Many ant species are territorial and aggressive. That maybe wouldn’t be so bad if ants were loners, but they tend to stick together in groups, sometimes tens of thousands strong. If you’re a bigger, tougher animal that eats ants, or one that stays safe by mimicking them, or makes your living as an ant parasite, those numbers are great. They make it that much easier to prey on them or blend in with a group.
The jumping spider Phintella piatensis neither eats ants nor mimics them. It’s not a parasite, either, but it still loves hanging out with ants. In one field study, 90 percent of all Phintella spiders that biologists found were in the company of ants, including the Asian weaver ant Oecophylla smaragdina. This has had puzzled scientists because Oecophylla is known as a formidable predator with a taste for arachnids, and yet Phintella is drawn to them and doesn’t seem to come into harm’s way when it’s around them. In experiments, Phintella’s survival rate among the ants was much higher than those of other regular jumping spiders, and closer to spiders that eat or mimic the ants.
What would drive Phintella into the company of an ant that it should be afraid of, scientists wondered, when there doesn’t seem to be any benefit? And why doesn’t the ant make a meal of it?
The answer to the first question is “something scarier than an ant.” For jumping spiders in the Philippines, that would be the spitting spiders of the genus Scytodes. While most spiders deliver venom from their teeth and spin silk into webs, these guys turn both into projectile weapons. They mix liquid silk and venom into a sticky fluid and “spit” it at prey, at once trapping it and poisoning it. The spider then approaches its prey and bites it, delivering another dose of venom that begins to liquefy its tissues. Phintella is a frequent victim of this hunting technique, and Scytodes spiders are known to build their webs directly over Phintella’s nests and attack the smaller spiders as they come and go from their homes.
Scytodes stays far away from weaver ants, though, which got New Zealand biologists Ximena Nelson and Robert Jackson thinking that the jumping spiders hang out with the ants for protection. When they got the three arthropods together in the lab, they found that jumping spiders were more likely to build nests when they could see or smell ants than when other insects were around or there was no sign of ants. The spitting spiders, on the other hand, were much less likely to build their webs when they could see the ants. They were also more likely to die when the ants were around, while the jumping spiders largely avoided becoming ant food.
To Nelson and Jackson, the differing nest-building responses and survival rates between the two spiders made it pretty clear that Phintella sticks close to the ants because they scare away or kill spitting spiders. And while ants would be happy to eat the jumping spiders, too, Phintella has a few tricks up its sleeves that turn enemy territory into a safe haven.
First, jumping spider nests aren’t like typical spider webs. They’re more like silken cocoons, and provide some resistance to intruders. Phintella then beefs up the nest’s security even more with a tough, dense weave of silk that the ants have a hard time tearing through. It also adds hinged flaps to either end of the nest that act like doors. The spider can get in and out of the nest as it pleases and close the flap behind it to keep the ants out.