Ingi Agnarsson

We designate a neotype for the Eurasian spider species Araneus angulatus Clerck, 1757, thereby fixing the objective application of the species name, the genus name Araneus Clerck 1757, the family name Araneidae Clerck 1757, and the superfamily name Araneoidea Clerck 1757. By etymology and arachnological tradition, it also unofficially fixes the application of the ordinal name Araneae Clerck 1757 and thus answers the uncontroversial question, “What’s a spider?” A. angulatus also happens to be historically the first animal species ever described because Clerck (1757) has priority over Linnaeus (1758). Neotype designation is justified for three reasons: the syntypes of A. angulatus are lost; “A. angulatus” contains significant COI sequence divergence, potentially representing intraspecific variation, misidentifications, or as many as four undescribed species; and, with the neotype anchored via a COI barcode to a reference genome, future researchers can use this species as the “exemplar spider” in analyses, knowing that their results will not be compromised in the future by nomenclatural changes. Because all 1757 Clerckian names are spiders, should the considerably less uncontroversial question ever arise, “What’s an animal?” one objective answer is, “It’s a spider, and probably the neotype of A. angulatus.”

Taxonomy is the foundation of biology, providing the currency (species) for biological studies and the handle (epithet) to refer to them. The task at hand is urgent: a race against extinction as most species remain undescribed and expertise declines. In response, various ‘rapid taxonomy’ approaches aim to hasten descriptions. Good examples exist that genuinely accelerate the growth of taxonomic knowledge, but shortcuts risk errors that may take more time to resolve than the original work took to produce. Authors must consider (1) the total time budget, including subsequent revisions necessitated by inadequate taxonomy; (2) the taxonomic literature on regional and widespread taxa; and (3) illustration quality as a critical and enduring component of species description. Reviewers and editors must also ensure minimum standards. I discuss a recent example where shortcuts, instead of accelerating taxonomy, merely borrowed time. Anelosimus blackgardens, Anelosimus covepond, Coleosoma roadsalt—all Sherwood, Mukhida & Connor, 2026—lacked sufficient reference to genus-level revisions and established genus-level synapomorphies and are proposed here as junior synonyms of widespread theridiids: T. dilucidum Simon, 1898 syn. nov., T. melanostictum O. Pickard-Cambridge, 1876 syn. nov., and C. blandum O. Pickard-Cambridge, 1882 syn. nov., respectively. The urgent task of documenting biodiversity can be sped up through effective allocation of effort, but not without due consideration of the taxonomic literature.

Taxonomy is the foundation of biology providing the currency (species) for biological studies and the handle (epithet) to refer to them. The task at hand is urgent: a race against extinction as most species remain undescribed and expertise declines. In response various ‘rapid taxonomy’ approaches aim to hasten descriptions. Good examples exist that genuinely accelerate the growth of taxonomic knowledge, but shortcuts risk errors that may consume more time to resolve than the original work took to produce. Authors must consider: 1) the total time budget including subsequent revisions necessitated by inadequate taxonomy; 2) taxonomic literature on regional and widespread taxa; 3) illustration quality as a critical and enduring component of species description. Reviewers and editors must also ensure minimal standards. I discuss a recent example where shortcuts instead of accelerated taxonomy merely borrowed time. Anelosimus blackgardens, Anelosimus covepond, Coleosoma roadsalt—all Sherwood, Mukhida & Connor, 2026—lacked reference to genus-level revisions and established synapomorphies and are here proposed as junior synonyms of widespread theridiids: T. dilucidum Simon, 1898 syn. nov., T. melanostictum O. Pickard-Cambridge, 1876 syn. nov., add C. blandum O. Pickard-Cambridge, 1882 syn. nov., respectively. The urgent task of documenting biodiversity can be sped up, but not without due consideration of taxonomic literature.

Obligate kleptoparasitic spiders (Argyrodinae, Theridiidae) live as thieves in the webs of larger hosts. Web size has long stood above all other variables in predicting how many kleptoparasites occupy a given web, best documented in the large orb webs of golden orb-weavers (Nephilidae). Yet no study has asked whether wind-a ubiquitous force acting on every aerial web-also influences kleptoparasite abundance. Prompted by the observation that exposed, wind-buffeted webs of Trichonephila inaurata in the spiny forest of Ifaty, southwestern Madagascar, appeared to carry fewer kleptoparasites than sheltered webs of similar size, we conducted a rapid, intensive survey of 60 webs along two transects in a single afternoon. We quantified wind effect as the maximum lateral sway of the web hub over one minute, controlling for web area. We found a strong negative effect of wind-induced web movement on kleptoparasite number; trumping the positive effect of web size. Wind effect thus emerges as a previously unrecognized axis structuring the distribution of spider kleptoparasites.

Specialized predator-prey interactions are key to understanding ecological and evolutionary dynamics. We document the predatory behaviour of Vigdisia praesidens, a theridiid spider recently discovered to feed on Nasutitermes canaliculatus termites during nest repair in Marojejy National Park, Madagascar. We induced damage to N. canaliculatus mounds in Analamazaotra National Park, >500 km south of Marojejy, promptly followed by mass arrival of V. praesidens to the nest. Based on direct observations and 800+ photographs and videos, we detail the predatory strategies of Vigdisia. Spiders arrive soon after termite nest disturbance, construct a silken scaffold, then hunt by flicking sticky silk towards termites at cavity edge or by fishing: casting silk into the nest interior. Spiders capture both workers and soldiers, the latter without eliciting defensive response, then haul them to nearby vegetation via the scaffold. Spiders aggressively competed for captured prey, with captors successfully defending their catch. Pheidole spinosa ants successfully stole dead termites temporarily abandoned by Vigdisia, in a three-way species interaction similar to the one documented in Marojejy. Male Vigdisia arrived in abundance to nearby vegetation, presumably seeking mating opportunities, but did not hunt. Our findings extend the known range of V. praesidens, support its specialized predation, and underscore that inducing termite-nest damage may reveal additional spider stenophagy worldwide.

