Certainly the most recognizable of all arachnids, spiders are ubiquitous little predators on every continent except Antarctica. Their lineage stretches back 380 million years, with some of the oldest known fossils such as Attercopus fimbriunguis dating from the Devonian period and still having segmented abdomens. This predates the arrival of the dinosaurs by 150 million years. One thing that hasn’t changed over all this time is their predatory behaviour.
With over 45 000 known species worldwide, spiders have evolved a myriad of strategies to capture prey. Most species use one of three techniques: wandering, ambush or silk webs. Early spiders were most likely active hunters. Today these spiders spend their time wandering around, actively searching for prey, using sight and tactile hairs to feel the vibrations of moving prey. They rarely use silk beyond building the occasional retreat or the female producing an egg sack. Wolf spiders, jumping spiders and the well-known, and often scary, rain spiders are all active hunters. This is why jumping spiders are often seen while gardening, wolf spiders are often found stuck in a sink or bathtub and rain spiders (also known as huntsman spiders) are often seen quietly sitting in a corner of the ceiling.
Trapdoor spiders and many tarantulas, including our endemic baboon spiders, prefer to construct burrows and ambush their prey. The inside and an area surrounding the opening of the burrow is lined with a fine, often almost invisible carpet of silk. While tarantulas usually have open burrows, trapdoor spiders go a step further and construct hinged covers for their burrows, camouflaging them from potential prey as well as other predators. In both cases, when a suitable prey item stumbles across the silken mat outside the burrow, vibrations are sent along the silk threads to the waiting spider that then quickly emerges, pounces on the prey and drags it back into the burrow to feed on in safety.
Web building is the most recognizable of the hunting techniques used by spiders, and web-builders can use up to seven different kinds of silk to construct webs, trap prey and even store them for later feeding. Spiders produce silk from glands in the abdomen and secret it from finger-like appendages located under the back of the abdomen called spinnerets. Orb weavers, daddy longlegs, button spiders and hairy field spiders are commonly seen using webs to catch prey. Here, the spiders either construct a permanent web, like orb weavers, or a temporary one that they build each evening and then devour in the morning, like hairy field spiders. In both cases the web works the same way. A prey item gets caught in the web’s sticky threads and the vibrations it produces while struggling is relayed to the waiting spider that then pounces on and subdues the prey, often cocooning it in a mat of silk for later feeding.
Some spiders go above and beyond these techniques by having developed special morphological and behavioural adaptations to catch their prey. A simple behavioural adaptation can be found in the spiders of the Argyrodes genus. These spiders, also known as dewdrop spiders, are small kleptoparasites that can invade another spider’s web without alerting its host. They then proceed to feed on prey items caught by the larger web-building spider. Although most spiders are lone hunters, community nest spiders form social nests that can contain dozens of individuals. These spiders then hunt together, enabling them to take down prey much larger than any one spider can manage.
Flower crab spiders have the peculiar ability to change colour. They sit on or just below a flower waiting for an insect to come and feed on the flower’s nectar. Invisible to the insect, it then pounces and paralyses its prey.
The bolas spider is a nocturnal predator that, instead of spinning a web, produces a single thread with a special sticky globule at the end. This globule contains an analogue of the mating pheromone secreted by female moths. The spider then sits under a twig on a tree and swings the globule around on the thread. Male moths, attracted by the pheromone, come to investigate and are then struck by the sticky globule and trapped. The spider reels in its prey.
Ogre-faced spiders have their own take on catching prey with web. This nocturnal predator spins a stretchable net held in its front four legs. The spider then perches just above the ground and waits for a prey item to walk past. When within range the spider thrusts the net over its prey and the hapless victim becomes entangled by the loose threads of the net.
Then we get to the spiders that adapt not only their behaviour, but their physical bodies as well. The spitting spider, for example, does not produce a web, and actively hunts for its prey. These spiders have an enlarged globous cephalothorax (head) which is modified to contain a myriad of contracting muscles and glands that produce a sticky, toxic glue. To catch its prey, the spider lives up to its name by spitting this cocktail onto a prey item. Trapped by this toxic glue, the victim is unable to escape. Pelican spiders take body modification to a new level and have greatly enlarged chelicerae and fangs that they carry on strangely elongate “necks”. They specialize in hunting other spiders and use their long mandibles and fangs to impale their arachnid prey and keep them at a safe distance, thus avoiding any retaliatory bites. Elongated mandibles are not unique to the pelican spider. Dysderid spiders, also known as woodlouse hunters, use their elongate fangs to prey on woodlouse. Their fangs allow them to find the soft underbellies of their prey even when the woodlouse curls up into a protective ball. Some jumping spiders that specialize in catching ants, also possess elongate fangs to keep their prey from stinging and biting them in self-defence.
The most extreme physical modifications are found in spiders that specialize in hunting ants. These spiders have modified their bodies through evolution to accurately mimic a specific host ant. This mimicry has evolved, to varying extents, in many spiders from different families, but are functionally the same. Each spider mimics only a specific host ant species and not only resembles them morphologically, but also produces analogues to pheromones used by the ants to recognize each other. This allows the spider to walk amongst its prey like a wolf in sheep’s clothing. And whenever it gets hungry, it just catches an ant, retreats to feed on it and then wanders back.
These are just some of the amazing predation adaptations that spiders have at their disposal. There are even spiders that hunt under water or live in retreats and under shells in the intertidal rocks on shores, and undoubtedly there are even more amazing hunting strategies that are waiting to be discovered.
References:
Dippenaar-Schoeman, A. 2014. Field guide to the spiders of South Africa. Lapa Publishers, Pretoria, R.S.A. 432pp.
Higgins, L. E. & Buskirk, R. E. 1998. Spider-web kleptoparasites as a model for studying producer-consumer interactions, Behavioral Ecology 9: 384–387.
Leslie, B. & Craig, C. L. 2010. Spider Silk: Evolution and 400 Million Years of Spinning, Waiting, Snagging, and Mating. Yale University Press, JSTOR, www.jstor.org/stable/j.ctt1nqbhw. Accessed 24 June 2021.
Michalko, R. & Pekár, S. 2016. Different hunting strategies of generalist predators result in functional differences. Oecologia 181: 1187–1197.
Wesołowska, W. & Haddad, C. R. 2009. Jumping spiders (Araneae: Salticidae) of the Ndumo Game Reserve, Maputaland, South Africa. African Invertebrates 50: 13-103.
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