The way food-chains usually work is that small things get eaten by bigger things, which in turn get eaten by even bigger things. But it’s not always that simple, says Stuart Blackman.
A review back in 2021 documented cases of spiders preying on snakes, turning up 319 reports from every continent except Antarctica. The average length of the unfortunate reptiles was 26cm, with the largest approximately a metre in length.
Surprisingly, perhaps, the predator list was not dominated by the largest spiders. The ‘tarantula’ family, which includes the species known as bird-eating spiders, made up only about 10 per cent of cases.
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Martin Nyffeler of Switzerland’s University of Basel, who co-wrote the review, points to experiments performed back in the 1920s showing that the Brazilian tarantula Grammostola actaeon prefers to catch snakes over insects. “Grammostola are expert vertebrate-catchers that feed heavily on frogs, snakes and possibly also lizards,” he says.
Half of the cases, though, involved a family of rather small spiders – the theridiids, which include black widows and redbacks. Not only are their tough, haphazard, 3D webs capable of subduing large prey, but their venom is particularly effective against vertebrate nervous systems. Back in 1933, a theridiid named Steatoda triangulosa was documented feeding on a garter snake 355 times its own weight.
Nyffeler doubts that theridiid webs are designed to catch snakes specifically. He suspects it’s more a happy accident (for the spider at least, who may spend days feeding on such a windfall, and will often not finish the meal) – a rare consequence of webs built for catching large insects.
“Not only do these spiders occasionally catch small vertebrates – mice, rats, frogs, lizards, snakes – they also catch bulky insects, such as huge beetles, cicadas and Jerusalem crickets, many times larger than the spiders themselves.
“The occasional catch of large and energetically rewarding prey may be essential in order to fulfil the reproductive needs of some web-building spiders,” says Nyffeler. “The ‘rare, large prey’ hypothesis could very well apply to the situation of black widow spiders.”
