Black widow spider
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- For other uses of the name "Black Widow", see Black Widow.
The black widow spider (Latrodectus spp.) is a spider notorious for its neurotoxic venom. It is a large widow spider found throughout the world and commonly associated with urban habitats or agricultural areas. Although the common name 'black widow spider' is most commonly used to refer to the three North American species best known for their dark coloration and red hourglass pattern, it is occasionally also applied to several other members of the Latrodectus (widow spider) genus in which there are 31 recognized species including the Australian red-back, brown widow spider (sometimes called the gray widow), and the red widow spider. In South Africa, widow spiders are also known as the button spiders.
Currently, there are three recognized species of black widow found in North America: The southern black widow (L. mactans), the northern black widow (L. variolus), and the western black widow (L. hesperus). As the name indicates, the southern widow is primarily found (and is indigenous to) the southeastern United States, ranging from Florida to New York, and west to Texas and Oklahoma. Specimens have been found further west as well. The northern widow is found primarily in the northeastern US and southeastern Canada, though its ranges overlap that of L. mactans quite a bit. The western widow is found in the western half of the United States, as well as in southwestern Canada and much of Mexico. Prior to 1970, when the current taxonomic divisions for North American black widows were set forth by Kaston, all three varieties were classified as a single species, L. mactans. As a result, there exist numerous references which claim that "black widow" (without any geographic modifier) applies to L. mactans alone. As common usage of the term "black widow" makes no distinction between the three species (and many laypersons are unaware of the differences between them), and as the three species have much in common, this article treats all three species of black widow equally. Except where otherwise indicated, the remainder of the article applies to all three of the above species.
Description
Adult female black widow spiders are shiny black with an hourglass shaped marking on the bottom of its abdomen which, although most commonly red, may range in color from white to yellow to various shades of orange and red. They also bear a small, usually red (colors vary) dot near the spinerettes, which is separate from the hourglass. In L. variolus, the two halves of the hourglass shape may be separated into two separate patches. A large female black widow spider can grow to 1.5 inches (38 mm), counting legspan. The body is about 0.5 inches (13 mm). Male black widow spiders are half the size of the female or smaller. They have longer legs and a smaller abdomen in relation to their body size. They are also usually dark brown with varying colors of stripes/dots, with no hourglass mark. Adult males can be distinguished from juvenile females by their more-slender body, longer legs and large pedipalps typical of most other male spiders. Juvenile black widow spiders start white, molting to dark brown to black exoskeletons with white, yellow, orange and red stripes and/or dots on their backs.
As with many poisonous creatures, the brightly coloured markings serve as a warning to predators. Eating a black widow will normally not kill a small predator (birds, et cetera), but the sickness that follows digestion is enough for the creature to remember that the bright red means not to eat. Because the adult female black widow typically hangs and moves about its web upside down, its hourglass is on its front. However, juvenile widows (female) spend a large quantity of time in search of an optimal environment. Once an optimal location is found, adult female widows often spend their entire lives in one place. However, because juvenile females must first find this optimal location, they bear brightly colored marks upon their backs, so that they may be seen by predators when the widow is travelling upon its legs. Males bear similar marks to the females to serve as warning while they are searching for mates, however, the marks are not as prominent (not as brightly colored, or as large). Males, being less poisonous, are less of a threat to predators, so having similar marks not as prominent helps predators to better judge their prey (some large birds can eat male widows without adverse effect, and so only avoid female widows. Those who cannot eat any widow without adverse effect eat nothing with the marks.) Female juveniles develop an hourglass before the dorsal markings are shed. As is characteristic of all arthropods, black widow spiders have a hard exoskeleton composed of chitin and protein (5).
Prey
Black widow spiders typically prey on a variety of insects, but occasionally they do feed upon woodlice, diplopods, chilopods and other arachnids (McCorkle, 2002). When the prey is entangled by the web, L. mactans quickly comes out of its retreat, wraps the prey securely in its strong web, then punctures and poisons its prey (Foelix, 1982). The venom takes about ten minutes to take effect, meanwhile the prey is held tightly by the spider (Foelix, 1982). When movements of the prey cease, digestive enzymes are released into the wound (Foelix, 1982). The black widow spider then carries its prey back to its retreat before feeding (Foelix, 1982).
Reproduction
When a male is mature, he spins a sperm web, deposits semen on it, and charges his palpi with the sperm (3). Black widow spiders reproduce sexually when the male inserts his palpus into the female's spermathecal openings (3). The female deposits her eggs in a globular silken container which they remain camouflaged and guarded (3). A female black widow spider can produce nine egg sacs in one summer, each containing about 400 eggs (1). Usually, eggs incubate for twenty to thirty days. Rarely do more than one hundred survive through this process, due to cannibalism. On average, thirty will survive through the first molting, due to cannibalism, lack of food, or lack of proper shelter. It takes two to four months for black widow spiders to mature enough to breed, however full maturation typically takes six to nine months. The females can live for up to five years, while a male's lifespan is much shorter. Lifespans depend upon environment, shelter being the greatest determining factor; food the second greatest.
