dorogawaensis Morikawa, 1956, stat. nov.; and Pseudotyrannochthonius undecimclavatus kishidai (Morikawa, 1960) = Spelaeochthonius kishidai (Morikawa, 1960), stat. nov. We emphasize that no epigean Spelaeochthonius have ever been found despite considerable search effort in the forests of Japan and Korea, and that past glacial cycling may have resulted in the total extirpation of surface species in this lineage. We also note that all species are narrow-range endemics that may require conservation management.

The pseudotyrannochthoniid genus Centrochthonius Beier, 1931 is one of the rarest and most poorly understood pseudoscorpion genera in the world. Only a handful of specimens are presently known, all from high-altitude habitats in China, Kyrgyzstan and Nepal. The inaccessibility of specimens has resulted in major taxonomic difficulties such as a poor generic diagnosis, but also a lack of understanding of evolutionary patterns in this psychrophilic fauna. Herein we diagnose Centrochthonius for the first time since its original description and include four Holocene species: one from Kyrgyzstan, two from China, and C. anatonus sp. nov. from Nepal. A fifth fossil species from Bitterfeld amber (Oligocene-Miocene) in central Europe highlights significant range retraction and continental extinction in this genus since the Neogene, leading us to suggest a climatic relict hypothesis for this fauna. Biogeographically, the genus provides further evidence for significant range shifts and extinction events in the Holarctic invertebrate biota during the Cenozoic. Close relatives of Centrochthonius are found in subterranean habitats throughout eastern Asia and warm temperate forests in the western U.S.A. which highlights the magnitude of range retraction caused by climatic cycling in an ancient fauna since at least the Paleogene. The subgenus Allochthonius (Urochthonius) Morikawa, 1954 from caves in Japan has been largely defined by the presence of troglomorphic features such as eye loss, which are here regarded as insufficient to retain as a separate clade from other species of Allochthonius. Therefore, Allochthonius (Urochthonius) is treated as a junior synonym of Allochthonius (syn. nov.).

Giant velvet mites, the largest living mites, are a familiar sight when they are present above ground, usually after the first summer rains, in warm arid areas. Despite their often-conspicuous presence in enormous numbers, little is known about their basic biology, life history strategies, or predators. The emergence of Dinothrombium magnificum (LeConte, 1852) in southern Arizona, USA usually occurs after the first summer rain that exceeds 1 cm. Of the 17 species of vertebrate predators offered velvet mites, only those that quickly engulf their prey intact (horned lizards and toads) successfully preyed upon the mites. Even those ate only one or few before rejecting subsequent mites. Of the 12 species of insect predators and 11 species of other arthropod predators offered mites, only the larvae of antlions were possibly meaningful predators. Nevertheless, antlion larvae only rarely succeeded in puncturing the rubbery integument of a mite and killing it. When sampled by an author, a velvet mite produced an exceedingly bitter, astringent, and spicy taste that endured in the mouth for about an hour. Overall, velvet mites appear to have no meaningful predators, likely because of their enormous suite of highly effective defensives: red aposematic coloration, aposematic odor, a tough puncture-resistant integument, unpleasant tasting chemicals on the integument, and exceedingly distasteful internal compounds.

Spider webs in general and orb webs in particular are delicate, ephemeral structures that are frequently damaged in nature. Some orb weavers respond to damage by quickly “shoring up” their webs with non-sticky dragline silk. This study of how Micrathena duodecimspinosa (O. Pickard-Cambridge, 1890) shores up damaged frame lines shows that repairs were largely based on a single, repeated pattern of attachments. These movements are shared with the distantly related family Pholcidae, suggesting a possibly ancient origin. Spiders tended to initiate repairs at the lower edge of a damaged sector, probably to reduce the damage produced when the spider's own weight caused sticky lines in slack portions of the web to sag into and adhere to each other. Repairs of lateral frames recuperated capture area more successfully than did those of upper frames, probably because damage caused by the spider's own weight during repair was reduced.

