Contents
The Journal of Arachnology - 2024
Volume 52 Number 1 - pp. n/a

Featured Articles

An assessment of the mechanosensory responses of peg sensilla on scorpion pectines

Abstract

Scorpions possess midventral touch/taste organs called pectines, which may be important for learning the nuances of the substrate during navigation as well as the detection of pheromones, spermatophores, and food. The pectines possess thousands of minute structures called peg sensilla that are responsive to both chemicals and mechanical deflection of the peg shaft. While much is known about the chemical responsiveness of the pegs, very little is known about their mechanosensory properties. Here we ask if the peg mechanosensory response is “all-or-nothing” or graded depending on the intensity of stimulation. We made electrophysiological recordings of neural activity from individual peg sensilla while deflecting the peg to elicit apparent mechanosensory responses. Our records show the presence of a rapid firing (.100 Hz), quickly adapting waveform that is indicative of a mechanoreceptor and appears to be independent of previously identified chemo-responsive cells. We tested mechanosensory response dynamics in two ways. The first test focused on a shorter-duration touch versus a longer-duration touch, while the second focused on a smaller deflection versus a larger deflection. Both pairs of stimulations (short vs long touch; small vs large touch) produced repeatable and statistically distinct responses in terms of spiking frequency. These results indicate the mechanosensory responses of peg sensilla are graded, which sheds light on the textural resolvability of the pectines and informs models of the type of information that scorpions obtain while assessing surfaces in their environment.

The natural history of Argiope anasuja (Araneae: Araneidae) with special reference to their mating behavior

Abstract

Argiope anasuja Thorell, 1887 is a species of orb-weaving spider found in tropical and subtropical regions of the world. Despite its widespread distribution, little is known about its natural history and mating behavior. In this study, I describe some aspects of the natural history and mating behaviors of A. anasuja in captivity and in natural habitats. The spiders were found to occupy less shady areas close to water bodies, building their webs between 50 to 200 cm above the ground level. Web decorations were common in female webs, primarily in the central area, and consisted of dense zigzag bands of silk. A significant difference in copulation duration was observed between cannibalized and non-cannibalized individuals. Furthermore, a significant difference in copulation duration was found between individuals in natural environments and laboratory conditions. However, no significant difference was observed in courtship duration between individuals in natural environment and laboratory conditions. Understanding the mating behaviors of A. anasuja is important because they play a key role in determining the success of mating. This study highlights the natural history and mating behavior of A. anasuja, contributing to our understanding of this species in Sri Lanka.

Supplementary Materials

Population fluctuations in Costa Rican golden silk orbweavers (Trichonephila clavipes)

Abstract

Globally, arthropod populations are declining at alarming rates, but the causes are rarely understood. Our research details and examines possible causes for fluctuations in the size of a Costa Rican population of golden silk orbweaver spiders (Trichonephila clavipes (Linnaeus, 1767)). Over a seven-year period from 2013–2019, we noted a sharp decline and then partial recovery of the study population during the wet season (June, July), but then failed to locate any spiders during a brief survey in June 2022 when they would otherwise be abundant. We monitored webs daily during 2013–2019 to test whether variation in prey capture, competitors, female size, male availability, predation, temperature, or rainfall related to population fluctuations. We were unable to explain T. clavipes population trends with the collected data. Future studies are needed to determine whether the extremely low population densities we witnessed in 2017 and 2022 can be interpreted as the lowest values of this species' normal population fluctuation cycle or whether these extremes are part of a long-term spider decline.

Supplementary Materials

Use of geometric morphometrics to distinguish trapdoor spider morphotypes (Mygalomorphae: Anamidae: Proshermacha): a useful tool for mygalomorph taxonomy

Abstract

Taxonomic studies have evolved greatly since their early stages and new techniques have been incorporated to improve species descriptions. Those involving the comparison of traits, either quantitatively or qualitatively, can be difficult because the identification of a species must rely on the experience of the observer and errors can occur when cryptic species are involved. Molecular methods have been used to fill these gaps, but morphological methods are still needed to match the recognized molecular species with an adequate taxonomic description. Focusing on the trapdoor spider genus Proshermacha Simon, 1908, we provide a case study using Geometric Morphometrics (GM) techniques to identify morphological divergence between species found in the south-western Australia region. We used GM to identify morphological divergence from museum-preserved specimens by examining shape variation of sexual characters from 39 male specimens from five different localities on a single mountain range. Variation in the shape of both the palpal bulb and tibia provided strong evidence to distinguish two morphotypes, while metatarsus shape showed fewer between-locality differences. Our results illustrate the utility of GM methods, when applied to a few taxonomically-informative structures, as a quantitative species delimitation tool for taxonomic studies.

Spinneret spinning field ontogeny and life history observations in the spider Palpimanus uncatus Kulczyński, 1909 (Araneae: Palpimanidae)

Abstract

As in other Palpimanidae, two pairs of posterior spinnerets present in typical Araneomorphae are vestigial in Palpimanus uncatus Kulczyński, 1909, with only the anterior lateral spinneret (ALS) pair prominent. Nevertheless, in late juvenile and adult females, spigots appear in the ancestral posterior spinneret region (PS). Consistent with these spigots serving cylindrical silk glands, females construct substantial egg sacs. While juveniles and adults exhibit a compressed PS, in postembryos it is fully extended. Piriform silk gland (PI) spigots form a linear array on ALSs from the 1st stadium, increasing in number during ontogeny by addition of PIs of the tartipore-accommodated (T-A) subtype (i.e., functional during proecdyses). The number of T-A PIs added from one stadium to the next and locations occupied by their spigots often exhibit a stereotypic pattern, especially consistent in early instars. The number of non-T-A PIs remains constant through ontogeny from the 1st stadium: one per ALS rather than the two per ALS inferred in a few araneoids. The secondary major ampullate silk gland (2° MaA) spigot, primitively uni-shafted among araneomorphs, has become modified into a multi-shafted spigot with extended base, the number of shafts increasing during ontogeny. However, the multiple ducts that connect to the shafts continue to be accommodated during proecdysis by a single enormous tartipore. Sexual dimorphism is present, with late stadium females having greater numbers of T-A PI spigots and 2° MaA spigot shafts. Observations are presented pertaining to feeding behavior, sexual cannibalism (absent), habitat, winter diapause, numbers of molts, and longevity.

Supplementary Materials

Developmental asynchrony: A potential cost of extreme sexual size dimorphism in seasonal environments

Abstract

Female-biased sexual size dimorphism reaches an extreme in the genus Trichonephila Dahl, 1911 (Araneae: Nephilidae). Large female size is accomplished through the addition of juvenile instars and is likely the result of fecundity selection. However, additional instars in only one sex could result in significant developmental asynchrony. To minimize asynchrony, males should grow more slowly and females more rapidly. To test this prediction, we reared spiders from six disjunct populations of Trichonephila clavipes (Linnaeus, 1767) on three different diets, inducing variation in growth rates. Males on all diets grew more slowly than females, but still matured significantly earlier. In the field, mature males cohabit with juvenile females, and in the laboratory, mean male age at maturity corresponded to the age at which females achieve the minimum size observed to host males. However, a significant fraction of males mature earlier than concurrently-emergent females reach that minimum size—and before any females reached sexual maturity. Thus, early-maturing males may perish before reproducing, especially in temperate environments. We propose that (1) sexual size dimorphism and early male maturation evolved in tropical environments characteristic for T. clavipes, where adult females are found all year round. Univoltinism is likely a secondary trait in highly seasonal environments. Seasonal populations are likely mixes of local individuals and recent tropical immigrants, which could mitigate the loss of early males and retain this apparently maladaptive trait outside the tropics. (2) Environmental variation generates asynchronous emergence at the beginning of the growing season, allowing later-emergent males to mate with early-emergent females.

Revalidation of Parasadocus Mello-Leitão, 1927 (Opiliones: Gonyleptidae), with its transference from Pachylinae to Roeweriinae

Abstract

A study was conducted to reevaluate the phylogenetic position of Discocyrtus catharinensis (Mello-Leitão, 1923) in the family Gonyleptidae Sundevall, 1833. Based on a maximum parsimony cladistic analysis of 22 terminal taxa and 92 morphologic characters (1766 scorings), (1) the genus Parasadocus Mello-Leitão, 1927 is restored from the synonym list of Discocyrtus Holmberg, 1878, and transferred from Pachylinae Sørensen, 1884 to Roeweriinae Carvalho & Kury, 2018, and (2) the specific combination Parasadocus catharinensis (Mello-Leitão, 1923) is also restored. P. catharinensis comb. rest. is here redescribed, and its geographical record of “Itatiaia” is contested. Finally, a lectotype is proposed here to solve questions about the type series of “Lycomedicus brasiliensis Soares & Soares 1949” (junior synonym of P. catharinensis comb. rest.).

Short Communications

Facultative iteroparity in a semelparous social spider, Stegodyphus dumicola (Araneae: Eresidae)

Abstract

Organisms can optimize their reproductive success by differential resource allocation. When adult survival is low, investment of all resources into a single reproductive event can be beneficial, favouring a semelparous strategy. In the spider genus Stegodyphus Simon, 1873 (Eresidae), all species are considered semelparous, based on observations of ancestral subsocial species. However, derived social species show task differentiation and helping by non-reproducing females. This could facilitate a facultative iteroparous strategy by providing sufficient resources to meet the threshold for repeated reproduction. We investigated the reproductive behaviour of groups with single breeding females in the cooperative breeding Stegodyphus dumicola Pocock, 1898. We found that mothers can depart from a strictly semelparous lifestyle by producing more than one clutch. The facultative iteroparity in S. dumicola may enhance colony growth and survival, and act as a mechanism to maintain sociality.

What's for dinner? Prey consumption by Neotropical scorpions across contrasting environments

Abstract

The overall assumption that scorpions are generalist predators is often based on conspicuous information from the literature. Here, we compiled a list of prey consumption by scorpions from different environments in Brazil to produce a documentation of predations by this taxon. This list is based on observations made under natural conditions in Atlantic Forest, Caatinga, and Cerrado formations. We compiled 135 predation instances including 11 scorpion species from field work through 14 years. The observed diet composition of the scorpions was mainly based on cockroaches, crickets, grasshoppers, spiders, and other scorpions. Such data highlights the generalist diet and cannibalism of scorpions with many cannibalistic events among the records of intraguild predation. Overall, this study broadens the knowledge of the diet composition of Brazilian scorpions under natural conditions.

Living with the enemy: behavioral study of Myrmecicultor chihuahuensis Ramírez, Grismado & Ubick (Araneae: Myrmecicultoridae)

Abstract

The spider, Myrmecicultor chihuahuensis Ramírez, Grismado & Ubick 2019 is a myrmecophage. In an earlier study, we found that the spider's cuticular hydrocarbon (CHC) profile overlapped with that of its prey. In the present study, we photographed nests of Novomessor ants to determine whether these spiders live inside the ant nest with their prey or in the vicinity of the colony. We set up two 35 mm cameras over two main entrances of one nest of N. albisetosus (Mayr 1886) such that images would be captured of the ant colony surface (including the entrances) every 15 – 60 sec over five consecutive nights. These images included five showing M. chihuahuensis directly at one of the nest entrances investigating dead ants. The spider was not seen away from the entrance. This study provides evidence suggesting that this myrmecophage may live inside the colony with the ants it eats.

Erratum

Erratum: Spiders feeding on vertebrates

Erratum: Morphology of the male reproductive tract of the harvestman Mischonyx cuspidatus (Roewer, 1913)