Beyond spiders: Harvestmen as predators of anurans in the Neotropics

Arthropods are traditionally viewed as invertebrate prey and predators for vertebrates, a paradigm increasingly challenged, especially among arachnids. While spiders are documented frog predators, the capacity of other groups like harvestmen (Opiliones) has remained anecdotal. We report novel field observations of anuran predation by multiple Cranaidae harvestmen species across Neotropical localities. These records include active capture and consumption of live frogs, demonstrating their role as opportunistic mesopredators. Alongside new spider predation records, our findings substantially advance Opiliones ecology by confirming vertebrate predation occurs in multiple, disjunct species. This reveals vertebrate consumption among arachnids is more taxonomically widespread than recognized, underscoring the need to include Opiliones as potential predators in Neotropical trophic models. Beyond spiders: Harvestmen as predators of anurans in the Neotropics Esteban Calvache 1, 2, 3, Osvaldo Villarreal 4, 5 *, Cynthia Ávila-Rojas 6, 7, Alexander G. Bentley 3, 8, 9, Kenny Brito 10, Chiara Correa-Zanotti 2, Maida Gutiérrez-Arboleda 7, Katherine Iñiguez 10, Juan Carlos Narváez 11, Lizardo Proaño 11, Mateo Reyes-Vizcaíno 10, Luís Fernando García 12 * 1 Departamento de Investigación y Biología, Mashpi Lodge, Pacto, Ecuador. 2 Fundación URU, Quito, Ecuador. 3 Instituto Nacional de Biodiversidad, Quito, Ecuador. 4 Centro de Ecología, Instituto Venezolano de Investigaciones Científicas (IVIC), km 11 carretera Panamericana, Altos de Pipe, edo. Miranda 1204-A, Venezuela. 5 Instituto y Museo del Instituto de Zoología Agrícola, Facultad de Agronomía, Universidad Central de Venezuela, Apartado 4579, Maracay 2101, Venezuela. 6 Grupo de Investigación Ecdysis, Universidad del Quindío. Armenia, Colombia. 7 Estudiante del programa de Biología, Universidad del Quindío, Armenia, Colombia. 8 Fundación Waska Amazonia, Mera, Ecuador. 9 Red de Investigación del Corredor de Conectividad Llanganates-Sangay. 10 Centro Jambatu de Investigación y Conservación de Anfibios, Fundación Jambatu, Quito, Ecuador. 11 Departamento de expediciones, Mashpi Lodge, Pacto, Ecuador. 12 CENUR Noreste, Sede Rivera, Universidad de la República, Rivera, Uruguay. *Correspondence: Luís F. García ([email protected]) and Osvaldo Villarreal ([email protected]) Running Title: Frog predation by harvestmen and spiders

Arthropods are traditionally viewed as invertebrate prey and predators for vertebrates, a paradigm increasingly challenged, especially among arachnids. While spiders are documented frog predators, the capacity of other groups like harvestmen (Opiliones) has remained anecdotal. We report novel field observations of anuran predation by multiple Cranaidae harvestmen species across Neotropical localities. These records include active capture and consumption of live frogs, demonstrating their role as opportunistic mesopredators. Alongside new spider predation records, our findings substantially advance Opiliones ecology by confirming vertebrate predation occurs in multiple, disjunct species. This reveals vertebrate consumption among arachnids is more taxonomically widespread than recognized, underscoring the need to include Opiliones as potential predators in Neotropical trophic models. Keywords: Anurophagy, trophic interactions, Neotropical food webs, Vertebrate consumption, Arachnida

