The prevalence of sexual cannibalism in Latrodectus females has inspired the common name "black widow spider". The females frequently eat their male partners after mating. The female's venom is at least three times more potent than that of the males, making a male's self-defense bite ineffective. Research at the University of Hamburg in Germany suggests this ultimate sacrifice strategy has evolved to promote the survival odds of the offspring.
DescriptionEditNot all adult black widows exhibit the red hourglass on the ventrum underside or top of the abdomen — some may have a pair of red spots or have no marking at all. Female black widows often exhibit various red markings on the dorsal or top side of the abdomen, commonly two red spots. However, black widow young are believed to have at least some sort of marking on their abdomens. Adult male black widows are half the size of the females, and are usually gray or brown rather than black and red; while they may sometimes have an hourglass marking on their ventral abdomen, it is usually yellow or white, not red. Variation in specifics by species and by gender is great; any spider exhibiting a red hourglass or a pair of large red round spots on the ventral abdomen with an otherwise black shiny body is an adult female black widow. The bright red hourglass and spots are never located on the dorsum, which is the more visible aspect; the identifying features are on the underside, anatomically known as ventrum; i.e., the spider must be lying on its back to reveal the markings.
In common with other members of the Theridiidae family, the widow spiders construct a web of irregular, tangled, sticky silken fibers. The spider very frequently hangs upside down near the center of its web and waits for insects to blunder in and get stuck. Then, before the insect can extricate itself, the spider rushes over to bite it and wrap it in silk. If the spider perceives a threat, it will quickly let itself down to the ground on a safety line of silk. As other web-weavers, these spiders have very poor eyesight and depend on vibrations reaching them through their webs to find trapped prey or warn them of larger threats. While some species are more aggressive, most are not; many injuries to humans are due to defensive bites delivered when a spider gets unintentionally squeezed or pinched. Some bites are thought to result from a spider mistaking a finger thrust into its web for its normal prey, or in cases where a female is protecting an egg sac, but ordinarily intrusion by any large creature will cause these spiders to flee.
Strength of Latrodectus silkEdit
Silk from L. hesperus spiders is reputed to be particularly strong compared with the silk of other spiders.However, the results of a study show that this is not the case.
The ultimate tensile strength and other physical properties of Latrodectus hesperus (western black widow) silk were found to be similar to the properties of silk from orb-weaving spiders that had been tested in other studies. The tensile strength for the three kinds of silk measured in the Blackledge study was about 1000 MPa. The ultimate strength reported in a previous study for Nephila edulis was b1290 MPa ± 160 MPa. The tensile strength of spider silk is comparable to that of steel wire of the same thickness. However, as the density of steel is about six times that of silk silk is correspondingly stronger than steel wire of the same weight.
ToxicologyEditVenom is produced by glands in the cephalothorax, and expelled venom travels through paired ducts from the cephalothorax, exiting through the tip of the spider's hollow fangs. The venom of the Black Widow spider is thought to be similar to other Latrodectus spiders, and contains a number of high-molecular-weight proteins, one of which, alpha-latrotoxin (a neurotoxin), is active in humans. In vertebrates, alpha-latrotoxin produces its effect through destabilization of cell membranes and degranulation of nerve terminals, resulting in the release of neurotransmitters; it causes uncontrolled release of acetylcholine, norepinephrine, and GABA. The release of these neurotransmitters leads to the clinical manifestations of envenomation. At concentrations of 200 ng/ml, the venom also deforms human red blood cells, an effect common to the venom of bees, the blue-ringed octupus, and a range of snakes.
Bites in humansEdit
The Black Widow spider bites rarely cause significant morbidity, and deaths are even more rare. Throughout Australian history, only 14 deaths from Black Widows have been recorded. A significant proportion of bites will not result in envenomation or any symptoms developing. Hundreds or even thousands of people are thought to be bitten each year across Australia, although only about 20% of bite victims require treatment. Children, the elderly, or those with serious medical conditions are at much higher risk of severe side effects and death resulting from a bite. No deaths have been reported since the introduction of antivenom in 1956.
In an Australian study of 750 emergency hospital admissions for spider bites where the spider was definitively identified, 56 were from Black Widows. Of these, 37 had significant pain lasting over 24 hours, but only six were treated with the antivenom.
The larger female spider is responsible for almost all cases of Widow spider bites in humans. The smaller male spider was thought to be unable to envenomate a human, although male bites have been reported. The rarity of male bite reports is probably due to the male's smaller size and proportionally smaller fangs, rather than the male being incapable of biting or lacking venom of potency similar to the female's. Cases have shown the male bite usually only produces short-lived, mild pain.
Most bites occur in the warmer months between December and April, in the afternoon or evening.As the female Widow is slow-moving, and rarely leaves its web, bites generally occur as a result of a person placing a hand or other body part too close to the web, such as when reaching into dark holes or wall cavities. Bites can also occur if a spider has hidden in clothes or shoes.
Bites from Black Widow spiders produce a syndrome known as latrodectism, with symptoms similar to bites from other Latrodectus spiders. The syndrome is generally characterised by extreme pain and severe swelling. The bite may be painful from the start, but sometimes only feels like a pinprick or mild burning sensation. Within an hour, victims generally develop more severe local pain with local swelling and sometimes piloerection (goosebumps). Pain, swelling and redness spread proximally from the site. Systemic envenoming is heralded by swollen or tender regional lymph nodes; associated features include malaise, nausea, vomiting, abdominal or chest pain, generalised sweating, headache, fever, hypertension and tremor. Rare complications include seizure, coma, pulmonary edema, respiratory failure or localised skin infection. Severe pain can persist for over 24 hours after being bitten.
Medical advice is recommended after being bitten by a Black Widow spider. Usually, this requires observation in or near a medical facility for six hours from time of the bite. Treatment is based on the severity of the bite; patients with localised pain, swelling and redness usually do not require any specific treatment apart from applying ice and routine analgesics. In more severe bites, the definitive treatment consists of administering Black Widow antivenom, which will usually relieve symptoms of systemic envenoming immediately.
Antivenom is indicated in anyone suffering symptoms consistent with Latrodectus envenoming. Particular indications for using antivenom are:
- Pain and swelling spreading proximally from site
- Chest pain
- Abdominal pain
- Unusual sweating
Currently, this antivenom is recommended to be given intramuscularly(IM) rather than intravenously(IV), although some have suggested IM antivenom is not as effective as IV antivenom,because IM antivenom takes longer to reach the blood serum. Adverse reactions to Black Widow antivenom are rare Antivenom may be effective for up to 3 months after a bite, but in the vast majority of cases, it is administered within 24 hours. Doses are the same for both children and adults.