Spiders eat flies, right? Everyone knows that the 45,000 or so spiders in the world are all obligate carnivores, more or less – eating other animals, mainly invertebrates.
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Are any spiders vegetarian?
Only one. It goes by the wonderful scientific name of Bagheera kiplingi, and its claim to fame is that its diet is – at least mostly – vegetarian.
B. kiplingi is a Central American member of the most charismatic family of spiders: the jumping spiders, also known as Salticidae. These are hunting spiders – instead of making a silken snare to catch their prey, they chase it down and pounce on it.
B. kiplingi looks very much like any other jumping spider and shares the same skillset. But, as a veggie, how it utilises those skills is very different. B. kiplingi lives on a particular kind of acacia plant that produces neat little packages called Beltian bodies on the tips of its leaflets. Accounting for around 90 per cent of the spider’s diet, these tiny bundles of protein and fat represent a complete ‘meal-deal’: they are nutritious, reliable and plentiful, and cannot run, hop, fly or crawl away.
Generally, spiders are antisocial, but B. kiplingi exists at surprisingly high population densities. With food readily available, there’s no need to compete, and dozens can live side by side on acacia bushes. Indeed, some scientists describe the species as quasi-social, with females sharing and guarding spiderlings and eggs in breeding nests.
So far, so good, until you understand that these Beltian bodies are not there for the spider; they are there for the resident Pseudomyrmex ants. The acacias are ‘ant plants’ and demonstrate one of the neatest examples of symbiosis in the natural world: they produce swollen, hollow thorns for the ants to nest in, as well as providing food in the form of Beltian bodies and extra-floral nectaries (nectar-producing glands) scattered around the stems.
In exchange for this bed and board, the ants provide their home plant with a ruthlessly effective security service, defending it with vigour. They will attack anything that comes into contact with the leaves – from a careless human to hungry insects – dishing out painful stings and bites. Handily, the ants even act as mini-gardeners, pruning competing plants under and around the acacia.
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It’s this beautiful textbook ecological arrangement between ant and plant that B. kiplingi is gate-crashing, cheating the system and stealing the ‘loot’ from right under the ants’ antennae. It does this by employing a highly effective avoidance strategy, side-stepping the ants by using those classic jumping spider hunting traits in a novel way.
It pays to be quick when you’re stealing food from troops of patrolling ants. An individual spider has been recorded eating up to 36 Beltian bodies in a single sitting, and putting away one in fewer than four minutes.
Jumping spiders have some of the best vision in the spider world, enabling B. kiplingi to see the ant patrols approaching from some distance. They are also renowned for their agility, allowing our spider to simply ‘ping’ out of harm’s way when the need arises. If things get really tight, it can easily abseil away on a silk thread, like Tom Cruise in Mission Impossible, dropping out of sight of the marauding ants and then hauling itself back when the coast is clear.
Even the spider’s home base (jumping spiders rest up in little silken hideouts) is positioned on old foliage that has lost its Beltian bodies and is therefore out of the way of any hungry ants.
How did this peculiar arachnid behaviour evolve in the first place? It’s thought to be nothing more complicated than the spider’s ancestors starting to eat the more sedentary stages of their insect prey’s life-cycle, such as eggs or larvae.
From this, it’s a short jump to consuming the protein-rich parts of the plant. However, how the spider deals with the fibre that comes with a plant-based diet is a mystery that is yet to be solved.
