Molecular phylogeny of moth-specialized spider sub-family Cyrtarachninae, which includes bolas spiders

Evolution and comparative morphology of raptorial feet in spiders

Spiders are among the most diverse animals, which developed different morphological and behavioral traits for capturing prey. We studied the anatomy and functionality of the rare and apomorphic raptorial spider feet using 3D reconstruction modeling, among other imaging techniques. The evolutionary reconstruction of the raptorial feet (tarsus plus pretarsus) features using a composite tree of spiders, indicating that similar traits emerged three times independently in Trogloraptoridae, Gradungulinae, and Doryonychus raptor (Tetragnathidae). The characteristics defining the raptorial feet are an interlocked complex merging of the base of the elongated prolateral claw with the pretarsal sclerotized ring, with the former clasping against the tarsus. Raptorial feet even flex over robust raptorial macrosetae forming a reduced tarsal version of a catching basket to encase prey during hunting. Our results show that Celaeniini (Araneidae) and Heterogriffus berlandi (Thomisidae), taxa previously compared with raptorial spiders, lack the raptorial feet key characteristics and the tarsal-catching basket. We make predictions about the possible behavior of the abovementioned taxa that will need to be tested by observing living specimens. We conclude that multiple morphological tarsal and pretarsal micro-structures define the raptorial foot functional unit and recommend a comprehensive evaluation before assigning this configuration to any spider taxa.

Connecting materials, performance and evolution: a case study of the glue of moth-catching spiders (Cyrtarachninae)

Morphological structures and extended phenotypes are made possible by materials that are encoded by the genome. Nearly all biomaterials are viscoelastic, which means that to understand performance, one must understand the strain rate-dependent properties of these materials in relevant ecological interactions, as the behavior of a material can vary dramatically and rapidly. Spider silks are an example of materials whose properties vary substantially intra- and inter-specifically. Here, we focus on aggregate silk, which functions as a biological adhesive. As a case study to understand how a material manifests from genome through organism to ecology, we highlight moth-specialist spiders, the Cyrtarachninae, and their glues as an ideal experimental system to investigate the relationship between genomics and ecologically variable performance of a biological material. There is a clear eco-evolutionary innovation that Cyrtarachne akirai and related species have evolved, a unique trait not found in other spiders, a glue which overcomes the scales of moths. By examining traditional orb-weavers, C. akirai and other subfamily members using biomechanical testing and genomic analysis, we argue that we can track the evolution of this novel bioadhesive and comment on the selection pressures influencing prey specialization. The importance of the ecological context of materials testing is exemplified by the poor performance of C. akirai glue on glass and the exceptional spreading ability and adhesive strength on moths. The genetic basis for these performance properties is experimentally tractable because spider silk genes are minimally pleiotropic and advances in genomic technologies now make possible the discovery of complete silk gene sequences.

Phylogenetic placement of the spider genus Taczanowskia (Araneae:Araneidae) and description of a new species from Ecuador

The genus Taczanowskia Keyserling, 1879 is one of the rarest groups of spiders in the orb-weaving family Araneidae, with only five species described and 17 specimens cited in publications. Our study provides new insights into the evolutionary relationships and diversity of Taczanowskia. Using morphological data, we tested the evolutionary relationships of the genus within the family Araneidae and propose the first phylogenetic hypothesis depicting the relationships among species of Taczanowskia. Our results place Taczanowskia as sister to Mastophora Holmberg, 1876, and confirm the monophyly of Taczanowskia. We describe the first species of Taczanowskia from Ecuador, collected at a Waorani community on the River Curaray basin, Amazonian lowlands of Ecuador. The new species can be easily diagnosed from all other species of Taczanowskia by having two tubercles in the opisthosoma; a distinct patchy dark–light colouration pattern with dark spots concentrated towards the anterior margin and on the lateral tips; small bundles of white setae forming a reticulum across the dorsal part of the opisthosoma, and the first two femora thick but lacking teeth on the margin. ZooBank registration: urn:lsid:zoobank.org:pub:46B8C1F7-A474-4DC3-90BC-940F84AC099D

Phylogenetic placement of the stone-nest orb-weaving spider Nemoscolus Simon, 1895 (Araneae : Araneidae) and the description of the first species from Australia

The spider genus Nemoscolus Simon, 1895 (Araneidae) has been neglected taxonomically despite the unique retreat that several species construct in their horizontal orb-webs, composed of pebbles and other detritus. The distribution of Nemoscolus is poorly known and the genus includes species from Africa and Europe. Nemoscolus is placed in Simon’s Cycloseae species group along with Cyclosa Menge, 1866, Acusilas Simon, 1895, Arachnura Vinson, 1863, Witica O. Pickard-Cambridge, 1895, among others. Here we describe a new species from Queensland, Australia, N. sandersi, sp. nov., drastically expanding the distribution range of the genus. We use nucleotide sequence data to phylogenetically place Nemoscolus using model-based inference methods within Araneidae and to explore its affinities to Simon’s Cycloseae. The data support propinquity of Nemoscolus with Acusilas and Arachnura but not with Cyclosa. Our analyses suggest that Cycloseae is not a clade, with Cyclosa, Acusilas, Witica and Nemoscolus not sharing a recent common ancestor. This use of an integrated granular retreat represents at least the second independent evolution of such a structure within Araneidae. These results improve our understanding of both phylogeny and retreat evolution in araneid spiders.

