Zodarion cyrenaicum: The Tiny Spider That Hunts Giant Ants

Most predators pick on something smaller than themselves. Zodarion cyrenaicum, a small ground spider of the Negev desert in Israel, does the opposite: it feeds almost exclusively on the harvester ant Messor arenarius and routinely kills ants up to almost four times its own size, according to the 2014 Springer release on the study.

The research behind that headline, led by Stano Pekár of Masaryk University in Brno, was published in Naturwissenschaften as David and Goliath: potent venom of an ant-eating spider (Araneae) enables capture of a giant prey. It answers a simple question with an elegant result: a tiny hunter can take giant prey when a careful attack is paired with very strong venom.

The David and Goliath study

The paper appeared in Naturwissenschaften volume 101, issue 7, pages 533 to 540, and went online on 31 May 2014, as listed in its PubMed record. The authors were Pekár, Šedo, Líznarová, Korenko and Zdráhal, working from Masaryk University's Department of Botany and Zoology.

The abstract opens with the point that makes this spider remarkable: it is rare to find a true predator that repeatedly and routinely kills prey larger than itself. The team studied the spider's trophic niche and the behavioural and venom adaptations it uses on ants, with observations made both in the field and in the laboratory, as the EurekAlert release describes.

They found that the spider captures mainly polymorphic Messor arenarius ants. Adult females took the large ant morphs and tiny juveniles took the smaller ones, yet in both cases the ants were giant compared with the spider.

Why even juveniles hunt giants

Juveniles of most Zodarion species start on ants close to their own size and move to larger species as they grow. The 2014 release reports that young Z. cyrenaicum break that pattern: they deliberately choose Messor arenarius over the other ants living in the dunes.

The researchers suggest the spiderlings may recognise a component of the ant's pheromone. That idea fits earlier work from the same group, which showed in a 2012 olfactometer study of Zodarion rubidum that the spider was drawn to chemical cues from only two ant genera and that the cue it detects is probably part of a recruitment or trail pheromone.

The juvenile solution to handling a giant is physical. Spiderlings climb onto the ant's back and bite the abdomen, which leaves the prey helpless at once and avoids any retaliation.

How the spider attacks

Adult females hunt with a fast attack from behind, then retreat out of reach of a counterattack. That tactic was described in detail in Pekár's 2004 study of predatory behaviour in two European Zodarion species, which found that the attack is a very rapid lunge from the rear followed by a bite on the most extended ant leg.

After the bite the spider retreats to a safe distance and waits until the ant stops moving, and only then moves in to handle it. Pekár called this limited-contact approach a clearly effective way of handling dangerous prey.

In the 2014 study, both juveniles and adults paralysed their prey with a single bite, and each giant ant was taken by a single spider. Earlier laboratory trials on the Israeli species, reported in the Journal of Arachnology natural history study, found that Z. cyrenaicum was the slowest of three Zodarion species to launch its first attack, taking on average 119 seconds against 30 seconds for the other two. The authors suggested that patience reflects its specialisation on slow-moving myrmicine ants such as Messor.

Potent venom from the start

The striking venom result concerns age. The venom glands of adults were more than 50 times larger than those of juveniles, yet the paralysis latency of juveniles was only 1.5 times longer, according to the paper's abstract. The authors concluded that the species carries very potent venom already at the juvenile stage.

A comparison of venom composition between juveniles and adults found no significant differences. The press release adds that the adults' greater success is likely because they can inject more venom, and quotes Pekár: "Specialized capture combined with very effective venom enables this ant-eating spider to capture giant prey."

A companion study on Zodarion rubidum, published in the Journal of Zoology in 2014, points the same way: spiders improved their capture efficiency as they grew on every diet tested, which the authors attributed to an increase in the amount of venom injected rather than a change in its makeup.

Spider versus ant by the numbers

The 2005 natural history study gives measurements for both animals. Adult Z. cyrenaicum are 3.5 to 8 mm long, with a uniformly blackish body and black legs apart from pale coxae and patellae.

Workers of Messor arenarius are polymorphic, ranging from 4 to 15 mm, and uniformly black. That size range is the point: a colony offers ants of many sizes, so a spider at almost any stage can find a giant worker suited to it.

The spider also looks the part. The same study found that adult Z. cyrenaicum mimic larger black ants such as the small workers of Messor arenarius, a form of Batesian mimicry, and that the spiders hunt the same ant species they imitate.

Life in the Negev

The World Spider Catalog lists Zodarion cyrenaicum Denis, 1935 from Libya, Egypt and Israel. In Israel it occurs in the northern part of the Negev desert, and the 2005 study collected it in the weedy margin of a melon field at the Bitronot-Be'eri Nature Reserve and on a semi-desert steppe slope near Lehavim.

By day the spiders rest in igloo-shaped retreats under stones, built from sand pebbles or pieces of snail shell and scaled to the spider's size, up to 2 cm across in females. In the daytime field observations they were active only in the morning, between 0900 and 1100, running among ants or hunting them.

Pitfall trapping across the Negev showed the species active through the year with a peak in July and little or no activity in January. It has two reproductive periods, one in May and one in November. Females lay a mean of 45 eggs per sac and guard the egg sac inside the retreat, and males carry a diploid chromosome number of 26.

The harvester ant on the menu

Messor arenarius (Fabricius, 1787) is a seed-harvesting ant. A study of its foraging in an arid Negev shrubland found that after the spring seed pulse its foragers specialised on a single annual grass, Stipa capensis, gathered mainly from open ground between shrubs, before widening their diet in summer and shifting to shrub seeds in autumn.

The 2005 spider study adds that Messor ants were active by day and by night, with a dip in activity between 0900 and 1400. As a myrmicine ant, Messor belongs to the group that Zodarion venom paralyses more slowly than formicine ants, which makes the spider's success on such large workers all the more telling.

Why prey specialisation matters

All genera of the subfamily Zodariinae are thought to feed on ants or termites, and Zodarion is among the most specialised. A 2013 study in the journal Zoology traced how this ant diet evolved gradually from a generalist ancestor by integrating a series of trophic traits, from behaviour to venom.

Specialisation can be fine-grained. Zodarion germanicum targets formicine ants and lives longer and grows faster on them, while a 2018 study of six Iberian Zodarion species found that each exploits different ants and that low molecular weight venom compounds correlate with capture success, pointing to prey-specific venom components.

The venom itself is tuned. A 2019 toxicity study found that the venom of the ant specialist Zodarion nitidum was far more potent towards its preferred prey than towards alternative prey, while generalist venoms worked similarly on both.

Prey choice can even split populations. In Zodarion styliferum, one form hunts mainly Messor ants and another mainly Camponotus, each with different venom and a reluctance to interbreed, a pattern the authors compare to host-shifting in parasites and herbivores. Z. cyrenaicum shows how far that specialisation can go: a predator whose whole life, from spiderling to adult, is built around one giant ant.