Plants are a vital resource for many animals that use them for food, shelter or protection. The best known plant–animal interactions involve insects and include negative interactions, such as herbivory, and positive interactions, such as pollination, and other mutualistic interactions. In many of these interactions, the insect shows specificity in that it only interacts with one, or a couple of plant species. These examples can be tightly co-evolved and include the food-for-protection mutualism between ants and swollen thorn acacias, where a specific species of ant is paired with a specific species of acacia [1], the extreme specificity of fig wasp pollinators to particular fig species hosts [2], and the specialisation of small groups of orchids to one species of bee pollinator, such as the South African guild of orchids (Coryciinae) exclusively relying on the oil-collecting bee (Rediviva peringueyi) for pollination [3].

Insects, as outlined above, and other arthropods, such as herbivorous and mutualistic mites [4], are well known for developing close associations with plants. Spiders, however, are usually thought of as generalist predators that only use vegetation indiscriminately for shelter or as a substrate for their webs. A study on a temperate grassland spider community, for example, showed that while some individual spider species showed a weak preference for a narrow range of host plants, the overwhelming preference was for tall and stable vegetation structures and not individual plant species [5]. Recently, the long-held notion that spiders have limited interactions with the vegetation in their surroundings have been challenged, especially by the surprising discovery that some species of spiders, and the first instars of web-building spiders in particular, rely on nectar, pollen and Beltian bodies as a significant component of their diets ^[6][7][8][6,7,8]. This prompted a review of spider–plant interactions in general, which revealed associations with plants across a much larger range of spider families than previously thought [9].

Very limited research is available on spiders that construct aerial webs, which predominantly consist of sheet-webs by members of the family Linyphiidae, tangle webs by members of the family Theriididae, and orb webs by members of the families Araneidae and Tetragnathidae. As the function of the webs to some degree depends on the substrate to which they are attached, it could be argued that they are more dependent on the correct choice of plant, and therefore, potentially should be more discerning than cursorial spiders. A relatively newly described species of linyphiid, Laetesia raveni, from Australia appears to exclusively build its webs on two thorny plant species, Calamus muelleri and Solanum inaequilaterum [10]. Similarly, one genus of araneid spiders, Eustala, seems a promising candidate for more in-depth research as several studies show close associations to individual species of acacias in the genus Vachellia ^[11][12][11,12]. These acacia species are in a mutualistic relationship with protective Pseudomyrmex ants, and the Eustala spiders probably associate closely with the acacias to exploit the ant–acacia mutualism for enemy-free space [13]. Another largely unresolved question is how spiders locate and identify their host plants. Insects generally locate their host plants using chemical cues from wind-dispersed plant volatiles ^[14][15][14,15]. In ant–plant associations, ants identify their mutualistic partner by chemical cues emitted from the plant [16]. However, the distance to which they rely on plant volatiles, or random searches for the location of host plants remains unclear. On the one hand, Pheidole minutula used plant volatiles to correctly locate their host plant Maieta guianensis during choice tests over distances of 15 cm in Y-maze experiments in the laboratory [17], while on the other hand, Crematogaster ants recognise their host Macaranga species only by direct contact with chemical compounds on the stem surface of saplings [18]. Spiders are also known to use chemical cues during mating behaviour [19], such as males using cues from silk to locate and evaluate females [20], and they use them to detect potential prey [21]. In addition, there are a few examples of spiders using chemical cues from plants, including two species of crab spiders in the genus Thomisus that were attracted to the clove oil flower fragrance [22] and the nectivorous spider Hibana futilis, which uses plant volatiles to recognise and potentially locate nectar sources [6].

2. Spider–Plant Associations

2.1. Cursorial Spiders

Bromeliads and other rosette-structured plants have a complex, three-dimensional architecture that presents a valuable microhabitat for a number of species [23], particularly members of Salticidae ^[24][25][26][24,25,26]. The best studied cursorial spider–plant association, and one of the few species-specific examples, is that of the bromeliad specialist Psecas chapoda and Bromelia balansae. Through a series of studies by Romero and Vasconcellos-Neto ^[27][28][29][27,28,29], P. chapoda was found exclusively on B. balansae across a large geographic range [26] (Table 1). Whilst B. balansae provides P. chapoda with a favourable microhabitat and microclimate, P. chapoda has been reported to contribute to the nutrition of B. balansae through the absorption of nitrogen from spider faeces deposited on the leaves of the bromeliad [30]. Romero et al. ^[31][32][31,32] evidenced that this interaction was indeed mutualistic as the leaves of B. balansae grew larger in the presence of P. chapoda. Some Thomisidae crab spiders, which have been documented as obligate Nepenthes pitcher-plant dwellers (Table 1), have likewise been reported to assist their host plant with nitrogen acquisition. The specialised leaves of pitcher-plants, which are used to attract, trap, and digest prey ^[33][34][33,34], also provide suitable microhabitats for the crab spiders Misumenops nepenthicola and Thomisus nepenthephilus ^[34][35][34,35]. These spiders feed on visiting insects drawn to the pitcher-plants ^[34][36][34,36], and in some circumstances, the spiders increase pitcher-plant prey consumption by dropping consumed prey remains into the pitchers. Interestingly, two studies by Lim et al. [34], and Lam and Tan [37] concluded that the type of association between crab spiders and pitcher-plants is environmentally context-dependent. Lam and Tan [37] demonstrated that T. nepenthephilus increased the prey capture rates of Nepenthes gracilis, offsetting the nitrogen loss from consumption by T. nepenthephilus, resulting in an overall net gain. However, this benefit only occurs under conditions where prey availability is low and is ultimately lost when prey availability increases, switching from a positively facilitative to a parasitic interaction [37]. Furthermore, a number of spider species have been reported to have unusually close associations with trichome-bearing plants ^{[9][38][39][40][41]}[9,38,39,40,41]. One genus from the Oxyopidae family, Peucetia, dominates such interactions and many species are considered to have strict, and perhaps obligatory, associations with glandular trichome-bearing plants ^[39][40][42][39,40,42]. Glandular trichomes are hair-like structures believed to have evolved as a direct biotic defence against herbivorous insects ^[43][44][43,44]. The insects and carrion (i.e., dead insects) trapped by the glandular hairs represent an energetically cost-free, accessible food source [45], which attracts arthropod predators, such as spiders, for added protection against herbivory ^[40][45][46][40,45,46]. In three complementary studies, Morais-Filho and Romero ^[39][40][47][39,40,47] observed Peucetia flava exclusively in association with Rhyncanthera dichotoma. During the latter study, Morais-Filho and Romero [40] physically removed the glandular trichomes from R. dichotoma and documented fewer Peucetia spiders occupying those plants compared to R. dichotoma with intact trichomes, further demonstrating the strong and potentially obligatory association Peucetia spiders have with glandular trichome-bearing plants [42]. Morais-Filho and Romero [40] reported that P. flava reduced herbivory in the buds and flowers of R. dichotoma and although this interaction did not increase fruit production, it also did not incur any significant costs to R. dichotoma fitness (i.e., through predation of pollinators), signifying a potential protective mutualism. Moreover, a recent study by Sousa-Lopes et al. [45] found that the presence of P. flava on the trichome-bearing Mimosa setosa var. paludosa positively correlated with an increase in trapped prey and carrion. Spider–plant associations that arise from an exploitable source of food are not uncommon. While some spiders may associate with plants that attract and/or trap insect prey, such as glandular trichome-bearing plants and pitcher-plants, other spiders species seek nutrition from the plant itself. The salticid, Bagheera kiplingi, for example, is exclusively associated with many myrmecophytic acacias ^[7][48][7,48]. These acacias produce Beltian bodies to attract ants that protect the plant, and in return, the ants gain nutritional rewards and refuge ^[1][7][1,7]. The spider exploits this ant–acacia mutualism and consumes the Beltian bodies as its primary food source, which in some cases constitute 90% of its diet [48]. Therefore, it is conceivable that access to a convenient source of prey is another primary driver of spider–plant associations, and perhaps the obligatory associations observed between Peucetia and glandular trichome-bearing plants and Thomisidae and Nepenthes pitcher-plants. Another potential driver of host plant selectivity in spiders could be crypsis (i.e., camouflage), whereby a spider may exhibit a preferential affinity for a substrate (e.g., flower, bark, and moss) that matches their body colouration/morphology, rendering them undetectable to potential predators or unsuspecting prey. Cryptic colouration is particularly well studied in Thomisidae crab spiders, which, in sit-and-wait predators, increases foraging success ^[49][50][51][49,50,51]. Certain species will preferentially select flowers, upon which they forage, that match their body colouration (i.e., background-matching) to avoid detection by pollinators and other visiting insects ^[41][49][52][41,49,52]. Moreover, there are some spider species that are also capable of changing their body colouration to match their chosen background, or in this instance, host plant. Such examples include the crab spiders Misumena vatia and Thomisus onustus that typically alternate between white and yellow ^[50][53][50,53]. It is evident that cryptic species will select specific substrates to ensure successful camouflage. However, there is a paucity of information to discern whether cryptic colouration is a resultant factor in specific spider–plant associations. Most crab spiders appear to be generalists, selecting a number of plant species that suit their needs. From the examples provided above, it is particularly apparent that Psecas chapoda facultatively relies on the microhabitat created by B. balansae for foraging, mating, and oviposition, as observed by Romero and Vasconcellos-Neto ^[28][29][28,29], and as a refuge and nursery site that can offer protection from predators and desiccation ^{[28][29][54][55]}[28,29,54,55]. Omena and Romero [56] inferred that this extreme fidelity was related to microhabitat structure, and observations by Romero and Vasconcellos-Neto ^[28][55][28,55] affirmed this after finding that P. chapoda seldom colonised bromeliads in forest habitats as leaves would often obstruct the rosette, hindering any use of the microhabitat. Likewise, some studies have reported that Peucetia spiders preferentially select larger plants as they offer more sites to forage and refuge, and attract and trap more insect prey ^[45][57][45,57]. Prey, and other sources of nutrients, are also key determinants, especially in terms of exploitable sources of food. In summary, it can be inferred that it is the availability of certain exploitable resources, together with a microhabitat structure and plant morphology that complements the ecological requirements, foraging the strategies and behavioural preferences of a spider ^{[9][23][56][58][59][60][61]}[9,23,56,58,59,60,61], which are the primary factors that drive specific spider–plant associations.