Silk structure rather than tensile mechanics explains web performance in the moth-specialized spider, Cyrtarachne

Orb webs intercept and retain prey so spiders may subdue them. Orb webs are composed of sticky, compliant spirals of capture silk spun across strong, stiff major ampullate silk threads. Interplay between differences in the mechanical properties of these silks is crucial for prey capture. Most orb webs depend upon insects contacting several radial and capture threads for successful retention. Moths, however, escape quickly from most orb webs due to the sacrificial scales covering their bodies. Cyrtarachne orb webs are unusual as they contain a reduced number of capture threads and moths stick unusually well to single threads. We aimed to determine how the tensile properties of the capture spiral and radial threads spun by Cyrtarachne operate in retention of moth prey. A NanoBionix UTM was used to quantify the material properties of flagelliform and major ampullate threads to test if Cyrtarachne's reduced web architecture is accompanied by improvements in tensile performance of its silk. Silk threads showed tensile properties typical of less-specialized orb-weavers, with the exception of high extensibility in radial threads. Radial thread diameters were 62.5% smaller than flagelliform threads, where commonly the two are roughly similar. We utilized our tensile data to create a finite element model of Cyrtarachne's web to investigate energy dissipation during prey impact. Large cross-sectional area of the flagelliform threads played a key role in enabling single capture threads to withstand prey impact. Rather than extraordinary silk, Cyrtarachne utilizes structural changes in the size and attachment of silk threads to facilitate web function.

A phylotranscriptomic backbone of the orb-weaving spider family Araneidae (Arachnida, Araneae) supported by multiple methodological approaches

The orb-weaving spider family Araneidae is extremely diverse (>3100 spp.) and its members can be charismatic terrestrial arthropods, many of them recognizable by their iconic orbicular snare web, such as the common garden spiders. Despite considerable effort to better understand their backbone relationships based on multiple sources of data (morphological, behavioral and molecular), pervasive low support remains in recent studies. In addition, no overarching phylogeny of araneids is available to date, hampering further comparative work. In this study, we analyze the transcriptomes of 33 taxa, including 19 araneids - 12 of them new to this study - representing most of the core family lineages, to examine the relationships within the family using genomic-scale datasets resulting from various methodological treatments, namely ortholog selection and gene occupancy as a measure of matrix completion. Six matrices were constructed to assess these effects by varying orthology inference method and gene occupancy threshold. Orthology methods used are the benchmarking tool BUSCO and the tree-based method UPhO; three gene occupancy thresholds (45%, 65%, 85%) were used to assess the effect of missing data. Gene tree and species tree-based methods (including multi-species coalescent and concatenation approaches, as well as maximum likelihood and Bayesian inference) were used totalling 17 analytical treatments. The monophyly of Araneidae and the placement of core araneid lineages were supported, together with some previously unsound backbone divergences; these include high support for Zygiellinae as the earliest diverging subfamily (followed by Nephilinae), the placement of Gasteracanthinae as sister group to Cyclosa and close relatives, and close relationships between the Araneus + Neoscona clade and Cyrtophorinae + Argiopinae clade. Incongruences were relegated to short branches in the clade comprising Cyclosa and its close relatives. We found congruence between most of the completed analyses, with minimal topological effects from occupancy/missing data and orthology assessment. The resulting number of genes by certain combinations of orthology and occupancy thresholds being analyzed had the greatest effect on the resulting trees, with anomalous outcomes recovered from analysis of lower numbers of genes.

Experimental evidence that high humidity is an essential cue for web building in Pasilobus spiders

Spiders in the subfamily Cyrtarachninae, including bolas spiders, are moth specialists, and it has been suggested that these spiders initiate web-weaving under high humidity. Here we used Pasilobus hupingensis to experimentally test whether Cyrtarachninae spiders build webs exclusively under high humidity. The results showed that humidity, as well as temperature and prey feeding history, affected web-building probability, but humidity had a much stronger effect. Moreover, spiders never constructed webs at under <70% humidity. We suggest that a mechanical property in sticky materials derived from moth specialization; namely, unusually high, yet rapidly degrading stickiness, is likely to have promoted the evolution of plastic foraging behaviour that varies with humidity.