Sydney funnel-web spiders are one of the most infamous arachnid species on Earth due to their potent venom that can prove fatal for humans if they’re unlucky enough to get bitten. Both sexes within the species produce venom, but it has long puzzled spider specialists as to why the males’ venom is so much deadlier than the females’. Now, new research published in the journal Proceedings of the National Academy of Sciences has discovered why the males adapted to be so dangerous, with hopes that an improved understanding of their venom can inform better treatments for people who find themselves on the receiving end of these eight-legged assassins.

Led by Associate Professor Bryan Fry, a team of researchers from the University of Queensland spent 20 years investigating delta-hexatoxins, the venom peptides that make funnel-web spider venom so deadly. This group of toxins cause fatal neurotoxic events in humans as it essentially suspends their nerves in an active state, firing over and over again. The resulting symptoms include profuse salivating, sweating, and muscle spasms followed by a spike in blood pressure and heart rate that when combined with respiratory distress can prove fatal.

"It has puzzled scientists why these toxins are so deadly to humans, when they and other primates, haven't featured as either prey or predator during the spider's evolution,” said Fry in a statement. “We couldn't understand why most human deaths were being caused by male funnel-web spiders, which seemingly had much deadlier venom than females."

They decided to take a closer look at the venom of 10 funnel-web species using molecular analysis and were able to describe and profile 22 new delta-hexatoxins from the samples. Previously, only eight had been analyzed meaning the team almost tripled the available data on funnel-web venom. The influx of information revealed how the males had arrived at their specific cocktail of delta-hexatoxins as the result of evolutionary pressures.

"These toxins had originally evolved to kill insects such as cockroaches and flies,” said Fry. “But, when male funnel-web spiders become sexually mature, they leave the safety of their burrow and wander quite considerable distances in search of females. This can be quite treacherous, and these male funnel-web spiders started to encounter dangerous vertebrate predators, such as the dunnart, a small nocturnal mouse-like marsupial.

"The data shows that natural selection put the necessary pressure on to switch an insect-specific venom into a vertebrate-specific defensive venom. And, unluckily for us, we're a vertebrate species which copped it in the process."

The team are hopeful their research will provide a better understanding of exactly what funnel-web spider venom does to the human body and how best to combat the symptoms with effective treatments. They also want to see if the discovery could inform new insect-specific toxins to create better insecticides that could eradicate invasive or unsafe infestations.

"They're dangerous as hell,” said Fry. “But male funnel-web spiders offer us some real opportunities."