In a dry, open field in New Mexico, US, a hungry lizard spots a brightly-coloured, hairy insect scurrying across the sandy soil. Thinking it has found a meal, the lizard sprints to catch the insect. But once it has the insect in its mouth, it finds it is too hard to chew.
The lizard then moves the insect around to find a softer chewing angle but gets nowhere. Meanwhile the insect starts to squeak and finally stings the luckless lizard in its mouth. Alarmed, the lizard spits it out.
The insect, still squeaking, gets away unscathed. The lizard is left with nothing but a sore mouth and a foul taste.
This sturdy insect is a female velvet ant. These females have an arsenal of defences unmatched by their male partners, or any other insect. The question is, what terrifying predator forced the females to evolve so many defences? And if they are in such dire threat from predators, why are they brightly coloured?
Let's get the confusing bit out of the way first. Velvet ants are not really ants: they are wasps. They got their name because the females look like large ants, albeit ants clothed in dense velvety hairs of various hues: they can be yellow, orange, red, white or black.
Female velvet ants have taken the idea of warning colouration to a whole new level
In most species, males are rather plain-looking. They are so unlike the bright and furry females, it can be hard to tell which male pairs with which female.
When faced with a predator, the males have an obvious advantage. They have wings, so they can simply fly away.
However, the females are both grounded and conspicuous. Their bright colours may seem like a dead giveaway, but they are actually a signal warning predators to stay away – just like the bright colours of a wasp warn predators that it can sting.
What's more, female velvet ants have taken the idea of warning colouration to a whole new level.
Different species of velvet ants have evolved to mimic each other. By resembling other velvet ant species from their neighbourhood, these solitary creatures have found strength in numbers.
When a velvet ant becomes aware of a threat, it starts to squeak
Naïve predators that try to tackle a velvet ant soon get the message, and steer clear of them in future. By all flashing the same signal, the velvet ants ensure that they all share in this protection.
Among the velvet ants of North America, the colour mimicry is extremely widespread. In a study published in August 2015, researchers grouped over 300 species into just 8 mimicry clusters, based on similarities in colour, hair density and location. They range from silvery and downy velvet ants found in the hot deserts to the brownish-red and bald species found east of the Rocky Mountains.
But this colour signalling is just the start of the velvet ants' arsenal. They also use a combination of audio and chemical signalling to deter predators.
When a velvet ant becomes aware of a threat, it starts to squeak or "stridulate".
Velvet ants can deter predators by releasing odours
It does this by moving different sections of its abdomen in and out. This motion rubs a tooth-like projection on the second section, the "scraper", against a ridged structure on the third section, the "file".
The resulting squeaky noise can warn off predators while they are still at a distance. But a velvet ant can also stridulate if it gets caught unawares and picked up in a predator's mouth.
To the predator, that might feel "like a mini jackhammer going zzzzzzzzzzz" in its mouth, says entomologist Justin Schmidt of the Southwest Biological Institute in Tucson, Arizona, US. If this sensation was unpleasant enough, the predator would open its mouth and the velvet ant would escape.
On top of their squeaking, velvet ants can deter predators by releasing odours.
They have well-developed glands that secrete smelly "allomones": chemicals that manipulate the behaviour of another species.
They are also agile and remarkably strong
Specifically, they make ketones, which ants are known to use as alarm pheromones. That makes sense: velvet ants often come across ants, so they may have evolved specific allomones to repel the ants, which Schmidt and his colleagues once described as "major potential predators" of velvet ants.
In experiments, tiny flags coated with one of these ketones were enough to trigger alarm behaviour among harvester and carpenter ant workers. The same flags also prompted fire ants to scatter until the ketones had evaporated. Similarly, fire ant workers fed less when their honey was laced with a cocktail of ketones.
So velvet ants look scary, sound scary and smell scary. They are also agile and remarkably strong.
In a study published in 1977, Schmidt and his PhD advisor Murray Blum presented velvet ants to a selection of predators, including ants, spiders, a praying mantis, lizards, birds and gerbils.
A velvet ant has a hard, slippery and rounded outer shell
When attacked by a few red fire ants, the velvet ants freed themselves by quickly scraping the ants off using their strong and muscular legs. However, when attacked by many ants at once, the velvet ant both removed the fire ants faster and ran faster to escape.
Schmidt and Blum repeatedly chased velvet ants with their fingers and estimated that they could scamper at about 0.5km/h (0.3mph). They can achieve these speeds because certain muscles, which in winged males control flight, are used to make the females' legs stronger, says Schmidt.
A female velvet ant's legs are so powerful, adds Schmidt, she can use them to wrestle her way out of a predator's mouth. You might think that a bigger animal like a lizard could easily crush her in its jaws, but the velvet ant is too tough for that.
A velvet ant has a hard, slippery and rounded outer shell, and this saves it from being crunched. Schmidt and Blum calculated that the force required for successfully crushing a velvet ant is about 11 times that for a worker honeybee, and almost twice that for a stag beetle.
What it lacks in toxicity it makes up for in sheer pain
In line with that, when insect collectors try to pin down a dead velvet ant, they often miss because the pin glances off and pierces their finger.
Schmidt's experiments show that a velvet ant's tough shell helps to protect it from spiders, which try to inject it with venom.
If none of that is enough to deter a predator, a velvet ant packs a legendarily painful sting.
Only females have stingers. That's because the stinger, which is highly flexible and half as long as the insect itself, is a modified egg-laying organ called an ovipositor.
Dasymutilla velvet ants are sometimes known as "cow killers", which is ridiculous as their sting is essentially harmless
The venom from a velvet ant's sting is only mildly toxic, being one of the least chemically active insect venoms compared to other stinging wasps, ants and bees. But what it lacks in toxicity it makes up for in sheer pain – something Schmidt can attest to.
In 2015, Schmidt was awarded an Ig Nobel Prize "for painstakingly creating the Schmidt Sting Pain Index, which rates the relative pain people feel when stung by various insects". In a study published in 1984, he rated insect stings from 1 to 4, where 4 is the most painful, based on how it felt when insects stung him. So he knows exactly how painful a velvet ant sting is.
One genus of velvet ants is particularly excruciating. Dasymutilla velvet ants are sometimes known as "cow killers", which is ridiculous as their sting is essentially harmless – apart from the pain.
Schmidt rated their stings between 2 and 3. He later described the sting of D. klugii females as causing "intense burning", with "variable reactions" lasting 5–30 minutes.
Given that they don't have stingers, males shouldn't be able to sting. But they have found a way to fake it.
Unlike a lot of velvet ant researchers, Joseph Wilson of Utah State University Tooele in the US has never been stung by a female. But he has been stung by males.
There's nothing that we have found that regularly eats velvet ants
"When you grab them they will aggressively poke you with the pointed parts of their genitalia," says Wilson. "They can feel like little needles sticking into your finger."
This fake sting can be enough to dupe a predator into letting a male go. "Generally, the pseudo-sting isn't too painful but it can surprise you if you aren't expecting it," says Wilson.
Clearly, velvet ants are not insects to mess with. But it has proved surprisingly difficult to identify the predator that they are defending themselves from.
Lizards have long been the prime suspects. They are active at the same times and in the same locations as velvet ants, and they eat insects that are similar to velvet ants.
The velvet ants almost always escaped unharmed
In the 1980s, two biologists studying the diets of collared lizards in the southern US found velvet ants in the stomachs of two females. Once in a while skinks and frogs have also been found to eat them. But such reports have been few and far between.
"There are some instances when potential predators will eat [velvet ants], but in general there's nothing that we have found that regularly eats velvet ants," says Wilson. He is currently trying to find out if western whiptail lizards eat them.
"There aren't really any true or meaningful predators," agrees Schmidt.
Schmidt could not identify any in his experiments. Ants, spiders, lizards and gerbils all attacked velvet ants, but the velvet ants almost always escaped unharmed.
Maybe their defences did not evolve to deal with predators after all
A tarantula and a gerbil did manage to eat one each, but that was it. Some predators gave up after one or two attempts and others after being stung, even though they habitually preyed upon stinging wasps and toxic ants.
In an intriguing experiment published in 2001, researchers dropped four velvet ant species into enclosures housing insect-eating Texas horned lizards. The lizards only ate the ones that looked like the harvester ants they normally eat, and ignored the rest.
That suggests velvet ants only get eaten by accident, by predators targeting the ants they resemble. So maybe their defences did not evolve to deal with predators after all.
Instead, it's conceivable that they evolved to protect the velvet ants when they lay their eggs.
You've got to have enough defences so that you can make it through
After mating, a female velvet ant sets off alone to find the closed underground nests of solitary wasps or bees. These insects seal their larvae inside their burrows with some food, and never return. The velvet ant female breaks into the nest and lays an egg inside.
When the velvet ant larva hatches, it feasts on the larva, munching its way into its innards. It then pupates inside the nest and makes its way out as a fully-grown adult.
According to Schmidt, this "parasitoid" lifestyle is fraught with risk.
"You have a pretty hard time finding your hosts, which are often widely dispersed," he says. "If you are spending hundreds and hundreds of hours exposed, with people looking at you like, 'oh yum, dinner', you've got to have enough defences so that you can make it through."
This is doubly true for velvet ants that, instead of targeting the undefended nests of solitary insects, break into the guarded nests of social wasps and bees.
These large nests are tempting targets, because they offer a lot of egg-laying opportunities in one place, but they are also more dangerous so only a few velvet ants tackle them.
One such species is Mutilla europaea.
It once targeted bumblebees, with great success. Records from the 19th century note instances when more velvet ants hatched from a bumblebee nest than did bumblebees. They also wreaked havoc in honeybee hives.
If the wasps do attack the velvet ant, its hardened shell protects it
Nowadays, M. europaea velvet ants are occasionally seen strolling into the nests of Polistes biglumis, a species of social paper wasp. They are there, not to lay eggs, but to eat – and they have picked an unusual diet.
These velvet ants suck the saliva from wasp larvae, without inflicting any visible damage upon them. "Larval saliva is highly nutritious," says Maria Cristina Lorenzi of the University of Turin, Italy.
Lorenzi found that an M. europaea female can sneak around a paper wasp nest unobserved. That's thanks to the chemical makeup of its outer shell, which has low concentrations of certain chemicals that cause the wasps to recognize and attack intruders. If the wasps do attack the velvet ant, its hardened shell protects it.
Clearly, velvet ants like M. europaea have good reason to evolve defences against the insects they are targeting. But that can't explain the other defences, like colour mimicry and squeaking, which play no role in this.
That brings us back to the idea of predators and the threat they pose to velvet ants.
It seems to me like it might be kind of a fun life to be a velvet ant
"We think it's just about predation," says Wilson. "Maybe there's an extinct predator, and these defences evolved in connection with some predator that we are not able to identify because it's no longer here."
Alternatively, it may be that velvet ants were faced with a multitude of predators, all using different strategies, so had to evolve a range of defences to safeguard against them all. "Different characteristics defend better against different predators," says Wilson.
In that case, maybe the reason so few animals eat them is simply that their defences are effective. Evolution has pushed them to have so many defences that, nowadays, most predators target easier prey.
"They are such masters of life," says Schmidt. "They really have figured [out] how to survive and do everything right. It seems to me like it might be kind of a fun life to be a velvet ant."
There's one last thing. For all that we have discovered about velvet ants, we have no idea about the most obvious question of all. Why are they so hairy?