Beyond polyphagy and opportunism: natural prey of hunting spiders in the canopy of apple trees

Spiders (Araneae) form abundant and diverse assemblages in agroecosystems such as fruit orchards, and thus might have an important role as natural enemies of orchard pests. Although spiders are polyphagous and opportunistic predators in general, limited information exists on their natural prey at both species and community levels. Thus, the aim of this study was to assess the natural prey (realized trophic niche) of arboreal hunting spiders, their role in trophic webs and their biological control potential with direct observation of predation events in apple orchards. Hunting spiders with prey in their chelicerae were collected in the canopy of apple trees in organic apple orchards in Hungary during the growing seasons between 2013 and 2019 and both spiders and their prey were identified and measured. Among others, the composition of the actual (captured by spiders) and the potential (available in the canopy) prey was compared, trophic niche and food web metrics were calculated, and some morphological, dimensional data of the spider-prey pairs were analyzed. Species-specific differences in prey composition or pest control ability were also discussed. By analyzing a total of 878 prey items captured by spiders, we concluded that arboreal hunting spiders forage selectively and consume a large number of apple pests; however, spiders’ beneficial effects are greatly reduced by their high levels of intraguild predation and by a propensity to switch from pests to alternative prey. In this study, arboreal hunting spiders showed negative selectivity for pests, no selectivity for natural enemies and positive selectivity for neutral species. In the trophic web, the dominant hunting spider taxa/groups (Carrhotus xanthogramma, Philodromus cespitum, Clubiona spp., Ebrechtella tricuspidata, Xysticus spp. and ‘Other salticids’) exhibit different levels of predation on different prey groups and the trophic web’s structure changes depending on the time of year. Hunting spiders show a high functional redundancy in their predation, but contrary to their polyphagous nature, the examined spider taxa showed differences in their natural diet, exhibited a certain degree of prey specialization and selected prey by size and taxonomic identity. Guilds (such as stalkers, ambushers and foliage runners) did not consistently predict either prey composition or predation selectivity of arboreal hunting spider species. From the economic standpoint, Ph. cespitum and Clubiona spp. were found to be the most effective natural enemies of apple pests, especially of aphids. Finally, the trophic niche width of C. xanthogramma and Ph. cespitum increased during ontogeny, resulting in a shift in their predation. These results demonstrate how specific generalist predators can differ from each other in aspects of their predation ecology even within a relatively narrow taxonomic group.

Evolution of the male palp morphology of the orb-weaver hunting spider Chorizopes (Araneae : Araneidae) revisited on a new phylogeny of Araneidae, and description of a third species from Madagascar

The genus Chorizopes (Araneae, Araneidae) includes over two dozen species of webless araneids found mainly in the Indomalayan region. They are characterised by a distinctive bulbous carapace and a specialised foraging behaviour: preying on other orb-weavers. Chorizopes casictones, sp. nov. (Araneae, Araneidae) is described based on specimens collected in northern Madagascar. This species represents the third member of Chorizopes known from the island. We conducted a total-evidence analysis based on morphology, behaviour and nucleotide sequence data, including this taxon for the first time. The palpal morphology of the male of Chorizopes is illustrated in detail. We discuss the palpal homologies and the evolution of araneid palpal sclerites based on the newly inferred family phylogenetic tree.

Markerless tracking suggests a tactile sensing role for forelegs of Dolomedes spiders during locomotion

Summary statementWe developed a machine vision technique for markerless tracking of locomotion in the spider Dolomedes aquaticus. Gait analysis suggests that each pair of legs plays a specific role in locomotion.AbstractBecause of their rigid exoskeleton with relatively simple joint mechanics, arthropods can provide useful models for studying the sensory-neural and mechanical design principles of agile animal locomotion. Gait analysis usually requires attaching markers or manually identifying reference points in video frames, which can be time consuming and inaccurate, especially with small animals. Here we describe a markerless motion capture technique and its application to gait analysis in the New Zealand semi-aquatic hunting spider, Dolomedes aquaticus. Our machine vision approach uses a model of the spider’s skeleton to infer the location of the centre of mass and the configuration of the skeleton in successive video frames. We found that stride length and frequency are correlated with running speed. Inter-limb coordination during the gait cycle suggests that different legs have specialized roles in locomotion. Phase relationships among the six hindmost legs exhibit an alternating tripod gait, as in hexapod insects. The middle two leg pairs appear to be primarily responsible for generating thrust, while the hind legs contribute more to stability. The front legs are not phase-coupled to the other legs and appear to be used as tactile probes during locomotion. Our machine vision approach has the potential to automate arthropod gait analysis, making it faster and easier. Our results indicate how specialization of limb function may contribute to locomotor efficiency and agility of a specialized hunting spider, and how arthropod design principles may contribute to developing efficient, agile legged robots.

The new Australian Ground-Hunting Spider Genus Leichhardteus (Araneae: Corinnidae)

The new corinnid genus Leichhardteus is described including eight new species from eastern Australia: L. albofasciatus, L. badius, L. bimaculatus, L. conopalpis, L. garretti, L. kroombit, L. reinhardi and L. terriirwinae. A key to the species is provided. Only one species is widely distributed, four species are collected only in rainforests and three species are recorded only from a single location.

Airflow elicits a spider's jump towards airborne prey. I. Airflow around a flying blowfly

The hunting spider Cupiennius salei uses airflow generated by flying insects for the guidance of its prey-capture jump. We investigated the velocity field of the airflow generated by a freely flying blowfly close to the flow sensors on the spider's legs. It shows three characteristic phases (I–III). (I) When approaching, the blowfly induces an airflow signal near the spider with only little fluctuation (0.013 ± 0.006 m s −1 ) and a strength that increases nearly exponentially with time (maximum: 0.164 ± 0.051 m s −1 s.d.). The spider detects this flow while the fly is still 38.4 ± 5.6 mm away. The fluctuation of the airflow above the sensors increases linearly up to 0.037 m s −1 with the fly's altitude. Differences in the time of arrival and intensity of the fly signal at different legs probably inform the spider about the direction to the prey. (II) Phase II abruptly follows phase I with a much higher degree of fluctuation (fluctuation amplitudes: 0.114 ± 0.050 m s −1 ). It starts when the fly is directly above the sensor and corresponds to the time-dependent flow in the wake below and behind the fly. Its onset indicates to the spider that its prey is now within reach and triggers its jump. The spider derives information on the fly's position from the airflow characteristics, enabling it to properly time its jump. The horizontal velocity of the approaching fly is reflected by the time of arrival differences (ranging from 0.038 to 0.108 s) of the flow at different legs and the exponential velocity growth rate (16–79 s −1 ) during phase I. (III) The air flow velocity decays again after the fly has passed the spider.