Sydney Funnel-Web Spider: The World's Deadliest Spider

Spiders can give many the creeps. Some of them are downright dangerous. We bring to you the deadliest of the lot.

Photo Credi: David Wilson/The Economist

Australia has a reputation for harbouring creepy critters ready to kill you. Reputations can be overblown sometimes. This one, however, seems to be well earned. Mostly found near the coasts of New South Wales, in moist habitats under logs or foliage and frequenting gardens, is the Sydney funnel-web spider or Atrax robustus. The male of the species was given the dubious honour of being officially recognized as the world’s most venomous spider by the Guinness World Records. Just 0.2 mg/kg of the male’s venom is a lethal dose for primates.The reason for the male being singled out maybe two-fold: female funnel-web spiders are sedentary and pass most of their lives inside their burrows, being less likely to encounter humans and most female funnel-web spiders have less potent venom than their male counterparts.

Although these spiders produce extraordinarily complex venoms, with each venom containing up to several thousand peptide toxins, their infamy for causing human fatalities can be attributed to venom peptides known as delta-hexatoxins (δ-HXTXs) that exert fatal neurotoxic effects in humans by inhibiting inactivation of voltage-gated sodium channels. These are one of the most important cellular features that facilitate communication signals in various systems, making them crucial in a wide range of coordination and physiological functions, from locomotion to cognition. An envenoming bite from a male Sydney funnel-web may cause adverse effects on the autonomic nervous system, cardiovascular effects, neurological effects (oral paraesthesia, muscle fasciculations, muscle spasm), and/or pulmonary oedema (fluid accumulation in the air spaces of the lungs).

One study investigating “species-specific envenoming rates and spectrum of severity of funnel-web spider bites” found the highest number of total envenoming bites and of those, the highest number of severe envenomations, to have involved the Sydney funnel-web spider. This could possibly be because the male spiders end up wandering into backyards, getting trapped in houses and seeking cover at dawn (dry daytime surface is hostile to the species) in dark, moist and cool hideaways  (like a shoe of an unsuspecting victim, left outdoors). The same study also found the rate of severe envenoming by the species to be 17%. The onset of severe envenoming is rapid, with deaths reported within an hour. “In our study, the median time to onset of envenoming was 28 minutes”, they reported.

However, they found no confirmed deaths after funnel-web spider bite since the antivenom was introduced in 1981. The antivenom produced against the venom of the Sydney funnel-web spider is effective against other species of funnel-web spiders as well.

Ironically, the deadliest spider for humans could not have evolved its lethality against humans through a direct contact with primates, since they evolved much later than funnel-web spiders did (Source: AusMuseum, HumanBiology). A study investigating the precise ecological role of δ-HXTXs in their venom concluded, “the lethal potency of δ-HXTXs against humans is an unfortunate evolutionary coincidence”.

Funnel-web envenomations, per the study, are mostly inflicted by male spiders in search of females during the mating season, suggesting the role of the lethal toxins to be predator deterrence, not hunting, since male spiders rarely feed during this period. That inducing of pain, a well-documented strategy for defensive toxins, is a common symptom following funnel-web envenomation, supports this theory, and so does the fact that the mostly sheltered females of the species have a bite with milder effects. 

Hence, these toxins likely evolved to affect negatively their vertebrate predators, like bandicoots, birds, and lizards. “Interestingly, in striking contrast to humans and other primates, some vertebrates such as dogs and cats are insensitive to funnel-web envenomation”, stated the study.

Nature isn’t kind. Life is, at its core, competitive. Although, we have been given many advantages, it seems like even we cannot escape some of nature’s darker jokes.

Editorial Support: Phoenix Content India