Among terrestrial predators, arthropods frequently occupy multiple trophic positions, serving as both intermediate predators and top-level carnivores in various ecosystems (Garrick et al., 2019; Miller-ter Kuile et al., 2022). For example, spiders’ diet consists largely of insects and other arthropods (Nyffeler and Birkhofer, 2017). Due to their relatively small size compared to vertebrates, arthropods are often portrayed mainly as prey of vertebrates, reinforcing a unidirectional view of trophic interactions between these groups (Mooney et al., 2010). As a result, most ecological literature depicts arthropods as occupying intermediate levels in food webs, with limited emphasis on their role as predators of vertebrates (McCormick and Polis, 1982). However, such simplifications overlook numerous documented cases in which arthropods consume vertebrates. A recent review by Valdez (2020) reported that several arthropod taxa, namely insects, arachnids, centipedes, and crustaceans, are capable of preying upon all major vertebrate classes. Remarkably, the same author compiled more than 1,300 records of vertebrate predation by arthropods. Although most records correspond to anecdotal observations, this suggests that the phenomenon may be more common than previously considered. Within arthropods, arachnids emerge as particularly significant vertebrate predators, and several arachnid orders have been reported feeding on mammals, reptiles, amphibians, and fish (Nyffeler and Altig, 2020; Nyffeler and Gibbons, 2021; Nyffeler and Knörnschild, 2013; Nyffeler and Pusey, 2014; Valdez, 2020). Spiders represent the most frequently documented arachnid predators of vertebrates, likely due to their diverse foraging strategies, large body size in some taxa, and greater detectability, which increases observational opportunities. In contrast, documented cases from other arachnid orders remain scarce and often incomplete. A notable early record is the report by (Hayes, 1983) of the harvestman Prionostemma frontale (Banks, 1909) preying on the eggs of the glass frog Hyalinobatrachium fleischmanni (Boettger, 1893). While other interactions between harvestmen and post-metamorphic anurans have been inferred (e.g., Villarreal et al., 2008; Menegucci et al., 2020), direct observations and comprehensive assessments of this behavior are lacking. Reports of amphibians as prey for non-spider arachnids are thus still limited and merit further investigation (Valdez, 2020). Anurans hold key positions in Neotropical food webs, functioning both as predators and prey (Souza et al., 2023b). As predators, they consume a wide range of animal taxa, including vertebrates; however, their diets predominantly consist of invertebrates, especially arthropods (Solé and Rödder, 2009). Conversely, their trophic role can be reversed, as anurans may fall prey to several arthropod groups at different life stages. For example, tadpoles are frequently targeted by aquatic insects such as odonate nymphs (Phuge et al., 2020) and arachnids (Babangenge et al., 2019; Mamede and Nomura, 2021), whereas juvenile and adult frogs may be captured and consumed by large arthropods, including insects and arachnids (Babangenge et al., 2019; Nyffeler and Altig, 2020; Souza et al., 2023a; Valdez, 2020). Despite increasing reports of arthropods as natural enemies of anurans, available data remain limited and strongly biased toward easily observable taxa (McCormick and Polis, 1982; Valdez, 2020). This underscores the importance of documenting interactions involving cryptic or understudied arthropod groups that may also play significant roles as anuran predators. Here, we document new records of arachnids preying upon anurans across multiple localities in South America. In addition to expanding known cases involving spiders, which are recognized as frequent anuran predators, we provide compelling evidence of predatory interactions by multiple species within the harvestman family Cranaidae, including observations of active capture and consumption of live frogs, a particularly notable contribution given the limited knowledge of their trophic ecology (Acosta and Machado, 2007). These findings substantiate earlier inferences of predation (e.g., Villarreal et al., 2008; Menegucci et al., 2020) and confirm the capacity of Opiliones to act as vertebrate predators. Records of spider predation are relatively common in nature, and numerous comprehensive reviews have compiled abundant cases (e.g., Valdez, 2020). In contrast, such trophic interactions are far less documented for harvestmen, with existing records being sporadic and representing a greater novelty. Therefore, our study provides new observations of trophic interactions between spiders and harvestmen with frogs, along with an updated compilation of literature records of harvestman–frog interactions. Finally, we provide a global distribution map depicting anurophagy in harvestmen.

The spider records were sourced from iNaturalist and original field observations, whereas harvestmen records were compiled from original field observations and a comprehensive review of the literature. All specimens were identified through photographic records to the highest possible taxonomic level, following the corresponding taxonomic keys and consulting experts from the different groups. Although direct measurements were unavailable, when possible we analyzed prey:predator’s size ratio by obtaining measurements in pixels. Images were analyzed using ImageJ software (Rueden et al., 2017). Figures were created in Adobe Photoshop CC 2017. The global distribution map was created using R v 4.5.2 (R Core Team, 2025).

