Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

The antagonist (Image: Alex Wild/Corbis)

Species: Myrmecia pyriformis and Delena cancerides

Habitat: the dry forests of eastern Australia, where spider and ant fight a perpetual war of attrition

When the end comes, it is sudden. Picture the scene: for months the mother spider has maintained her home while her offspring grow to adulthood. The youngest ones still need many more months of care, but the eldest are almost ready to leave. It has all been going well, until in one night they are driven out.


Bulldog ants battle their way into the spiders’ nest, and they show no mercy. The mother spider manages to kill some, but eventually she is overwhelmed. Retreat is the only option.

But that is not enough for the ants. These spiders are their competitors, and they don’t want a new family moving into the nest. So before they leave, they fill the nest with twigs and leaves, ensuring it is completely useless. This “scorched Earth” tactic ensures that the ants will not have to compete with spiders for a long time.

Destroyer of worlds

The bulldog ant is the most dangerous ant on Earth according to Guinness World Records, having killed at least 3 people since 1936. They are big ants, sometimes reaching more than 2.5 centimetres long, with a powerful bite and toxic venom.

“I’ve never been stung, but I’ve been told it’s an incredibly painful experience,” says Eric Yip of the Ben-Gurion University of the Negev in Israel.

Bulldog ants live in colonies dug into the soil, which can be 1400 strong. The worker ants start foraging in the evening twilight and often spend the entire night out. They seem to share food with their fellows when they return to the nest.

That lifestyle puts them on a collision course with Avondale spiders, which forage at the same time and in the same places. The two species are mortal enemies.

Spider army

Avondale spiders, also known as flat huntsman spiders, are unwittingly famous. They were used in the film Arachnophobia, where they “played” the deadly spiders infiltrating a US town. The producers chose them partly because they are harmless to humans and so were safe to work with.

But they do rather come as a shock when you first see them. “An adult female can be the size of your palm,” says Yip. A width of 20 centimetres – mostly legs – is quite normal.

What’s more, where you find one Avondale spider, you will find many. They are one of the few spiders that live in social groups, typically consisting of around 100 individuals but sometimes reaching 300. Each group is made up of an adult female and several generations of her offspring that take a year to reach maturity.

The young spiders sometimes move to neighbouring colonies, so around 47 per cent of colonies have at least one immigrant living alongside the natives. But the spiders are judicious about who they accept. They can recognise unrelated spiders, and if the strangers are close to breeding age, they are likely to be attacked and even cannibalised.

Most social spiders build their colonies around large webs for catching prey, but the Avondale spiders don’t build webs. Instead they make their homes under the loose bark of dead trees. Each spider normally hunts alone, but the older juveniles do share food with their younger siblings, who grow larger as a result.

She’s leaving home

Yip has been studying Avondale spiders for almost a decade, and on two occasions he saw bulldog ants entering the spiders’ nests – and in one case seemingly driving them out. Wondering if this was a regular occurrence, he set up 132 nest boxes, 120 of which were colonised by Avondale spiders.

Over two months, seven of the nests were invaded by bulldog ants. In each case the spiders were driven out and the nests were filled with debris like twigs, leaves and pieces of bark. Yip even caught groups of ants in the act of carrying in the debris.

“The retreat is useless once it fills up with debris,” says Yip. “They have to have the space to grow.” None of the nests were re-colonised during the study.

The mother spider was “pretty good” at fighting back against the ants, says Yip. “In a couple of instances the adult female was able to kill all the ants.” But her offspring were not very good, as “their fangs can’t puncture the ant exoskeleton.”

Avondale spiders need nests to live in and suitable shelters are scarce, so losing one is a major blow. This might make it seem like the spiders are losing out to the ants, and are on the road to extinction. But Yip suspects that the ants are actually the underdogs.

“Spiders famously have a slow metabolism,” says Yip. That may give them an advantage, allowing them to survive while eating less. If the ants have a faster metabolism and thus need more food, having spiders around may pose a real threat to them. Their only option may be to drive the spiders out. A behaviour that looks like an act of dominance may actually be one of desperation.

Journal reference: Insectes Sociaux, DOI: 10.1007/s00040-014-0368-0