Rhaucoides Roewer, 1912 is herein revisited and re-diagnosed based on somatic and genital morphology, including the differences of both expanded and unexpanded glans complex shown for the first time in Cosmetidae. The genus is characterized mainly by abdominal ornamentation, coloration and by having the two basalmost tarsomeres of leg I fused. The genus Cumbalia Roewer, 1963 is newly considered a junior synonym of Rhaucoides. The species Cumbalia octomaculata Roewer, 1963, Rhaucoides festae Roewer 1925 and Rhaucoides sulfureus Mello-Leitão 1939 are considered junior subjective synonyms of Rhaucoides riveti Roewer, 1919. Erginus devillei Simon 1879 (currently in Metarhaucus) is transferred to Rhaucoides, resulting in Rhaucoides devillei (Simon 1879), comb. nov. Rhaucoides atahualpa sp. nov. and Rhaucoides nasa sp. nov. are described from Ecuador and Colombia, respectively. Comments about the genital and tarsal morphology in Cosmetidae are provided. A map with the known distribution of the genus, in the Andes of southern Colombia and northern Ecuador, is given. Finally, emended diagnoses are provided for the genera Rhauculus Roewer, 1927 and Reimoserius Roewer 1947. After the type species of Metarhaucus Pickard-Cambridge, 1905 was transferred to Rhaucus Simon, 1879, a few species remained without formal generic allocation. All of those are herein combined either with Rhaucoides (1 species), Rhauculus (5 species) or Reimoserius (1 species).

Two new pseudoscorpion species, Neobisium (Ommatoblothrus) wettereri Nassirkhani sp. nov. and Neobisium (Ommatoblothrus) karmanae Nassirkhani sp. nov., are described from Socotra (Yemen). The family Neobisiidae is reported for the first time from Socotra. Ecological notes on these two new species and a distribution map of all known Neobisium (Ommatoblothrus) species are presented.

Two new species of pelican spiders (family Archaeidae) are described from rainforest habitats in south-eastern Queensland, Australia – Austrarchaea davidi Rix, sp. nov. and A. laidlawae Rix, sp. nov. – and the female of A. clyneae Rix & Harvey, 2011 is described for the first time. Phylogenetic analysis of a mitochondrial molecular dataset for the genes cytochrome c oxidase subunits I and II reveals that these two new species are the closest relatives of A. judyae Rix & Harvey, 2011 and A. clyneae, respectively, with mitochondrial sequencing also used to identify newly-collected specimens of Austrarchaea from populations throughout south-eastern Queensland. These recent collections were largely made during dedicated survey work conducted after the devastating 2019–20 summer bushfires in eastern Australia. We further provide a synopsis of the archaeid fauna of south-eastern Queensland, which is comprised of 11 species in two monophyletic regional-endemic clades. A revised key to species is presented for both clades, along with updated distributional information, and live habitus images for nine of the 11 species.

The spider Myrmecicultor chihuahuensis Ramírez, Grismado & Ubick (Myrmecicultoridae) was described in 2019 and hypothesized to be a myrmecophile, living inside the nests of Novomessor (Myrmicinae) and perhaps also Pogonomyrmex (Myrmicinae) ants. To test the hypothesis that M. chihuahuensis are chemical mimics of their host ants, we carried out behavioral bioassays to observe interactions between the spiders and the host ants. We compared the cuticular hydrocarbon (CHC) profiles of the spiders and the ants. We discovered that this new species of spider is a myrmecophage, displaying hunting strategies similar to other myrmecophagous spiders, particularly those in the genus Zodarion Walckenaer, 1826 (Zodariidae). The CHC profile of M. chihuahuensis did not match the profile of the ants with which it associates, Novomessor albisetosus (Mayr), but ants and spider shared several compounds, potentially involved in species recognition.

Sociality in spiders has evolved independently multiple times, resulting in convergently evolved cooperative breeding and prey capture. In all social spiders, prey is captured by only a subset of group members and then shared with other, non-attacking group members. However, spiders' propensity to attack prey may differ among species due to species-specific trade-offs between risks, costs and benefits of prey capture involvement. We explored whether engagement in prey attack differs among three social Stegodyphus species, using orthopteran prey, and found substantial differences. Stegodyphus mimosarum Pavesi, 1883 had a low prey acceptance rate, was slow to attack prey, and engaged very few spiders in prey attack. In S. sarasinorum Karsch, 1892, prey acceptance was high, independently of prey size, but more spiders attacked when prey was small. While medium-sized prey had higher acceptance rate in S. dumicola Pocock, 1898, indicating a preference, the number of attackers was not affected by prey size. Our results suggest that the three species may have different cooperative prey capture strategies. In S. mimosarum and S. dumicola, whose geographical ranges overlap, these strategies may represent niche specialization, depending on whether their respective cautious and choosy approaches extend to other prey types than orthopterans, while S. sarasinorum may have a more opportunistic approach. We discuss factors that can affect social spiders' foraging strategy, such as prey availability, predation pressure, and efficiency of the communal web to ensnare prey. Future studies are required to investigate to which extent species-specific cooperative foraging strategies are shaped by ontogeny, group size, and plastic responses to environmental factors.