. Feeding Records 3.1.1.Harvestmen On 5 February 2020 at 20:00 h, during a field survey on the “Verrugosa” trail (0.1665°, -78.8774°, 874 m a.s.l.) in the Mashpi-Tayra Wildlife Refuge, Ecuador, a male Holocranaus aff. angulus (Roewer, 1932) was observed preying on a live Pristimantis sp. (Fig. 1A, B). The harvestman, approximately 10 mm in dorsal scute length, held the frog by its hind limb about 75 cm above the ground on a leaf. Although prey capture was not recorded, the frog remained alive during partial consumption. A similar event was recorded on 6 August 2022 at 01:15 h by AGB along the Cueva del Tigre trail (-1.4259°, -78.0361°, 1,235 m a.s.l.) in the José Fiallos finca within the Ecominga Foundation’s Anzu Reserve, Ecuador. A large Phareicranaus lucifer (Pinto-da-Rocha & Kury, 2003), approximately 8 mm in body length, was observed gripping a partially eaten clown frog, Dendropsophus parviceps (Boulenger, 1882) (Fig. 1C), on a leaf about 80 cm above the ground, near a shallow pond above the Cueva del Tigre cave. Only half of the frog remained at the time of observation. In another case, an individual of Phareicranaus sp. was observed feeding on an Atelopus sp. ( spumarius-pulcher complex) (Fig. 1D), on 3 September 2025 at 20:15 h along the Río Napinaza (Quebrada Napinaza) in Limón Indanza, Morona Santiago Province, Ecuador (-2.9228°, -78.4089°, 1,146 m a.s.l.). The event occurred on the foliage of understory vegetation approximately 150 cm above the ground near the riverbank. The harvestman was actively consuming the frog when first observed and moved away shortly afterward, leaving the body headless and partially consumed. The frog was part of a reintroduced population of Atelopus sp. ( spumarius-pulcher complex) within its historical range, from which the species had previously disappeared. This frog species is currently undergoing formal description (Coloma pers. com.). Additionally, on 25 July 2025 at 21:00 h in the village of Pedregales Alto, Génova, Quindío, Colombia (4.173047° N, -75.734339° W; 2,100 m a.s.l.), an individual of the harvestman Phareicranaus sp. was observed preying on a Pristimantis sp. (Fig. 1E, F). The harvestman was holding the live frog by its right hind limb. The harvestman was similar in size to the frog (prey:predator size ratio=1.29), although exact measurements could not be taken. Considering the amphibian predation by harvestmen, we provide an updated list focusing on post-metamorphic anurans (Table 1), highlighting the main families involved. 3.1.2. Spiders A direct observation was made on 5 February 2025 at 20:00 h along the Verrugosa trail (0.1665°, -78.8774°, 874 m a.s.l.) in Ecuador, where LP documented an orb-weaver, Eriophora nephiloides (O. Pickard-Cambridge, 1889), preying upon a frog, Pristimantis latidiscus (Boulenger, 1898) (Fig. 1G, H). The frog was larger than the spider’s prosoma (prey:predator size ratio length=4.60), was fully enwrapped in silk, and partially consumed during the observation. Another event, based on an iNaturalist record, using search criteria Smilisca phaeota, (voucher available https://www.inaturalist.org/observations/316253623), depicts an unidentified araneid, likely Araneus sp., preying upon a masked tree frog, Smilisca phaeota (Cope, 1862) (Fig. 1I), in Carepa, Antioquia, Colombia (7.7876°, -76.7331°, 32 m a.s.l.). Frog was several times larger than the spider’s prosoma (prey:predator size ratio length=4.69) .

