Diet composition and prey size

I found 49 images that show Tmarus spiders holding prey items in their chelicerae (H' = 0.298). 46 of all depicted prey items represent (individual) ants (Fig. 1); one image shows a Tmarus specimen holding a salticid spider, one a dipteran (Brachycera) and one an unidentified, four-winged insect. In conclusion, non-ant prey items in Tmarus images represent 6.1% of all cases in this dataset. For Monaeses, 20 images showing spiders with prey in their chelicerae were available (H' = 0.199). In 19 cases, spiders hold individual ants, and in one case (5% of all prey items), an individual Monaeses was photographed with an undetermined spider in its chelicerae.

Observed spider species

Three Western Palaearctic Tmarus species were identified from available images. Tmarus piger (Walckenaer, 1802) (the type species of Tmarus) was identified holding ants, a dipteran and a salticid (n = 13). Tmarus staintoni (O. Pickard-Cambridge, 1873) was identified from a single image with an ant as prey, the same as Tmarus stellio Simon, 1875 (see also  Appendix Tab. S1 (AM62_61_66_Appendix_TableS1.xlsx)). 19 out of 20 cases of Monaeses spiders show M. paradoxus (Lucas, 1846), the type species of the genus, and one shows a female specimen of M. israeliensis Levy, 1973 (Fig. 1). The only image with a non-ant prey item (spider) is of M. paradoxus.

Fig. 2:

Position of the bite in Tmarus (n = 46) and Monaeses (n = 19) crab spiders holding ants in their chelicerae, observed in online-accessible images of feeding specimens

Position of chelicerae in ant capture

For Tmarus (Fig. 2), I found 25 cases of ants held near or at the anterior-dorsally articulation of head and pronotum (cf. Fig. 1), in contrast to 11 incidents were the spider held the ant at a different position on the remaining body. In two cases, the spider held the ant at a leg. In Monaeses (Fig. 2), the results were similar. In ten images, the spider held the ant at the area of the articulation between head and pronotum, while in six cases, ants were held at other parts of the body. In one case, the ant was held by the spider at a leg.

Predator-prey size ratio

The ratio of ant body size to spider body size varied in images from 0.99 to 2.16 (n = 7; mean ± SD: 1.43 ± 0.46) in Monaeses from 0.84 to 1.26 (n = 4, mean ± SD: 1.1 ± 0.19). On average, both spider genera preyed upon ants that exceeded their own body length.

Analysis of prey types

Ants were the main photographed prey in images showing Western Palaearctic species of both genera, Monaeses and Tmarus. Consulting the thresholds for stenophagy in Pekár et al. (2012), both genera appear as stenophagous in nature (0 ≤ H' ≤ 0.3; but see also Huseynov et al. (2008) and Huseynov (2014). Based on reports in the literature (e.g. Lubin 1983, Wunderlich 1995), this behaviour was to be expected in Tmarus. However, the observations show that Palaearctic Tmarus also feed on other arthropods, including spiders. A preference for ants over spiders was also observed by Šoltysová (2020) in laboratory experiments with Tmarus stellio, but no additional non-ant prey was reported by Lubin (1983) for Tmarus galapagosensis from the Galapagos Islands. It is possible that the degree of trophic specialisation on ants in Tmarus varies, as observed for the woodlouse-eating genus Dysdera Latreille, 1804 (Řezáč et al. 2008, 2021), ranging from euryphagous species to specialists that feed only on a limited number of ant species. Given that Tmarus is a widespread and species-rich genus (Ileperuma Arachchi & Benjamin 2019), it might have a similar potential as Dysdera for the study of trophic specialisation in spiders, and arthropod predators in general. For Monaeses, the observation of Dentici & Amata (2018) already showed that spiders of this genus are potentially able to prey on ants.The data presented here corroborate this hypothesis and suggest that at least Monaeses paradoxus is a stenophagous predator of ants in its natural habitat, which occasionally preys on other arthropods.

Lubin (1983) reported that Tmarus galapagosensis avoided feeding on introduced myrmicine ants with a sting. In two cases ( Appendix, Tab. S1 (AM62_61_66_Appendix_TableS1.xlsx)) a Tmarus fed on a myrmicine ant of an unknown genus and another was observed feeding on a queen belonging to the genus Myrmica that has shed its wings (Fig. 1), which suggests that European Tmarus include, at least occasionally, myrmicine ants in their natural diet. However, it is important to note that I did not distinguish between different ontogenetic stages or sexes. It is possible that early instars of both genera prey on a different spectrum of arthropods compared to adults (see e.g. Bartos 2011), which might include a higher number of non-ant insects or other spiders.

Hunting strategy

The majority of images for both genera show spiders that hold ants somewhere at the anterior-dorsal articulation of the head and the pronotum (Fig. 1). Because of their mandibles, sting (most myrmicine ant workers), ability to spray chemicals such as formic acid (formicine ant workers) and cooperative behaviour, most spiders and other arthropod predators avoid preying upon ants (Cushing 2012). Hence, an attack to the dorsal part of the ant's “neck” seems to be a favourable option for a crab spider and keeps the ant's gaster and mandibles away from the spider's body. On the other hand, Foelix (1996) showed that euryphagous crab spiders first attack any available part of the prey's body, and later switch position to the often-observed”neck“hold. Specialised prey capture behaviour, however, evolved independently in different spider groups (e.g. Heller 1976, Jackson et al. 1998, Pekár 2004), which suggests that Monaeses and Tmarus species might show some specialised behavioural traits towards ants that differ from that of euryphagous crab spider species. Indeed, Lubin (1983, sub. T. stolzmanni) reported a very sophisticated hunting behaviour in the nocturnal Tmarus galapagosensis. After dropping from a twig to which the spider is still attached by a silken thread, T. galapagosensis swings in the air for some time (possibly monitoring ant movements from a safe distance) and releases further fine threads from its spinnerets, which become attached to twigs or branches in the vicinity due to light air movements. Then the spider climbs along on one of these threads (and produces a visible dragline), reaches a new position on a different twig or branch and assumes a hunting position. Lubin also speculated that the dragline, which was expanded by one observed spider to a position 2 cm down the twig, might even serve as an alert system. The ant itself is caught by a bite positioned anterior-dorsally of the thorax (“nape of the neck”) and the spider later remains in this position while feeding. In addition, the spiders were never observed manipulating prey with their legs. In laboratory trials with Tmarus stellio and other crab spider taxa, Šoltysová (2020) observed most often a “head-on” attack on ants in Tmarus, in contrast to euryphagous thomisids, which also attacked from the side. This is in accordance with my analysis of images, in which most spiders of both genera held the ants anterior-dorsally somewhere at the articulation between head and pronotum, which corresponds to the observations in Lubin (1983). The spider probably changes the position later (and maybe not in every case) only to feed on other parts of the body, possibly to balance nutrient intake (Pekár et al. 2010). Further, a direct attack to the “neck” of the ant is in accordance with Pekár (2004) as well, who suggested that robust, inaccurate or non-ant mimicking spiders attack ants head-on (like the gnaphosid Callilepis nocturna (Linnaeus, 1758); Heller 1976), while more fragile, ant-mimicking spiders like Zodarion Walckenaer, 1826 attack from the rear, followed immediately by a retreat. However, there is some variation in the hunting behaviour and position of attack during the ontogenetic development of the ant-eating and ant-mimicking thomisid Aphantochilus rogersi O. Pickard-Cambridge, 1871 (Castanho & Oliveira 1997, Oliveira & Sazima 1985). Strophius nigricans Keyserling, 1880, another ant-mimicking thomisid, uses dead ants as a protective shield and distraction against living ants, which the spider also preys upon (Oliveira & Sazima 1984).The ant-mimicking Amyciaea forticeps (O. Pickard-Cambridge, 1873) also seems to utilize different behaviour to attack its model, Oecophylla smaragdina (Fabricius, 1775), but usually attacks from behind (Mathew 1954). Given the microhabitat of Tmarus and Monaeses, there might be a simple explanation for the frontal attack. Twigs and stalks do not provide the necessary area to perform a sophisticated “attack and retreat” behaviour such as known from Zodarion, and bitten prey would simply drop off the twig or stalk on which the spiders sit, and be lost. Hence, the spider has to hold onto its dangerous prey until it is completely immobilised, which happens quickest when a bite is positioned close to the central nervous system.

Predator-prey size ratio

The measurements taken from images suggest that Tmarus and Monaeses frequently catch ants that have the same or a greater body size than the spider itself. In Monaeses, the elongated opisthosoma leads to relatively small ratios between ants and spiders, despite a frequently higher biomass of the caught ant (Fig. 1), so that measurements of the prosoma, which were not possible in available images, probably better reflect the ability of Monaeses to overcome large and oversized prey. In summary, this fits to the hypothesis that both genera are specialised ant predators, because stenophagous spiders tend to be able to subdue prey larger than their own body size more frequently (Michálek et al. 2017, Pekár et al. 2014, 2017). Such behaviour is also known from other myrmecophagous spiders (Pekár 2004), with an extreme case of oversized ant prey observed in Zodarion cyrenaicum Denis, 1935 (Pekár et al. 2014). However, euryphagous crab spiders are known to be able to frequently catch prey that is much larger than themselves (e.g. Huseynov 2007a, 2007b), which may also include ants (Huseynov 2007b: fig. 2d).

Ant-eating in related groups

Ant-eating behaviour within the Oval Calamistrum Clade (Wheeler et al. 2017) seems to be mostly limited to the Thomisidae and Oxyopidae (e.g. Cushing 2012, Huseynov 2006). Compared to Tmarus and Monaeses, but also to other crab spiders, some oxyopids inhabit a similar microhabitat. In Azerbaijan, the ant-eating Oxyopes globifer Simon, 1876 dwells mostly on Salsola-shrubs (Huseynov 2006). Several species of Tmarus are often found on branches of shrubs as well (Ileperuma Arachchi & Benjamin 2019, Lubin 1983, Bauer unpubl.). In Oxyopes lineatus Latreille, 1806, a lynx spider that inhabits herbaceous vegetation in sunny open areas, ants can represent around 20% of the natural prey (Huseynov 2007c). In Oxyopes salticus Hentz, 1845 and Peucetia viridans (Hentz, 1832), ants were also found to be part of their diet (Nyffeler et al. 1992). In addition, various species of the lynx spider genus Hamataliwa Keyserling, 1887, for instance Hamataliwa rufocaligata Simon, 1898, exhibit a similar cryptic colour pattern and possess an elongated opisthosoma not unlike that of many Tmarus species (e.g. Tmarus staintoni), and are often found sitting on branches (Dippenaar-Schoeman et al. 2020). They resemble small twigs and, on the first glance, might even be mistaken for a Tmarus species.