Kleptoparasitism, the theft of resources from another organism, is a widespread foraging strategy amongst animals. Many Argyrodinae cobweb spiders (Theridiidae) are obligatory kleptoparasites that have abandoned web building, relying instead on webs of larger hosts (kleptotany). Theory predicts that kleptoparasites are not randomly distributed amongst host webs and prior studies indicate that web architecture (size and tenure) and prey availability impact kleptoparasite abundance and host choice. This study is the first to examine argyrodine biology in Madagascar and provides an insight into a multi-species endemic community of spider kleptoparasites and their in-situ distribution across contrasting host webs: Nephilingis (Nephilidae, large nocturnal orb weavers), Caripetella (Pisauridae, large nocturnal sheet web builders) and Anelosimus (Theridiidae, small cathemeral social spiders constructing 3D tangle webs). We found a striking diversity of nine kleptoparasite species in six genera that, remarkably, are not conspecific with the five species that were previously known from all of Madagascar. Kleptoparasite species composition and abundance varied across the three hosts: some appeared host-specific, while others are versatile. In general, argyrodine kleptoparasites evidently discriminate amongst hosts, but differ in the degree of host preference. At the community level, we speculate that species assembly into host webs involves a complex interplay between host preference and species competitive hierarchy. Future field research should investigate this system as a community of multiple interacting species to gain a more comprehensive understanding of the rules that may govern the assembly of diverse kleptoparasites into equally variable host webs.

Obligate argyrodine kleptoparasites (Theridiidae, Araneae) exploit heterospecific spider webs for food and shelter. Argyrodine spiders are a model lineage for the study of kleptoparasitism and related strategies, yet data on the behaviour of the majority of the over 250 argyrodine species is lacking. Here, we help fill that knowledge gap by documenting the natural history of two poorly-known species. We studied Argyrodes pluto and Neospintharus baboquivari in the webs of two other cobweb spiders, the western black widow ( Latrodectus hesperus ) and Tidarren sisyphoides , across four sites in southern Arizona. Argyrodes pluto completes its life cycle in L. hesperus webs and specialises on host egg sacs, displacing them to the periphery and feeding on eggs and juveniles; this behaviour appears essential for its reproduction. Neospintharus baboquivari occurs gregariously in both host webs, gleaning small prey. In contrast, N. baboquivari is reportedly a solitary araneophage in the colonial orb webs of Philoponella oweni . We quantified egg sac displacement and describe foraging, mating, egg sac construction and interactions with parasitoids and predators. These findings reveal novel natural history information and expand our understanding of argyrodine behavioural plasticity

Obligate argyrodine kleptoparasites (Theridiidae, Araneae) exploit heterospecific spider webs for food and shelter. Argyrodine spiders are a model lineage for the study of kleptoparasitism and related strategies, yet data on the behavior of the majority of the over 250 argyrodine species is lacking. Here we help fill that knowledge gap by documenting the natural history of two poorly known species. We studied Argyrodes pluto and Neospintharus baboquivari in the webs of two other cobweb spiders, the western black widow ( Latrodectus hesperus ) and Tidarren sisyphoides , across four sites in southern Arizona. Argyrodes pluto completes its life cycle in L. hesperus webs and specializes on host egg sacs, displacing them to the periphery and feeding on eggs and juveniles; this behavior appears essential for its reproduction. Neospintharus baboquivari occurs gregariously in both host webs, gleaning small prey. In contrast, N. baboquivari is reportedly a solitary araneophage in the colonial orb webs of Philoponella oweni . We quantified egg-sac displacement, and describe foraging, mating, egg-sac construction, and interactions with parasitoids and predators. These findings reveal novel natural history information and expand our understanding of argyrodine behavioral plasticity

Among terrestrial animals, spiders exhibit the most striking examples of sexual size dimorphism (SSD) but better understanding of its evolution requires improved taxonomy and phylogeny. Many sexually dimorphic spiders lack adequate description, phylogenetic placement, and natural history observations. In South Africa, we documented the natural history of a poorly known spider, Megaraneus gabonensis (Lucas, 1858), with extreme, female-biased SSD (eSSD, female:male approximately 4:1). Here, we redescribe M. gabonensis, place Megaraneus Lawrence, 1968 phylogenetically for the first time, assess whether the observed eSSD represents an independent evolutionary origin, and test whether the macroevolutionary pattern is better explained by male dwarfism or female gigantism. The recovered phylogenetic placement of Megaraneus in the araneid ‘backobourkiines’, a clade previously considered as restricted to East Asia and Australasia, extends the range of this clade to the Afrotropics. We find that eSSD was present in the common ancestor of the ‘backobourkiines’, with further increases in female body length occurring independently in Megaraneus, Backobourkia Framenau, Dupérré, Blackledge & Vink, 2010, and the currently misplaced Parawixia dehaani (Doleschall, 1859). We conclude that the evolution of eSSD reflects a complex pattern of sex-specific size changes across spider phylogeny, but that in Megaraneus it results from female gigantism.