2.2. Web-Building Spiders

Research on web-building spider–plant associations is far less numerous than on their non-web-building counterparts. Currently, there are only a few examples of exclusive spider–plant associations, represented by Eustala (Araneidae) and Laetesia raveni (Linyphiidae). The research on cursorial spider–plant associations indicates that the suitability of a plant as a microhabitat to find shelter or food resources (i.e., prey, carrion or nectar) are the main determinants of host plant selection and subsequent spider–plant associations. This also applies to web-building species, where it is vital to select a web-building site that maximises foraging success [58]. For these sit-and-wait predators this is ultimately dependent on the density of prey ^[69][70][69,70], which as mentioned is a key driver in host plant selection. However, the key driver of foraging success for a web-building spider is the optimal construction of its web; hence, the majority of available research on web-building spiders documents preferential, facultative associations with plants that provide suitable structural features for web construction ^{[48][71][72][73][74]}[48,71,72,73,74]. Two neotropical spider species, the theridiid Latrodectus geometricus and the araneid Alpaida quadrilorata, are both found in association with Paepalanthus bromelioides [73]. This rosette-structured plant provides the spiders with the structural necessities for web construction and may also offer refuge and protection from predators ^[23][75][23,75]. More importantly, P. bromelioides is considered to be a protocarnivorous plant that obtains nutrients from insects with the aid of digestive mutualists, namely L. geometricus and A. quadrilorata [73]. This plant apparently possesses features that attract insect prey, such as leaves that reflect ultraviolet light and a phytotelma (i.e., a water-filled cavity) with specialised fluid that also digests captured prey ^[73][76][73,76]. Similar to the pitcher-plant dwelling Thomisidae crab spiders that forage at the mouth of the pitcher, L. geometricus and A. quadrilorata build their webs above the phytotelma [76], providing easy access to incoming prey. Both spider species capture prey, while discarding carcasses and faeces into the rosette of P. bromelioides effectively, and thereby channelling a more bioavailable form of nitrogen directly to the plant [31]. Nishi et al. [73] observed A. quadrilorata strictly on P. bromelioides within the study area in Morro da Pedreira, Brazil. However, no other research is available to determine how exclusive this association is, and since L. geometricus has been documented on other plant species (e.g., [68]), both should be considered facultative digestive mutualists. As previously discussed, carnivorous plants present spiders with a suitable microhabitat ^[34][37][34,37]. However, aside from Nishi et al. [73], there are no reports of unequivocal web-building spider associations with carnivorous plants. Cresswell [77] observed an unidentified species of linyphiid occupying the pitcher-plant Sarracenia purpurea as an apparent kleptoparasite. Milne and Waller [78] similarly observed linyphiids interacting with S. purpurea, using the pitchers as substrates to build their horizontal sheet webs. However, Milne and Waller [78] noted that many of the linyphiids constructed their webs at a height similar to the pitchers, implying that this a spatial coincidence rather than an association. The theridiid Theridion decaryi has also been observed inhabiting a different pitcher-plant species, Nepenthes madagascariensis, according to Fage [79]. The available research on these interactions is evidently scarce and ambiguous. However, considering that several other spider species have been found in association with pitcher-plants and other carnivorous plants (Table 1), the possibility that there are species of web-building spiders closely associated with pitcher-plants cannot be ruled out. In addition, web-building spiders in the genus Stegodyphus (Eresidae) have strong affinities for thorny plants ^[54][72][80][54,72,80]. A recent study by Rose et al. [54] determined that Stegodyphus dumicola nests occurred more frequently on tall thorny plants and were observed on several different genera. Lubin et al. [80] also found that S. lineatus preferred to inhabit tall, thorny, and even poisonous plants. Thorny plants offer protection against predators (e.g., birds) and reduce the risk of disturbances from large herbivorous animals (e.g., cattle and other browsing/grazing mammals) that can damage or destroy spider webs ^{[54][72][75][80]}[54,72,75,80]. Ruch et al. [72] demonstrated that S. tentoriicola, which inhabits both thorny and thornless plants, constructed larger webs when inhabiting thorny plants, and were less likely to relocate, compared to spiders in thornless vegetation. As larger webs are more costly to build, it is evident that thorny plants provide S. tentoriicola, and likely other spider occupants, with favourable microhabitats that enable spiders to invest more energy into building larger webs, increasing their foraging success, whilst receiving refuge and protection from animal-related disturbances ^[54][72][54,72]. Extreme specificity and fidelity toward host plants is evidently not as common among web-building spiders. Many web-building spiders often interact with and inhabit multiple plant species from different families and orders, as described, for example, by Rose et al. [54] and Whitney [71]. A recent study conducted by Cuff et al. [81] in England evaluated the leaf and habitat preferences for oviposition in the candy-striped spiders Enoplognatha ovata and E. latimana in the family Theridiidae. These spiders create a retreat, or nest, for oviposition by rolling a leaf with silk [81]. Enoplognatha appeared to preferentially select the leaves of bramble (Rubus fruticosus), nettle (Urtica dioica), hogweed (Heracleum sphondylium), and have also been found using fireweed (Chamaenerion angustifolium) for their leaf-roll nests. Plant preferences were not taxon-related, nor was the size and structure of leaves important; however, certain traits, such as the length–width ratio, were thought to influence leaf selection [81]. Cuff et al. [81] even suggested that the spiders could possibly provide a degree of protection from herbivorous insects in a mutualistic association.