Our observations align with previous reports indicating that spiders are significant anuran predators (Menin et al., 2005; Toledo, 2005; Nyffeler and Altig, 2020; Prémel and Torres, 2021). Most documented cases involve large wandering spiders in semi-aquatic or riparian environments where frogs are abundant (Toledo, 2005; Babangenge et al., 2019). However, web-building spiders also engage in anurophagy, despite their webs being primarily adapted for capturing flying insects. Although the specific capture mechanisms are not well understood, frog predation by orb-weavers and other web-building spiders likely occurs when amphibians climb vegetation and inadvertently encounter aerial webs (Nyffeler and Altig, 2020). Given that araneid webs are strong enough to intercept small vertebrates such as bats (Nyffeler and Knörnschild, 2013), it is plausible that they can retain relatively large frogs, as observed in the present records. In two cases, frogs were nearly five times larger than the spider’s prosoma size, consistent with evidence that orb-weavers are capable of subduing prey several times their own size (Nentwig and Wissel, 1986). Additionally, araneid venoms appear sufficiently potent to immobilize small vertebrates, suggesting that their venom efficacy warrants further investigation in the context of vertebrate predation. Beyond spiders, we highlight anurophagy as a noteworthy trophic strategy in harvestmen. While a harvestman ( Prionostemma frontale ) has been reported preying on anuran eggs (Hayes, 1983), active predation on post-metamorphic anurans has remained anecdotal and largely overlooked. Traditionally regarded as broad-spectrum omnivores, consuming plant material, fungi, vertebrate feces, insects, and conspecifics, these arachnids are demonstrated here to be capable of actively preying on small vertebrates (Acosta and Machado, 2007). Within this extensive trophic niche, active predation on vertebrates like anurans represents a rare but ecologically significant behavior (Villarreal et al., 2008; Menegucci et al., 2020) . Its occurrence is phylogenetically clustered in large-bodied families, such as Cranaidae and Gonyleptidae (Table 1), which suggests a trait constrained by morphology, a key prerequisite that likely facilitates the subjugation of vertebrate prey. Crucially, the documentation of anurophagy in multiple genera across disjunct Neotropical regions, from the Andes of Colombia and Ecuador, to the Coastal Range of Venezuela, and the Atlantic Forest of Brazil (Fig. 2), suggests this behavior is not a rarity restricted to a single lineage. While these families are phylogenetically related within Gonyleptoidea (Kury, 2014; Kury and Villarreal M., 2015; Pinto-da-Rocha et al., 2014), their considerable taxonomic and functional diversity, combined with this broad geographical amplitude, indicates that vertebrate predation may have evolved independently in different clades. This emerging pattern confirms that anurophagy represents a genuine, recurrent ecological strategy in these harvestmen, rather than a sampling artifact. It underscores that this behavior is a more prevalent and significant trophic strategy than traditionally assumed, though its frequency and drivers remain largely unexplored. Further studies should investigate whether it also occurs among other large-bodied harvestmen beyond these families. Although not all harvestmen species were observed capturing live prey, and scavenging remains a plausible explanation in some cases due to their omnivorous habits, our records support previous evidence that certain Opiliones are capable of subduing relatively large vertebrates. This ability appears to be enabled by several key morphological traits such as heavily sclerotized body armour, spined pedipalps, and robust chelicerae, which combined with their considerable body size facilitate the restraint of struggling anurans. The fact that the prey size was comparable to that of the predator aligns with patterns seen in other arthropod predators of frogs, such as carabid beetles of the genus Epomis (Wizen and Gasith, 2011) and many spiders (Meneses et al., 2020), and is likely facilitated by the soft integument of amphibians (Valdez, 2020). Nevertheless, the ability of these typically slow-moving arachnids to capture evasive vertebrates raises important questions about their predatory strategies, including potential reliance on ambush, substrate advantage, or mechanical immobilization. Future behavioral studies will be essential to elucidate how harvestmen overcome the mobility and escape responses of amphibian prey. Beyond offering substantial biomass, vertebrate prey may provide a distinct nutritional profile compared to arthropods. Although some studies have suggested that certain vertebrates, such as lizards, constitute low-quality prey for spiders (Wilder and Simpson, 2022), contrasting evidence indicates that anurans can supply essential micronutrients that have a positive effect on spider fitness, even at the cost of reduced lipid intake (Hunsucker et al., 2025). Given the similarities in physiological and digestive traits between harvestmen and spiders, it is reasonable to hypothesize that frogs confer nutritional advantages when consumed. In light of the increasing recognition of micronutrients as key drivers of food web dynamics (Kaspari, 2021), elucidating the causes and consequences of vertebrate feeding in arachnids may uncover overlooked pathways of nutrient transfer and refine current assumptions regarding their trophic position. Further studies should analyze vertebrate predation in other arachnid groups and investigate whether arthropods show selectivity toward certain vertebrate groups.

