Spiders take a leaf out of Spielberg’s book in dealing with potentially deadly adversaries, according to recent research. ”Keep absolutely still.. his vision’s based on movement.“
What if you had an unassailable personal army of bodyguards, but who would kill you the instant they knew they could? What if your only available prey were potentially vicious killers? You have to be able to catch one, and at the same time avoid becoming an hors d’oeuvre on *their* menu.
Such is the problem for any arthropod that decides to rely on ants for its survival. To put it mildly, they’re not the world’s easiest, plumpest or least-resisting meal. They can get a bit, shall we say, argumentative.
But if you can crack it, boy, you’ve got it made.
Plenty of insects have given this a shot – so many that the habit is given its own name, “myrmecophily”.
For example, the butterfly family Lycaenidae have this down to a fine art (explored in this review by Pierce et al 2002).
Take Liphyra, which I remember most clearly because it was featured on one of David Attenborough’s marvellous programs. The caterpillars are shaped like impregnable tanks and slowly gorge themselves on ant larvae while the ants are helpless to stop them. Once emerged, the adults are covered in waxy scales that the ants cannot grip, and waltz out of the nest to fly away unharmed.
Some Lycaenids have evolved glands (called “cupola glands”) that secrete perfume that mimics the ants’ own secretions. Consider for example the caterpillars of Niphanda, which cloak themselves in the perfume of ant larvae, and then sit back and relax as they are enthusiastically fed to adulthood by the ant workers. Hojo et al (2009) found that the ants would even feed glass beads that had been experimentally rolled in the correct perfume.
Beetles of the tribe Paussini have arrived at a similar set of solutions, reviewed here: some are shaped like little tanks while others cloak themselves in protective chemicals. In all cases the beetles munch happily away on the ant larvae while the ants can do nothing to stop them.
Spiders can also use chemical trickery: Cosmophasis spiders live within ant nests, again safely cloaked in “Eau de Fourmi” acquired (in a macabre twist shown by this study) directly from the ant larvae on which they feast.
Aphantochilus spiders, although they are not chemically or physically protected, have evolved a very particular set of skills to allow them to pick off individual ants. They are still vulnerable to attack, though, especially when small. Accordingly the mother defends small offspring vigorously against attacks by their future prey.
Many arthropods use ants as protection. No-one’s likely to attack you if you are surrounded by thousands of angry, biting micro-warriors. But the same principle applies; nothing’s stopping those warriors from turning on you.
Aphids, treehoppers and caterpillars provide “protection money”. They are well-known to be farmed by ants, a great example of a protection mutualism where the insect produces a sticky, sugary “gift” in return for the ants’ protection. Treehoppers that normally provide parental care known to turn over responsibility for their offspring entirely to ants. This recent study found that treehoppers can even summon ants using signals made by vibrating their wings when they are attacked by predators. These associations run so deep that ants will even protect aphid eggs, which do not produce honeydew – in effect the ants are investing effort for a future reward.
Sweet rewards are cheap for aphids and treehoppers (since honeydew is waste material to them) but caterpillars must produce this substance de novo, making it rather expensive. Lycaenids do this by means of “tentacular glands” (see Pierce et al‘s review).
The old Jurassic Park trick
Eustala spiders are not so generous. A new study in the journal Ecological Entomology by researchers Loriann Garcia and John Styrsky shows that Eustala like to keep it simple; no complex chemicals, super-thick armour, or twinkle-toes defence techniques – and definitely no sugary gifts for the ants.
Instead, like that nerve-jangling scene from Jurassic Park, what Eustala does is to stay really, really, really, really still. Even with ants crawling all over it, feeling and probing it with their antennae. And the ants don’t notice it.
By day, Eustala spiders rest on Acacia trees in Panama. They sit on the thorns, and oddly are not attacked by the ants that swarm over the plant and over the body of the spider.
Why are they not attacked? The authors did an experiment using both Eustala and Argiope, a spider that lives on ant-free trees. They placed recently-killed (frozen and just-thawed) spiders of both species onto branches containing ants, and watched what happened. For comparison, they released moving spiders (still agitated after being handled) of both species onto nearby branches.
Playing sleeping lions… with actual lions
Regardless of species, the immobilised spiders were not attacked, but the moving spiders were. Moving
Eustala had no particular advantage over moving Argiope. That shows that Eustala is not primarily relying on any kind of chemical camouflage to defend against the ants.
Furthermore, Eustala had one more trick up its sleeve. If discovered, Eustala jumped to safety on a line of silk, allowing them to cautiously rappel back up the line once the ant attack subsided and resume their position. Once in this position, they did not react at all to patrolling ants, even when tickled by ants’ antennae and lunged at by ants’ jaws.
Argiope, on the other hand, tended to try and run away, which further enraged the ants and ended up with them being killed or driven off the plant entirely.
One huge Tyrannosaur or millions of tiny ones: clearly the advice is the same.