Black widows: does their reputation precede fact?

By Shreyas Kuchibhotla

There are few things that man abhors more than the spider. For millennia, our perception of these creatures has been one driven by disgust, hate and unbridled horror. They possess all too many unblinking eyes, eight hairy legs that scale walls with ease, and a tendency to appear out of nowhere. About 3-15% of the population suffers from arachnophobia – a crippling fear of spiders¹ – and the main culprit is one of the most infamous members of this group – the black widow. 

It would not be an exaggeration to consider the black widow the celebrity of its ilk, instantly recognisable from its menacing black body adorned with an iconic red hourglass. The quintessential femme fatale, this spider’s common name comes from the female’s macabre habit of killing her mate after copulation, an infrequent² yet surprisingly widespread occurrence among arachnids that is termed sexual cannibalism.³ All of this, however, is trumped by the black widow’s specialty – its lethal venom. It is often touted as the mother of all toxins, killing within minutes and leaving one writhing in pain as their life flashes before their eyes. In reality, very little common knowledge about the black widow is true. To begin with, the ‘black widow’ is not a unique identity. The name ‘black widow’, or more precisely ‘widow’, has been co-opted to describe not one but over 30 species of spiders living in temperate regions across the world. The genus Latrodectus (Greek for ‘biting in secret’) consists of numerous species all sharing the same body template⁴ – meaning that the resemblance of the notorious Australian Redback (Latrodectus hasselti) to the American black widows is not just superficial.⁵Additionally, some of these spiders sport brightly coloured markings to warn predators to stay away (a phenomenon known as aposematic colouration).⁶

Almost all spiders are venomous,⁷ but only some are truly dangerous. Except for heavy-bodied hunters like the wolf spiders and huntsmen, which rely on their speed and strength to make a kill, the vast majority of spiders are weak, soft-bodied and slow-moving. To survive in the chaotic world of the undergrowth, they require biological weaponry in the form of silk and venom. Against humans, though, the efficacy of venom as a defensive tool is limited. To cause severe envenomation in a creature several thousand times a spider’s mass, there are three factors that need to work in its favour. It must have sufficiently virulent venom, fangs that can penetrate skin, and glands able to deliver large enough doses. The widows fall into this elite class (quite literally, as only females are known to deliver life-threatening bites),² and so they are widely thought to be some of the most dangerous spiders in the world.  

Nevertheless, the final (and arguably most important) factor that determines a spider’s danger to humans is its disposition. A dangerously venomous spider that does not like to bite is akin to a stationary knife inches away from your skin. While harmful in theory,it is entirely benign until you press your finger onto it. Black widows are extremely adaptable spiders and are often synanthropic (i.e. they live close to people),² with many homeowners blissfully unaware of their presence. For about 300 million people in the United States, only about 2500 bites are reported annually, of which a paltry 4-8 are fatal.⁸ For a spider confronted by a giant ape, the first sensible course of action is to flee. And this is precisely what most will do. The rule of thumb (pun intended) for spider bites is that unless they are forcefully pressed against a barrier or squeezed, they will always prefer to escape. Black widows are no exception, and most bites are caused due to accidental contact. A past study by researchers from Loma Linda University showed that even when prodded several times by a finger-like object, these spiders simply retreated. Instead, only when they were squeezed between jelly fingers (certainly piques curiosity regarding the experimental method) did they even attempt to bite.⁹ This is practically demonstrated by wildlife educators like Nathaniel “Coyote” Peterson of the Brave Wilderness YouTube channel, who lets a black widow walk along his hands for several minutes without receiving anything remotely close to a bite.¹⁰ After all, as far as a spider is concerned, a human is simply a slightly motile tree – until the tree starts pinching its abdomen. 

This is not to say the venom is not powerful, however. The venom of the Southern Black Widow (Latrodectus mactans) has an LD50 (Lethal Dose-50, the dose of venom per unit body mass required to kill 50% of a group of experimental animals)¹¹ of 0.26 mg/kg when injected intraperitoneally (into the abdominal cavity).¹² This is almost 15 times stronger than the 3.5-4 mg/kg dose for the Western Diamondback Rattlesnake (Crotalus atrox) and comparable to the 0.11-0.24 mg/kg dose for the Mojave Rattlesnake (Crotalus scutulatus), the most toxic species of rattlesnake.¹³ It is worth noting that LD50 by itself is not a complete metric of potency, for it only depends on whether death has occurred and not the time taken to perish – an equally important gauge of predatory usefulness. Even so, the fact that the black widow has a powerful venom has never been under question; simply the public perception of the risk it poses.

Like many other venomous creatures, widows use a neurotoxin to incapacitate prey. The primary active component of their venom, a-Latrotoxin, attacks nerves by stimulating excessive release of neurotransmitters such as acetylcholine and norepinephrine and preventing their vital reuptake. This causes spastic paralysis¹⁴ in important muscles, which can eventually lead to death. While this is likely the effect elicited in prey such as insects, healthy adult humans are unlikely to die from a typical venom dose.¹⁵ The characteristic symptoms of a black widow bite – sometimes referred to as Latrodectism – are a pinprick-like pain at the bite site, followed by muscular pain, headaches, nausea and a slew of other ailments. Nevertheless, if a patient bitten by a black widow is promptly taken to the hospital, chances of death are close to nil.¹⁶

The black widow has a fearsome reputation. The connotation of their name has historically been one of terror, and not without reason. Widow spiders would much rather use their biological arsenal to rid homes of pesky flies than to attack the giant creatures that inhabit them. In an age where ominously named but inoffensive Noble False Widows (Steatoda nobilis) rapidly expand their range into the United Kingdom and the mass media thrives on sensationalising their ‘invasion’,¹⁷ it is perhaps time for us to step back and reflect on the meaning of the word ‘widow’ – and remember that the real victims here are the tiny males of a species named for their murderous spouses. 

References: 

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16. Latrodectism: Diagnosis and Treatment · California Poison Control System (CPCS) [Internet]. California Poison Control System (CPCS). 2006. Available from: https://calpoison.org/news/latrodectism-diagnosis-treatment
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