We thank the Mashpi Lodge and Ecominga Foundation for permitting access to their ecological reserves. We are also deeply grateful to Daniel Vargas and Sergio Cruz, who generously allowed the use of their pictures for this study. Special thanks are due to Adriano Kury for his invaluable role in facilitating the collaboration between our research groups. We express our most sincere gratitude to the Fundación ProAves de Colombia for its invaluable work in biodiversity conservation and for providing the necessary support to conduct field activities at the “Loro Coroniazul” Nature Reserve (Génova, Quindío), where one of the records reported in this study was obtained. We extend special recognition to Sergio Cruz, the reserve’s Park Ranger, whose dedication, assistance, unconditional support, and photographic record contribution during the fieldwork were indispensable for the accurate documentation of the record he contributed. We also acknowledge Re:wild, whose support and funding for the Atelopus sp. ( spumarius-pulcher complex) conservation project made possible the fieldwork during which one of the records included in this study was obtained. We are furthermore grateful to Luis A. Coloma and Jaime Culebras for their valuable assistance in the identification of frog records included in this study. The iNaturalist platform was instrumental in providing access to essential photographic records for this work, and we are grateful to the vast community of contributors and developers who make this resource possible.

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Iñiguez, K.: Taxonomic identification, Writing – review & editing. Narváez, J.C.: Methodology, Taxonomic identification, Investigation, Resources, Writing – review & editing. Proaño, L.: Taxonomic identification, Investigation, Resources, Writing – review & editing. Reyes-Vizcaíno, M.: Methodology, Taxonomic identification, Investigation, Resources, Writing – review & editing. García, L.F.: Conceptualization, Methodology, Investigation, Resources, Writing – original draft, Tables, Writing – review & editing. CONFLICT OF INTEREST STATEMENT The authors declare no conflict of interest. DATA AVAILABILITY STATEMENT: All the required data are uploaded as supplementary material. Table 1 . Documented cases of anurophagy by harvestmen (Opiliones) on post-metamorphic anurans. A record of predation on anuran eggs by Prionostemma frontale (Hayes, 1983) is discussed in the text but not included here. | Harvestman family | Harvestman species | Anuran family | Anuran species | Date | Locality | Anuran status | Reference | | Gonyleptidae | Neosadocus maximus (Giltay, 1928) | Hylidae | Metamorph of Boana bischoffi (Boulenger, 1887) | 2001 | Parque Estadual Carlos Botelho, São Paulo, Brazil | Partially consumed | Mendes & Pinto-da-Rocha, 2005 | | N. maximus | Unrecognizable frog | 2002 | Consumed | |||| | Cranaidae | Phareicranaus curvipes (Roewer, 1916) | Hemiphractidae | Flectonotus pygmaeus (Boettger, 1893) | Unspecified | Henry Pittier National Park, Aragua, Venezuela | Alive | Villarreal et al., 2008 | | P. curvipes | Eleutherodactylidae | Eleutherodactylus Duméril & Bibron, 1841 | Unspecified | Consumed | ||| | Gonyleptidae | Heteromitobates discolor (Sørensen, 1884) | Hylodidae | Hylodes phyllodes (Heyer & Corocoft, 1986) | 2017 | Private reserve in Ubatuba, São Paulo, Brazil | Alive | Menegucci et al., 2020 | | H. discolor | Juvenile H. phyllodes | 2018 | Dead | |||| | Cranaidae | Holocranaus aff . angulus (Roewer, 1932) | Craugastoridae | Pristimantis sp.1 | 2020 | Mashpi-TayraWildlife Refuge, Ecuador | Alive | This study | | Phareicranaus lucifer (Pinto-da-Rocha & Kury, 2003) | Hylidae | Dendropsophus parviceps (Boulenger, 1882) | 2022 | EcoMinga Foundation’s Río Anzu Reserve, Pastaza Province, Ecuador | Dead | || | Phareicranaus sp.1 | Craugastoridae | Pristimantis sp.2 | 2025 | ProAves Reserve, Génova, Quindío, Colombia | Alive | || | Phareicranaus sp.2 | Bufonidae | Atelopus sp. ( spumarius-pulcher complex) | 2025 | Río Napinaza (Quebrada Napinaza), Limón Indaza, Morona Santiago Province, Ecuador | Partially consumed | Information & Authors Information Version history Copyright This work is licensed under a Non Exclusive No Reuse License. Collection

Authors Metrics & Citations Metrics Article Usage 462views 209downloads Citations Download citation Esteban Calvache, Osvaldo Villarreal, Cynthia Ávila-Rojas, et al. Beyond spiders: Harvestmen as predators of anurans in the Neotropics. Authorea. 03 December 2025. DOI: https://doi.org/10.22541/au.176475087.70315061/v1 DOI: https://doi.org/10.22541/au.176475087.70315061/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu.