My hungry eyes were scanning leaves and twigs for anything ‘bug’. It was evening and the light was fading fast in the Arizona forest, particularly here in the understory. I don’t know what made me do a double-take but I’m glad I did.
- The weirdest insects in the world – including a moth that resembles a poodle and a fly with eyes on stalks
- With deadly long jaws that drain their prey dry, this is one of the world’s most spine-chilling spiders — and it's known as a lethal assassin
A fresh and particularly succulent bird dropping sat next to the midrib of a leaf. Then I noticed it had eyes… and legs – eight of them.
I had just found a bolas spider, an arachnid master of both disguise and a rare and remarkable skill.
The species I was looking at was the southern bolas spider (Mastophora cornigera), sitting with her legs pulled in and her body parts all perfectly interlocked.
- Can spiders really be beautiful? Oh yes... meet the prettiest spiders on the planet, from metallic-blue tarantulas to dancing peacock spiders
- Why do spiders have 8 eyes?
How do bolas spiders protect themselves from predators?
This unspidery profile was combined with a body colouration of rich browns with swirls of white on the abdomen (like the white uric acid in a bird dropping); a cephalothorax with two horns, and a surface textured with multiple rounded beads resembling post-digested seeds.
The whole thing was made to look even more repugnant by being high-gloss, giving the appearance of a bird dropping of the messiest kind. No avian in its right mind would take a peck at that, which is, of course, the whole point.
But it is not just their unappealing mimesis that makes bolas spiders so interesting, and the clue is in their name.
How do bolas spiders hunt prey?
A bola is a traditional hunting missile from the Americas, consisting of two or more weights on a leather cord or strap, hurled with great accuracy at fleeing prey, so as to entangle its legs or wings.
Bolas spiders don’t build a web to ensnare their prey, like most members of the orb weaver family. Instead, they ambush prey using a device that superficially resembles a bola.
It isn’t really used in the same way as the human device, but it is just as lethal. By day, a bird turd; by night, an agile and accurate flinger of projectiles.
- Creatures that shoot to kill: Discover nature's deadliest missile launchers
- Deadliest ambush predators – the animals with killer techniques such as corpse camouflage and impalement
In preparation for the night’s hunting, the bolas spider drops down on a line of silk. When in position, she produces another single line of strong silk with a weighty globule of liquid silk at its end.
The silk strand is then supported over a cocked leg, in much the same way an angler fingers his fishing line, allowing precise control of the line and its movement.
- How do animals make silk? And why is it so strong?
- Is spider silk - pound for pound - as strong as steel?
She now waits for a moth to arrive. She knows it will be a moth because that is what she is calling. As it turns out, resembling bird poo is not the bolas spider’s only mimetic genius – it also uses ‘aggressive chemical mimicry’.
How does it attract moths?
It deploys an allomone (a type of chemical) from a gland on the abdomen, and this perfume is a replica of that produced by a female moth. The male moths are fooled and lured in.
The extra genius is that each spider can switch between up to three different chemical analogues, depending on which species of moth is on the wing.
Detecting the vibrations of incoming wingbeats, Mastophora is ready and accurately swings at the moth with her ball of sticky goo.
Now, moths are not ‘sticky’ kinds of insects. Their wings are covered in a slippery coating of microscopic and loosely attached scales. The secret to the spider’s success is that the 2.5mm bola is more than just a dollop of goo.
It has an intricate microstructure comprising multi-folded fibre enclosed in a viscous fluid, which is, in turn, embedded in another layer of less viscous fluid.
As the bola hits the moth wing, the low-viscosity liquid flows between and past the moth scales and adheres to the cuticle below. The more viscous fluid then acts as a glue, bonding this to the silk.
The folded thread acts as elastic, allowing the whole droplet to deform when it is flung, increasing the spider’s range.
The accurate deployment of this droplet of doom occurs in a split second, in the dark. It is incredibly precise.
But if no luck comes the spider’s way, she simply consumes the silk, bola and all, and with her resources recycled, tries again the following night.
Top image: bolas spider (Mastophora cornigera). Credit: Brett_Hondow/Getty Images