Natural Enemies
There are various parasites and predators of widow spiders in North America, though apparently none of these have ever been evaluated in terms of augmentation programs for improved biocontrol. Parasites of the egg sacs include the flightless scelionid wasp Baeus latrodecti, and members of the chloropid fly genus Pseudogaurax. Predators of the adult spiders include a few wasps, most notably the blue mud dauber Chalybion californicum, and the spider wasp Tastiotenia festiva. Other species will occasionally and opportunistically take widows as prey, but the preceding all exhibit some significant specific preference for Latrodectus.
Venom components and effects
Although their venom is extremely potent, these spiders are not especially large. Compared to many other species of spiders, their chelicerae are not very large or powerful. In the case of a mature female, the hollow, needle shaped part of each chelicera, the part that penetrates the skin, is approximately 1.0 mm (around .04 inch) long, sufficiently long to inject the venom to a dangerous depth. The males, being much smaller, can inject far less venom and inject it far less deeply. The actual amount injected, even by a mature female, is very small in physical volume. When this small amount of venom is diffused throughout the body of a healthy, mature human, it usually does not amount to a fatal dose. Deaths in healthy adults from Latrodectus bites are relatively rare in terms of the number of bites per thousand people. Only sixty-three deaths were reported in the United States between 1950 and 1989 (Miller, 1992). On the other hand, the geographical range of the widow spiders is very great. As a result, far more people are exposed, world-wide, to widow bites than are exposed to bites of more dangerous spiders, so the highest number of deaths world-wide are caused by members of their genus. Widow spiders have more potent venom than most spiders, and prior to the development of antivenom, 5% of reported bites result in fatalities. Black widow venom spreads rapidly throughout the body and acts by causing the release of the neurotransmitter acetylcholine, which is involved in muscular contraction.
There are a number of active components in the venom. There are a few high-molecular mass components (These proteins, called latrotoxins, cause a massive transmitter release from a diversity of nerve endings), and a number of smaller polypeptides, toxins interacting with cation channels which display spatial structure homology. They can affect the functioning of calcium, sodium, or potassium channels. There are also a few even more simple molecules, adenosine (1), guanosine (2), inosine (3), and 2,4,6-trihydroxypurine (4). The mechanism of the venom relates to the toxin initially being carried by the lymphatic system until it reaches the blood stream. Once in the blood, the toxin is moved by the circulation and deposited in the nerve ends where they insert into the muscle. Most strongly affected are back, abdomen, and thigh muscle areas. The venom acts at the nerve ends to prevent relaxation of the muscle, causing tetany - or constant, strong, painful contractions of the muscles. Standard treatments usually involve symptomatic therapy with pain medication, muscle relaxants, and, rarely, antivenin. The venom does not typically cause problems at the bite site itself, unless a secondary skin infection occurs.
Antivenom is generally reserved for use only when absolutely necessary. Due to the low mortality rate of victims, the risk of major injuries or death due to serum sickness are greater than the risk of death due to the neurotoxins. In past years, injections of calcium gluconate were used to replace calcium rendered unusable by the neurotoxins, but hospitals have been switching over to the use of prescription painkillers, especially opiates, such as morphine and others, to reduce symptoms, as the calcium injections do very little for the pain, and do not significantly reduce the recovery time.
More photos
References
- Chesapeake Bay Program partner. March 2002. [Black Widow Spider]
- Foelix, R. 1982. Biology of Spiders. Harvard University: USA. pp. 162-163
- Jones, Susan C. Black Widow Spider Fact Sheet. Ohio State University: USA. [link]
- McCorkle, Matthew. October 17th, 2002. [Latrodetus Mactans]
- Miller, T. 1992. Latrodectism: bite of the black widow spider. Am. Fam. Phys. 45:181
- Solomon et al. 2002.Biology.Thompson Learning, Inc:USA. pp. 629-630,1209-1214
- Wigmore, B. 2003. Venom 'Viagra'. The Mirror. News section, p.31
- R. Burton, MD; Emergency Medicine. Lectures on Toxins and Venom. 1989.
External links
- [Latrodectus Mactans on Pterodattilo]
- [Black widow spider diagnostic photographs, descriptions, natural history]
- [Photos of female with egg sac, Veracruz, Mexico]
- [Black widow spider Pictures]
- [Black widow spider and habitat pictures, Veracruz, Mexico]
- [Virginia Cooperative Extension: widow spiders]
- [University of Washington Burke Museum spider myths]
- [Bites and stings of medically important venomous spiders - UCR]
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