Male opportunistic mating increases with intensity of female sexual cannibalism in three web-building spiders

Sexual conflict is common in animals, and female sexual cannibalism represents an extreme form of sexual conflict. Males in many species have evolved a variety of strategies to circumvent or decrease the risk of female sexual cannibalism. Opportunistic mating, by which a male mates with a female when she is disturbed or when she is feeding or undertaking moulting, is one of such kinds of strategies, and widely occurs in many animals, especially in spiders. However, whether the occurrence of male opportunistic mating depends on the intensity of female sexual cannibalism remains largely unexplored. We predicted a positive correlation between them. In this study, we tested this prediction by performing a series of mating trials in the laboratory using three species of web-building spiders with different intensities of female sexual cannibalism: Nephila pilipes, Nephilengys malabarensis, and Parasteatoda tepidariorum. We found that the occurrence of male opportunistic mating was positively, though not statistically significantly, correlated with the intensity of female sexual cannibalism, thus supporting our hypothesis. All together, we provide evidence that male opportunistic mating may have evolved to respond to the selection pressure posed by female sexual cannibalism.

Cross-Ecosystem Linkages: Transfer of Polyunsaturated Fatty Acids From Streams to Riparian Spiders via Emergent Insects

Polyunsaturated fatty acids (PUFAs) are essential resources unequally distributed throughout landscapes. Certain PUFAs, such as eicosapentaenoic acid (EPA), are common in aquatic but scarce in terrestrial ecosystems. In environments with low PUFA availability, meeting nutritional needs requires either adaptations in metabolism to PUFA-poor resources or selective foraging for PUFA-rich resources. Amphibiotic organisms that emerge from aquatic ecosystems represent important resources that can be exploited by predators in adjacent terrestrial habitats. Here, we traced PUFA transfer from streams to terrestrial ecosystems, considering benthic algae as the initial PUFA source, through emergent aquatic insects to riparian spiders. We combined carbon stable isotope and fatty acid analyses to follow food web linkages across the ecosystem boundary and investigated the influence of spider lifestyle (web building vs. ground dwelling), season, and ecosystem degradation on PUFA relations. Our data revealed that riparian spiders consumed considerable amounts of aquatic-derived resources. EPA represented on average 15 % of the total fatty acids in riparian spiders. Season had a strong influence on spider PUFA profiles, with highest EPA contents in spring. Isotope data revealed that web-building spiders contain more aquatic-derived carbon than ground dwelling spiders in spring, although both spider types had similarly high EPA levels. Comparing a natural with an anthropogenically degraded fluvial system revealed higher stearidonic acid (SDA) contents and Σω3/Σω6 ratios in spiders collected along the more natural river in spring but no difference in spider EPA content between systems. PUFA profiles of riparian spiders where distinct from other terrestrial organism and more closely resembled that of emergent aquatic insects (higher Σω3/Σω6 ratio). We show here that the extent to which riparian spiders draw on aquatic PUFA subsidies can vary seasonally and depends on the spider’s lifestyle, highlighting the complexity of aquatic-terrestrial linkages.

Evidence for competition and cannibalism in wormlions

Trap-building predators, such as web-building spiders and pit-building antlions, construct traps to capture their prey. These predators compete over sites that either enable the construction of suitable traps, are prey rich, or simply satisfy their abiotic requirements. We examined the effect of intraspecific competition over suitable space in pit-building wormlions. As expected, the ability of wormlions to select their favorable microhabitats—shaded or deep sand over lit or shallow sand—decreased with increasing density. Favorable microhabitats were populated more frequently by large than by small individuals and the density of individuals in the favorable microhabitat decreased with their increase in body mass. The advantage of large individuals in populating favorable microhabitats is nevertheless not absolute: both size categories constructed smaller pits when competing over a limited space compared to those constructed in isolation. The outcome of competition also depends on the type of habitat: deep sand is more important for large wormlions than small ones, while shade is similarly important for both size classes. Finally, in contrast to previous reports, cannibalism is shown here to be possible in wormlions. Its prevalence however is much lower compared to that documented in other trap-building predators. Our findings show that the advantage of large individuals over small ones should not be taken for granted, as it can depend on the environmental context. We present suggestions for the relative lack of competitive advantage of large wormlion individuals compared to other trap-building predators, which may stem from the absence of obvious weaponry, such as sharp mandibles.

Distinct movement patterns generate stages of spider web-building

The geometric complexity and stereotypy of spider webs have long generated interest in their algorithmic origin. Like other examples of animal architecture, web construction is the result of several assembly phases that are driven by distinct behavioral stages coordinated to build a successful structure. Manual observations have revealed a range of sensory cues and movement patterns used during web construction, but methods to systematically quantify the dynamics of these sensorimotor patterns are lacking. Here, we apply an analytical pipeline to quantify web-making behavior of the orb-weaver Uloborus diversus. Position tracking revealed stereotyped stages of construction that could occur in typical or atypical progressions across individuals. Using an unsupervised clustering approach, we identified general and stage-specific leg movements. A Hierarchical Hidden Markov Model revealed that stages of web-building are characterized by stereotyped sequences of actions largely shared across individuals, regardless of whether these stages progress in a typical or atypical fashion. Web stages could be predicted based on action-sequences alone, revealing that web-stages are a physical manifestation of underlying behavioral phases.

Know your foe: synanthropic spiders are deterred by semiochemicals of European fire ants

Many ants prey on spiders, suggesting that web-building spiders may avoid micro-locations near ant colonies or frequented by foraging ants. Here we tested the hypothesis that ant-derived semiochemicals deter synanthropic spiders. To generate stimuli, we exposed filter paper for 12 h to workers of European fire ants, Myrmica rubra, black garden ants, Lasius niger, or western carpenter ants, Camponotus modoc , and then offered select urban spiders in three-chamber olfactometer bioassays a choice between ant-exposed filter paper and unexposed control filter paper. Semiochemical deposits of M. rubra , but not of L. niger or C. modoc , had a significant deterrent effect on subadults of the false black widow, Steatoda grossa, the black widow, Latrodectus hesperus , and the hobo spider, Eratigena agrestis, as well as a moderate (but statistically not significant) deterrent effect on the cross spider, Araneus diadematus . The deterrent effect caused by semiochemical deposits of M. rubra may be attributable to the aggressive nature and efficient foraging of M. rubra in its invaded North American range, exerting selection pressure on community members to recognize M. rubra semiochemicals and to avoid micro-locations occupied by M. rubra .

Dynamic environments do not appear to constrain spider web building behaviour

Many laboratory experiments demonstrate how orb-web spiders change the architecture of their webs in response to prey, surroundings and wind loading. The overall shape of the web and a range of other web parameters are determined by frame and anchor threads. In the wild, unlike the lab, the anchor threads are attached to branches and leaves that are not stationary but move, which affects the thread tension field. Here we experimentally test the effect of a moving support structure on the construction behaviour and web-parameters of the garden cross spider Araneus diadematus. We found no significant differences in building behaviour between rigid and moving anchors in total time spent and total distance covered nor in the percentage of the total time spent and distance covered to build the three major web components: radials, auxiliary and capture spirals. Moreover, measured key parameters of web-geometry were equally unaffected. These results call for re-evaluation of common understanding of spider webs as thread tensions are often considered to be a major factor guiding the spider during construction and web-operation.

Shifts in morphology, gene expression, and selection underlie web loss in Hawaiian Tetragnatha spiders

Background A striking aspect of evolution is that it often converges on similar trajectories. Evolutionary convergence can occur in deep time or over short time scales, and is associated with the imposition of similar selective pressures. Repeated convergent events provide a framework to infer the genetic basis of adaptive traits. The current study examines the genetic basis of secondary web loss within web-building spiders (Araneoidea). Specifically, we use a lineage of spiders in the genus Tetragnatha (Tetragnathidae) that has diverged into two clades associated with the relatively recent (5 mya) colonization of, and subsequent adaptive radiation within, the Hawaiian Islands. One clade has adopted a cursorial lifestyle, and the other has retained the ancestral behavior of capturing prey with sticky orb webs. We explore how these behavioral phenotypes are reflected in the morphology of the spinning apparatus and internal silk glands, and the expression of silk genes. Several sister families to the Tetragnathidae have undergone similar web loss, so we also ask whether convergent patterns of selection can be detected in these lineages. Results The cursorial clade has lost spigots associated with the sticky spiral of the orb web. This appears to have been accompanied by loss of silk glands themselves. We generated phylogenies of silk proteins (spidroins), which showed that the transcriptomes of cursorial Tetragnatha contain all major spidroins except for flagelliform. We also found an uncharacterized spidroin that has higher expression in cursorial species. We found evidence for convergent selection acting on this spidroin, as well as genes involved in protein metabolism, in the cursorial Tetragnatha and divergent cursorial lineages in the families Malkaridae and Mimetidae. Conclusions Our results provide strong evidence that independent web loss events and the associated adoption of a cursorial lifestyle are based on similar genetic mechanisms. Many genes we identified as having evolved convergently are associated with protein synthesis, degradation, and processing, which are processes that play important roles in silk production. This study demonstrates, in the case of independent evolution of web loss, that similar selective pressures act on many of the same genes to produce the same phenotypes and behaviors.